diff --git a/.dart_tool/package_config.json b/.dart_tool/package_config.json
index 8bd78518f79..87d1ada8f42 100644
--- a/.dart_tool/package_config.json
+++ b/.dart_tool/package_config.json
@@ -427,12 +427,12 @@
       "name": "observatory",
       "rootUri": "../runtime/observatory",
       "packageUri": "lib/",
-      "languageVersion": "2.9"
+      "languageVersion": "2.2"
     },
     {
       "name": "observatory_test_package",
       "rootUri": "../runtime/observatory/tests/service/observatory_test_package",
-      "languageVersion": "2.9"
+      "languageVersion": "2.7"
     },
     {
       "name": "package_config",
diff --git a/pkg/test_runner/lib/src/compiler_configuration.dart b/pkg/test_runner/lib/src/compiler_configuration.dart
index 8113cc5c7fb..97be6bd17fd 100644
--- a/pkg/test_runner/lib/src/compiler_configuration.dart
+++ b/pkg/test_runner/lib/src/compiler_configuration.dart
@@ -962,6 +962,7 @@ class AnalyzerCompilerConfiguration extends CompilerConfiguration {
       "ffi_2",
       "language_2",
       "lib_2",
+      "service",
       "standalone_2"
     };
 
diff --git a/runtime/observatory/BUILD.gn b/runtime/observatory/BUILD.gn
index 053164ecd54..1762e1aee03 100644
--- a/runtime/observatory/BUILD.gn
+++ b/runtime/observatory/BUILD.gn
@@ -37,7 +37,6 @@ prebuilt_dart2js_action("build_observatory") {
     "-o",
     rebase_path(output),
     "--packages=" + rebase_path("../../.packages"),
-    "--enable-experiment=non-nullable",
   ]
   if (is_debug) {
     args += [ "--enable-asserts" ]
diff --git a/runtime/observatory/analysis_options.yaml b/runtime/observatory/analysis_options.yaml
index 13bad8a6816..6babf28a427 100644
--- a/runtime/observatory/analysis_options.yaml
+++ b/runtime/observatory/analysis_options.yaml
@@ -1,10 +1,7 @@
 analyzer:
-  enable-experiment:
-    - non-nullable
   errors:
     dead_code: ignore
     unused_local_variable: ignore
-    getter_not_subtype_setter_types: ignore
   exclude:
     - tests/service/bad_reload/v2/main.dart
     - tests/service/complex_reload/v2/main.dart
diff --git a/runtime/observatory/bin/heap_snapshot.dart b/runtime/observatory/bin/heap_snapshot.dart
index b5aaf93fafd..22d3ce53f14 100644
--- a/runtime/observatory/bin/heap_snapshot.dart
+++ b/runtime/observatory/bin/heap_snapshot.dart
@@ -22,14 +22,14 @@ Future<SnapshotGraph> load(String uri) async {
 
   reader.onProgress.listen(print);
 
-  ws.listen((dynamic dynResponse) {
-    if (dynResponse is String) {
-      final response = json.decode(dynResponse);
+  ws.listen((dynamic response) {
+    if (response is String) {
+      response = json.decode(response);
       if (response['id'] == 1) {
         getVM.complete(response['result']['isolates'][0]['id']);
       }
-    } else if (dynResponse is List<int>) {
-      final response = new Uint8List.fromList(dynResponse);
+    } else if (response is List<int>) {
+      response = new Uint8List.fromList(response);
       final dataOffset =
           new ByteData.view(response.buffer).getUint32(0, Endian.little);
       dynamic metadata = new Uint8List.view(response.buffer, 4, dataOffset - 4);
diff --git a/runtime/observatory/bin/shell.dart b/runtime/observatory/bin/shell.dart
index d6c551fef7d..a0cb274be4a 100644
--- a/runtime/observatory/bin/shell.dart
+++ b/runtime/observatory/bin/shell.dart
@@ -30,7 +30,7 @@ void repl(VM vm, Isolate isolate, String lastResult) {
 void main() {
   String addr = 'ws://localhost:8181/ws';
   new WebSocketVM(new WebSocketVMTarget(addr)).load().then((serviceObject) {
-    VM vm = serviceObject as VM;
+    VM vm = serviceObject;
     Isolate isolate = vm.isolates.first;
     repl(vm, isolate, 'isolate ${isolate.id}');
   });
diff --git a/runtime/observatory/lib/event.dart b/runtime/observatory/lib/event.dart
index 71bed827806..e79b16b3c61 100644
--- a/runtime/observatory/lib/event.dart
+++ b/runtime/observatory/lib/event.dart
@@ -108,7 +108,7 @@ class PauseBreakpointEvent implements M.PauseBreakpointEvent {
   final bool atAsyncSuspension;
 
   /// [optional]
-  final M.Breakpoint? breakpoint;
+  final M.Breakpoint breakpoint;
   PauseBreakpointEvent(
       this.timestamp,
       this.isolate,
@@ -128,7 +128,7 @@ class PauseBreakpointEvent implements M.PauseBreakpointEvent {
 class PauseInterruptedEvent implements M.PauseInterruptedEvent {
   final DateTime timestamp;
   final M.IsolateRef isolate;
-  final M.Frame? topFrame;
+  final M.Frame topFrame;
   final bool atAsyncSuspension;
   PauseInterruptedEvent(
       this.timestamp, this.isolate, this.topFrame, this.atAsyncSuspension) {
@@ -141,7 +141,7 @@ class PauseInterruptedEvent implements M.PauseInterruptedEvent {
 class PausePostRequestEvent implements M.PausePostRequestEvent {
   final DateTime timestamp;
   final M.IsolateRef isolate;
-  final M.Frame? topFrame;
+  final M.Frame topFrame;
   final bool atAsyncSuspension;
   PausePostRequestEvent(
       this.timestamp, this.isolate, this.topFrame, this.atAsyncSuspension) {
@@ -168,7 +168,7 @@ class PauseExceptionEvent implements M.PauseExceptionEvent {
 class ResumeEvent implements M.ResumeEvent {
   final DateTime timestamp;
   final M.IsolateRef isolate;
-  final M.Frame? topFrame;
+  final M.Frame topFrame;
   ResumeEvent(this.timestamp, this.isolate, this.topFrame) {
     assert(timestamp != null);
     assert(isolate != null);
@@ -309,71 +309,69 @@ class ServiceUnregisteredEvent implements M.ServiceUnregisteredEvent {
   }
 }
 
-M.Event? createEventFromServiceEvent(S.ServiceEvent event) {
+M.Event createEventFromServiceEvent(S.ServiceEvent event) {
   switch (event.kind) {
     case S.ServiceEvent.kVMUpdate:
-      return new VMUpdateEvent(event.timestamp!, event.vm);
+      return new VMUpdateEvent(event.timestamp, event.vm);
     case S.ServiceEvent.kIsolateStart:
-      return new IsolateStartEvent(event.timestamp!, event.isolate!);
+      return new IsolateStartEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kIsolateRunnable:
-      return new IsolateRunnableEvent(event.timestamp!, event.isolate!);
+      return new IsolateRunnableEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kIsolateUpdate:
-      return new IsolateUpdateEvent(event.timestamp!, event.isolate!);
+      return new IsolateUpdateEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kIsolateReload:
       return new IsolateReloadEvent(
-          event.timestamp!, event.isolate!, event.error!);
+          event.timestamp, event.isolate, event.error);
     case S.ServiceEvent.kIsolateExit:
-      return new IsolateExitEvent(event.timestamp!, event.isolate!);
+      return new IsolateExitEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kBreakpointAdded:
       return new BreakpointAddedEvent(
-          event.timestamp!, event.isolate!, event.breakpoint!);
+          event.timestamp, event.isolate, event.breakpoint);
     case S.ServiceEvent.kBreakpointResolved:
       return new BreakpointResolvedEvent(
-          event.timestamp!, event.isolate!, event.breakpoint!);
+          event.timestamp, event.isolate, event.breakpoint);
     case S.ServiceEvent.kBreakpointRemoved:
       return new BreakpointRemovedEvent(
-          event.timestamp!, event.isolate!, event.breakpoint!);
+          event.timestamp, event.isolate, event.breakpoint);
     case S.ServiceEvent.kDebuggerSettingsUpdate:
-      return new DebuggerSettingsUpdateEvent(event.timestamp!, event.isolate!);
+      return new DebuggerSettingsUpdateEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kResume:
-      return new ResumeEvent(event.timestamp!, event.isolate!, event.topFrame);
+      return new ResumeEvent(event.timestamp, event.isolate, event.topFrame);
     case S.ServiceEvent.kPauseStart:
-      return new PauseStartEvent(event.timestamp!, event.isolate!);
+      return new PauseStartEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kPauseExit:
-      return new PauseExitEvent(event.timestamp!, event.isolate!);
+      return new PauseExitEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kPausePostRequest:
-      return new PausePostRequestEvent(event.timestamp!, event.isolate!,
-          event.topFrame, event.atAsyncSuspension!);
+      return new PausePostRequestEvent(event.timestamp, event.isolate,
+          event.topFrame, event.atAsyncSuspension);
     case S.ServiceEvent.kPauseBreakpoint:
       return new PauseBreakpointEvent(
-          event.timestamp!,
-          event.isolate!,
-          event.pauseBreakpoints!,
-          event.topFrame!,
-          event.atAsyncSuspension!,
+          event.timestamp,
+          event.isolate,
+          event.pauseBreakpoints,
+          event.topFrame,
+          event.atAsyncSuspension,
           event.breakpoint);
     case S.Isolate.kLoggingStream:
-      return new LoggingEvent(
-          event.timestamp!, event.isolate!, event.logRecord!);
+      return new LoggingEvent(event.timestamp, event.isolate, event.logRecord);
     case S.ServiceEvent.kPauseInterrupted:
-      return new PauseInterruptedEvent(event.timestamp!, event.isolate!,
-          event.topFrame, event.atAsyncSuspension!);
+      return new PauseInterruptedEvent(event.timestamp, event.isolate,
+          event.topFrame, event.atAsyncSuspension);
     case S.ServiceEvent.kPauseException:
       return new PauseExceptionEvent(
-          event.timestamp!, event.isolate!, event.topFrame!, event.exception!);
+          event.timestamp, event.isolate, event.topFrame, event.exception);
     case S.ServiceEvent.kInspect:
-      return new InspectEvent(
-          event.timestamp!, event.isolate!, event.inspectee!);
+      return new InspectEvent(event.timestamp, event.isolate, event.inspectee);
     case S.ServiceEvent.kGC:
-      return new GCEvent(event.timestamp!, event.isolate!);
+      return new GCEvent(event.timestamp, event.isolate);
     case S.ServiceEvent.kServiceRegistered:
       return new ServiceRegisteredEvent(
-          event.timestamp!, event.service!, event.method!, event.alias!);
+          event.timestamp, event.service, event.method, event.alias);
     case S.ServiceEvent.kServiceUnregistered:
       return new ServiceUnregisteredEvent(
-          event.timestamp!, event.service!, event.method!);
+          event.timestamp, event.service, event.method);
     case S.ServiceEvent.kNone:
-      return new NoneEvent(event.timestamp!, event.isolate!);
+      return new NoneEvent(event.timestamp, event.isolate);
     default:
       // Ignore unrecognized events.
       Logger.root.severe('Unrecognized event: $event');
diff --git a/runtime/observatory/lib/object_graph.dart b/runtime/observatory/lib/object_graph.dart
index 8a62d01eeb2..ce48c8bf694 100644
--- a/runtime/observatory/lib/object_graph.dart
+++ b/runtime/observatory/lib/object_graph.dart
@@ -22,7 +22,7 @@ abstract class SnapshotReader {
 
 class _SnapshotReader implements SnapshotReader {
   bool _closed = false;
-  List<Uint8List>? _chunks = <Uint8List>[];
+  var _chunks = <Uint8List>[];
   final _onProgress = new StreamController<String>.broadcast();
   final _done = new Completer<SnapshotGraph>();
 
@@ -32,8 +32,8 @@ class _SnapshotReader implements SnapshotReader {
     if (_closed) {
       throw new StateError("Stream is closed");
     }
-    _chunks!.add(chunk);
-    _onProgress.add("Receiving snapshot chunk ${_chunks!.length}...");
+    _chunks.add(chunk);
+    _onProgress.add("Receiving snapshot chunk ${_chunks.length}...");
 
     // TODO(rmacnak): Incremental loading.
   }
@@ -45,7 +45,7 @@ class _SnapshotReader implements SnapshotReader {
     _closed = true;
 
     var graph = new _SnapshotGraph._new();
-    var chunks = _chunks!;
+    var chunks = _chunks;
     _chunks = null; // Let the binary chunks be GCable.
     _done.complete(graph._load(chunks, _onProgress));
     return _done.future;
@@ -270,39 +270,39 @@ class _SnapshotObject implements SnapshotObject {
   int get hashCode => _id ^ _graph.hashCode;
 
   int get shallowSize => internalSize + externalSize;
-  int get internalSize => _graph._internalSizes![_id];
-  int get externalSize => _graph._externalSizes![_id];
-  int get retainedSize => _graph._retainedSizes![_id];
+  int get internalSize => _graph._internalSizes[_id];
+  int get externalSize => _graph._externalSizes[_id];
+  int get retainedSize => _graph._retainedSizes[_id];
 
   String get description => _graph._describeObject(_id);
-  SnapshotClass get klass => _graph._classes![_graph._cids![_id]]!;
+  SnapshotClass get klass => _graph._classes[_graph._cids[_id]];
 
   Iterable<SnapshotObject> get successors sync* {
     final id = _id;
-    final cid = _graph._cids![id];
-    final startSuccIndex = _graph._firstSuccs![id];
-    final limitSuccIndex = _graph._firstSuccs![id + 1];
+    final cid = _graph._cids[id];
+    final startSuccIndex = _graph._firstSuccs[id];
+    final limitSuccIndex = _graph._firstSuccs[id + 1];
     for (var nextSuccIndex = startSuccIndex;
         nextSuccIndex < limitSuccIndex;
         nextSuccIndex++) {
       final index = nextSuccIndex - startSuccIndex;
-      final succId = _graph._succs![nextSuccIndex];
+      final succId = _graph._succs[nextSuccIndex];
       final name = _graph._edgeName(cid, index);
       yield _SnapshotObject._new(succId, _graph, name);
     }
   }
 
   Iterable<SnapshotObject> get predecessors sync* {
-    var firstSuccs = _graph._firstSuccs!;
-    var succs = _graph._succs!;
+    var firstSuccs = _graph._firstSuccs;
+    var succs = _graph._succs;
     var id = _id;
-    var N = _graph._N!;
+    var N = _graph._N;
     for (var predId = 1; predId <= N; predId++) {
       var base = firstSuccs[predId];
       var limit = firstSuccs[predId + 1];
       for (var i = base; i < limit; i++) {
         if (succs[i] == id) {
-          var cid = _graph._cids![predId];
+          var cid = _graph._cids[predId];
           var name = _graph._edgeName(cid, i - base);
           yield _SnapshotObject._new(predId, _graph, name);
         }
@@ -314,12 +314,12 @@ class _SnapshotObject implements SnapshotObject {
     if (_id == _ROOT) {
       return this;
     }
-    return _SnapshotObject._new(_graph._doms![_id], _graph, "");
+    return _SnapshotObject._new(_graph._doms[_id], _graph, "");
   }
 
   Iterable<SnapshotObject> get children sync* {
-    var N = _graph._N!;
-    var doms = _graph._doms!;
+    var N = _graph._N;
+    var doms = _graph._doms;
     var parentId = _id;
     for (var childId = _ROOT; childId <= N; childId++) {
       if (doms[childId] == parentId) {
@@ -374,7 +374,7 @@ abstract class SnapshotMergedDominator {
 class _SnapshotMergedDominator implements SnapshotMergedDominator {
   final int _id;
   final _SnapshotGraph _graph;
-  final _SnapshotMergedDominator? _parent;
+  final _SnapshotMergedDominator _parent;
 
   _SnapshotMergedDominator._new(this._id, this._graph, this._parent);
 
@@ -389,78 +389,70 @@ class _SnapshotMergedDominator implements SnapshotMergedDominator {
 
   String get description => "$instanceCount instances of ${klass.name}";
 
-  SnapshotClass get klass => _graph._classes![_graph._cids![_id]]!;
+  SnapshotClass get klass => _graph._classes[_graph._cids[_id]];
 
   int get shallowSize => internalSize + externalSize;
 
   int get internalSize {
-    var cids = _graph._cids!;
-    var internalSizes = _graph._internalSizes!;
-    var mergedDomNext = _graph._mergedDomNext!;
+    var cids = _graph._cids;
     var size = 0;
     var sibling = _id;
     while (sibling != _SENTINEL && cids[sibling] == cids[_id]) {
-      size += internalSizes[sibling];
-      sibling = mergedDomNext[sibling];
+      size += _graph._internalSizes[sibling];
+      sibling = _graph._mergedDomNext[sibling];
     }
     return size;
   }
 
   int get externalSize {
-    var cids = _graph._cids!;
-    var externalSizes = _graph._externalSizes!;
-    var mergedDomNext = _graph._mergedDomNext!;
+    var cids = _graph._cids;
     var size = 0;
     var sibling = _id;
     while (sibling != _SENTINEL && cids[sibling] == cids[_id]) {
-      size += externalSizes[sibling];
-      sibling = mergedDomNext[sibling];
+      size += _graph._externalSizes[sibling];
+      sibling = _graph._mergedDomNext[sibling];
     }
     return size;
   }
 
   int get retainedSize {
-    var cids = _graph._cids!;
-    var retainedSizes = _graph._retainedSizes!;
-    var mergedDomNext = _graph._mergedDomNext!;
+    var cids = _graph._cids;
     var size = 0;
     var sibling = _id;
     while (sibling != _SENTINEL && cids[sibling] == cids[_id]) {
-      size += retainedSizes[sibling];
-      sibling = mergedDomNext[sibling];
+      size += _graph._retainedSizes[sibling];
+      sibling = _graph._mergedDomNext[sibling];
     }
     return size;
   }
 
   int get instanceCount {
-    var cids = _graph._cids!;
-    var mergedDomNext = _graph._mergedDomNext!;
+    var cids = _graph._cids;
     var count = 0;
     var sibling = _id;
     while (sibling != _SENTINEL && cids[sibling] == cids[_id]) {
       count++;
-      sibling = mergedDomNext[sibling];
+      sibling = _graph._mergedDomNext[sibling];
     }
     return count;
   }
 
   Iterable<SnapshotObject> get objects sync* {
-    var cids = _graph._cids!;
-    var mergedDomNext = _graph._mergedDomNext!;
+    var cids = _graph._cids;
     var sibling = _id;
     while (sibling != _SENTINEL && cids[sibling] == cids[_id]) {
       yield _SnapshotObject._new(sibling, _graph, "");
-      sibling = mergedDomNext[sibling];
+      sibling = _graph._mergedDomNext[sibling];
     }
   }
 
   SnapshotMergedDominator get parent => _parent ?? this;
 
   Iterable<SnapshotMergedDominator> get children sync* {
-    var next = _graph._mergedDomNext!;
-    var cids = _graph._cids!;
+    var next = _graph._mergedDomNext;
+    var cids = _graph._cids;
     var prev = _SENTINEL;
-    var child = _graph._mergedDomHead![_id];
+    var child = _graph._mergedDomHead[_id];
     // Walk the list of children and look for the representative objects, i.e.
     // the first sibling of each cid.
     while (child != _SENTINEL) {
@@ -512,11 +504,9 @@ class _SnapshotClass implements SnapshotClass {
   int get instanceCount => liveInstanceCount;
 
   Iterable<SnapshotObject> get instances sync* {
-    final N = _graph._N!;
-    final cids = _graph._cids!;
-    final retainedSizes = _graph._retainedSizes!;
+    final N = _graph._N;
     for (var id = 1; id <= N; id++) {
-      if (cids[id] == _cid && retainedSizes[id] > 0) {
+      if (_graph._cids[id] == _cid && _graph._retainedSizes[id] > 0) {
         yield _SnapshotObject._new(id, _graph, "");
       }
     }
@@ -566,27 +556,26 @@ const _kRootName = "Root";
 const _kUnknownFieldName = "<unknown>";
 
 class _SnapshotGraph implements SnapshotGraph {
-  List<Uint8List>? _chunks;
-  List<Uint8List> get chunks => _chunks!;
+  List<Uint8List> _chunks;
+  List<Uint8List> get chunks => _chunks;
 
   _SnapshotGraph._new();
 
-  String get description => _description!;
+  String get description => _description;
 
-  int get size => _liveInternalSize! + _liveExternalSize!;
-  int get internalSize => _liveInternalSize!;
-  int get externalSize => _liveExternalSize!;
-  int get capacity => _capacity!;
+  int get size => _liveInternalSize + _liveExternalSize;
+  int get internalSize => _liveInternalSize;
+  int get externalSize => _liveExternalSize;
+  int get capacity => _capacity;
 
   SnapshotObject get root => _SnapshotObject._new(_ROOT, this, "Root");
   SnapshotMergedDominator get mergedRoot =>
       _SnapshotMergedDominator._new(_ROOT, this, null);
 
   Iterable<SnapshotObject> get objects sync* {
-    final N = _N!;
-    final retainedSizes = _retainedSizes!;
+    final N = _N;
     for (var id = 1; id <= N; id++) {
-      if (retainedSizes[id] > 0) {
+      if (_retainedSizes[id] > 0) {
         yield _SnapshotObject._new(id, this, "");
       }
     }
@@ -600,7 +589,7 @@ class _SnapshotGraph implements SnapshotGraph {
       return _kRootName;
     }
     var cls = _className(oid);
-    var data = _nonReferenceData![oid];
+    var data = _nonReferenceData[oid];
     if (data == null) {
       return cls;
     } else {
@@ -609,8 +598,8 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   String _className(int oid) {
-    var cid = _cids![oid];
-    var cls = _classes![cid];
+    var cid = _cids[oid];
+    var cls = _classes[cid];
     if (cls == null) {
       return "Class$cid";
     }
@@ -618,7 +607,7 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   String _edgeName(int cid, int index) {
-    var c = _classes![cid];
+    var c = _classes[cid];
     if (c == null) {
       return _kUnknownFieldName;
     }
@@ -630,7 +619,7 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   Iterable<SnapshotClass> get classes sync* {
-    for (final c in _classes!) {
+    for (final c in _classes) {
       // Not all CIDs are occupied.
       if (c != null) {
         yield c;
@@ -641,9 +630,9 @@ class _SnapshotGraph implements SnapshotGraph {
   final processPartitions = new Map<String, int>();
 
   Future<SnapshotGraph> _load(
-      List<Uint8List>? chunks, StreamController<String> onProgress) async {
+      List<Uint8List> chunks, StreamController<String> onProgress) async {
     _chunks = chunks;
-    _ReadStream? stream = _ReadStream._new(chunks!);
+    var stream = _ReadStream._new(chunks);
     chunks = null;
 
     // The phases of loading are placed in explicit `new Future(compuation)` so
@@ -651,16 +640,16 @@ class _SnapshotGraph implements SnapshotGraph {
     // defer to the microtask loop.
 
     onProgress.add("Loading classes...");
-    await new Future(() => _readClasses(stream!));
+    await new Future(() => _readClasses(stream));
 
     onProgress.add("Loading objects...");
-    await new Future(() => _readObjects(stream!));
+    await new Future(() => _readObjects(stream));
 
     onProgress.add("Loading external properties...");
-    await new Future(() => _readExternalProperties(stream!));
+    await new Future(() => _readExternalProperties(stream));
 
     onProgress.add("Loading process partitions...");
-    await new Future(() => _readProcessPartitions(stream!));
+    await new Future(() => _readProcessPartitions(stream));
     stream = null;
 
     onProgress.add("Compute class table...");
@@ -706,46 +695,46 @@ class _SnapshotGraph implements SnapshotGraph {
     return this;
   }
 
-  Uint8List? _encoded;
+  Uint8List _encoded;
 
-  String? _description;
+  String _description;
 
-  int? _kStackCid;
-  int? _kFieldCid;
-  int? _numCids;
-  int? _N; // Objects in the snapshot.
-  int? _Nconnected; // Objects reachable from root.
-  int? _E; // References in the snapshot.
+  int _kStackCid;
+  int _kFieldCid;
+  int _numCids;
+  int _N; // Objects in the snapshot.
+  int _Nconnected; // Objects reachable from root.
+  int _E; // References in the snapshot.
 
-  int? _capacity;
-  int? _liveInternalSize;
-  int? _liveExternalSize;
-  int? _totalInternalSize;
-  int? _totalExternalSize;
+  int _capacity;
+  int _liveInternalSize;
+  int _liveExternalSize;
+  int _totalInternalSize;
+  int _totalExternalSize;
 
-  List<_SnapshotClass?>? _classes;
+  List<_SnapshotClass> _classes;
 
   // Indexed by node id, with id 0 representing invalid/uninitialized.
   // From snapshot.
-  List? _nonReferenceData;
-  Uint16List? _cids;
-  Uint32List? _internalSizes;
-  Uint32List? _externalSizes;
-  Uint32List? _firstSuccs;
-  Uint32List? _succs;
+  List _nonReferenceData;
+  Uint16List _cids;
+  Uint32List _internalSizes;
+  Uint32List _externalSizes;
+  Uint32List _firstSuccs;
+  Uint32List _succs;
 
   // Intermediates.
-  Uint32List? _vertex;
-  Uint32List? _parent;
-  Uint32List? _semi;
-  Uint32List? _firstPreds; // Offset into preds.
-  Uint32List? _preds;
+  Uint32List _vertex;
+  Uint32List _parent;
+  Uint32List _semi;
+  Uint32List _firstPreds; // Offset into preds.
+  Uint32List _preds;
 
   // Outputs.
-  Uint32List? _doms;
-  Uint32List? _retainedSizes;
-  Uint32List? _mergedDomHead;
-  Uint32List? _mergedDomNext;
+  Uint32List _doms;
+  Uint32List _retainedSizes;
+  Uint32List _mergedDomHead;
+  Uint32List _mergedDomNext;
 
   void _readClasses(_ReadStream stream) {
     for (var i = 0; i < 8; i++) {
@@ -759,7 +748,7 @@ class _SnapshotGraph implements SnapshotGraph {
     _totalExternalSize = stream.readUnsigned();
 
     var K = stream.readUnsigned();
-    var classes = new List<_SnapshotClass?>.filled(K + 1, null);
+    var classes = new List<_SnapshotClass>.filled(K + 1, null);
     classes[0] = _SnapshotClass._new(this, 0, "Root", "", "");
 
     for (var cid = 1; cid <= K; cid++) {
@@ -802,7 +791,7 @@ class _SnapshotGraph implements SnapshotGraph {
 
     var internalSizes = _newUint32Array(N + 1);
     var cids = _newUint16Array(N + 1);
-    var nonReferenceData = new List<dynamic>.filled(N + 1, null);
+    var nonReferenceData = new List(N + 1);
     var firstSuccs = _newUint32Array(N + 2);
     var succs = _newUint32Array(E);
     var eid = 0;
@@ -878,7 +867,7 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   void _readExternalProperties(_ReadStream stream) {
-    final N = _N!;
+    final N = _N;
     final externalPropertyCount = stream.readUnsigned();
 
     final externalSizes = _newUint32Array(N + 1);
@@ -904,11 +893,11 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   void _computeClassTable() {
-    final N = _N!;
-    final classes = _classes!;
-    final cids = _cids!;
-    final internalSizes = _internalSizes!;
-    final externalSizes = _externalSizes!;
+    final N = _N;
+    final classes = _classes;
+    final cids = _cids;
+    final internalSizes = _internalSizes;
+    final externalSizes = _externalSizes;
     var totalInternalSize = 0;
     var totalExternalSize = 0;
 
@@ -919,7 +908,7 @@ class _SnapshotGraph implements SnapshotGraph {
       var externalSize = externalSizes[oid];
       totalExternalSize += externalSize;
 
-      var cls = classes[cids[oid]]!;
+      var cls = classes[cids[oid]];
       cls.totalInternalSize += internalSize;
       cls.totalExternalSize += externalSize;
       cls.totalInstanceCount++;
@@ -930,9 +919,9 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   void _dfs() {
-    final N = _N!;
-    final firstSuccs = _firstSuccs!;
-    final succs = _succs!;
+    final N = _N;
+    final firstSuccs = _firstSuccs;
+    final succs = _succs;
 
     final stackNodes = _newUint32Array(N);
     final stackCurrentEdgePos = _newUint32Array(N);
@@ -1020,11 +1009,11 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   void _buildPredecessors() {
-    final N = _N!;
-    final Nconnected = _Nconnected!;
-    final E = _E!;
-    final firstSuccs = _firstSuccs!;
-    final succs = _succs!;
+    final N = _N;
+    final Nconnected = _Nconnected;
+    final E = _E;
+    final firstSuccs = _firstSuccs;
+    final succs = _succs;
 
     // This is first filled with the predecessor counts, then reused to hold the
     // offset to the first predecessor (see alias below).
@@ -1079,17 +1068,17 @@ class _SnapshotGraph implements SnapshotGraph {
   // Fold the size of any object with in-degree(1) into its parent.
   // Requires the DFS numbering and predecessor lists.
   void _buildOwnedSizes() {
-    final N = _N!;
-    final Nconnected = _Nconnected!;
+    final N = _N;
+    final Nconnected = _Nconnected;
     final kStackCid = _kStackCid;
     final kFieldCid = _kFieldCid;
 
-    final cids = _cids!;
-    final internalSizes = _internalSizes!;
-    final externalSizes = _externalSizes!;
-    final vertex = _vertex!;
-    final firstPreds = _firstPreds!;
-    final preds = _preds!;
+    final cids = _cids;
+    final internalSizes = _internalSizes;
+    final externalSizes = _externalSizes;
+    final vertex = _vertex;
+    final firstPreds = _firstPreds;
+    final preds = _preds;
 
     final ownedSizes = _newUint32Array(N + 1);
     for (var i = 1; i <= Nconnected; i++) {
@@ -1134,11 +1123,11 @@ class _SnapshotGraph implements SnapshotGraph {
 
     // TODO(rmacnak): Maybe keep the per-objects sizes to be able to provide
     // examples of large owners for each class.
-    final classes = _classes!;
+    final classes = _classes;
     for (var i = 1; i <= Nconnected; i++) {
       final v = vertex[i];
       final cid = cids[v];
-      final cls = classes[cid]!;
+      final cls = classes[cid];
       cls.ownedSize += ownedSizes[v];
     }
   }
@@ -1220,14 +1209,14 @@ class _SnapshotGraph implements SnapshotGraph {
   // T. Lengauer and R. E. Tarjan. "A Fast Algorithm for Finding Dominators
   // in a Flowgraph."
   void _buildDominators() {
-    final N = _N!;
-    final Nconnected = _Nconnected!;
+    final N = _N;
+    final Nconnected = _Nconnected;
 
-    final vertex = _vertex!;
-    final semi = _semi!;
-    final parent = _parent!;
-    final firstPreds = _firstPreds!;
-    final preds = _preds!;
+    final vertex = _vertex;
+    final semi = _semi;
+    final parent = _parent;
+    final firstPreds = _firstPreds;
+    final preds = _preds;
 
     final dom = _newUint32Array(N + 1);
 
@@ -1236,7 +1225,7 @@ class _SnapshotGraph implements SnapshotGraph {
     for (var i = 1; i <= N; i++) {
       label[i] = i;
     }
-    final buckets = new List<dynamic>.filled(N + 1, null);
+    final buckets = new List(N + 1);
     final child = _newUint32Array(N + 1);
     final size = _newUint32Array(N + 1);
     for (var i = 1; i <= N; i++) {
@@ -1263,7 +1252,7 @@ class _SnapshotGraph implements SnapshotGraph {
       // w.semi.bucket.add(w);
       var tmp = vertex[semi[w]];
       if (buckets[tmp] == null) {
-        buckets[tmp] = [];
+        buckets[tmp] = new List();
       }
       buckets[tmp].add(w);
 
@@ -1295,17 +1284,17 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   void _calculateRetainedSizes() {
-    final N = _N!;
-    final Nconnected = _Nconnected!;
+    final N = _N;
+    final Nconnected = _Nconnected;
 
     var liveInternalSize = 0;
     var liveExternalSize = 0;
-    final classes = _classes!;
-    final cids = _cids!;
-    final internalSizes = _internalSizes!;
-    final externalSizes = _externalSizes!;
-    final vertex = _vertex!;
-    final doms = _doms!;
+    final classes = _classes;
+    final cids = _cids;
+    final internalSizes = _internalSizes;
+    final externalSizes = _externalSizes;
+    final vertex = _vertex;
+    final doms = _doms;
 
     // Sum internal and external sizes.
     for (var i = 1; i <= Nconnected; i++) {
@@ -1315,7 +1304,7 @@ class _SnapshotGraph implements SnapshotGraph {
       liveInternalSize += internalSize;
       liveExternalSize += externalSize;
 
-      var cls = classes[cids[v]]!;
+      var cls = classes[cids[v]];
       cls.liveInternalSize += internalSize;
       cls.liveExternalSize += externalSize;
       cls.liveInstanceCount++;
@@ -1346,18 +1335,18 @@ class _SnapshotGraph implements SnapshotGraph {
     _liveInternalSize = liveInternalSize;
     _liveExternalSize = liveExternalSize;
 
-    print("internal-garbage: ${_totalInternalSize! - _liveInternalSize!}");
-    print("external-garbage: ${_totalExternalSize! - _liveExternalSize!}");
-    print("fragmentation: ${_capacity! - _totalInternalSize!}");
-    assert(_liveInternalSize! <= _totalInternalSize!);
-    assert(_liveExternalSize! <= _totalExternalSize!);
-    assert(_totalInternalSize! <= _capacity!);
+    print("internal-garbage: ${_totalInternalSize - _liveInternalSize}");
+    print("external-garbage: ${_totalExternalSize - _liveExternalSize}");
+    print("fragmentation: ${_capacity - _totalInternalSize}");
+    assert(_liveInternalSize <= _totalInternalSize);
+    assert(_liveExternalSize <= _totalExternalSize);
+    assert(_totalInternalSize <= _capacity);
   }
 
   // Build linked lists of the children for each node in the dominator tree.
   void _linkDominatorChildren() {
-    final N = _N!;
-    final doms = _doms!;
+    final N = _N;
+    final doms = _doms;
     final head = _newUint32Array(N + 1);
     final next = _newUint32Array(N + 1);
 
@@ -1411,10 +1400,10 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   void _sortDominatorChildren() {
-    final N = _N!;
-    final cids = _cids!;
-    final head = _mergedDomHead!;
-    final next = _mergedDomNext!;
+    final N = _N;
+    final cids = _cids;
+    final head = _mergedDomHead;
+    final next = _mergedDomNext;
 
     // Returns the new head of the sorted list.
     int sort(int head) {
@@ -1448,10 +1437,10 @@ class _SnapshotGraph implements SnapshotGraph {
   }
 
   void _mergeDominatorSiblings() {
-    var N = _N!;
-    var cids = _cids!;
-    var head = _mergedDomHead!;
-    var next = _mergedDomNext!;
+    var N = _N;
+    var cids = _cids;
+    var head = _mergedDomHead;
+    var next = _mergedDomNext;
     var workStack = _newUint32Array(N);
     var workStackTop = 0;
 
diff --git a/runtime/observatory/lib/service_common.dart b/runtime/observatory/lib/service_common.dart
index 2dd171811ca..f0748918b72 100644
--- a/runtime/observatory/lib/service_common.dart
+++ b/runtime/observatory/lib/service_common.dart
@@ -26,9 +26,9 @@ class WebSocketVMTarget implements M.Target {
   bool get standalone => !chrome;
 
   // User defined name.
-  late String name;
+  String name;
   // Network address of VM.
-  late String networkAddress;
+  String networkAddress;
 
   WebSocketVMTarget(this.networkAddress) {
     name = networkAddress;
@@ -89,7 +89,7 @@ abstract class CommonWebSocketVM extends VM {
 
   String get displayName => '${name}@${target.name}';
 
-  CommonWebSocket? _webSocket;
+  CommonWebSocket _webSocket;
 
   CommonWebSocketVM(this.target, this._webSocket) {
     assert(target != null);
@@ -120,7 +120,7 @@ abstract class CommonWebSocketVM extends VM {
   void disconnect({String reason: 'WebSocket closed'}) {
     if (_hasInitiatedConnect) {
       if (_webSocket != null) {
-        _webSocket!.close();
+        _webSocket.close();
       }
     }
     // We don't need to cancel requests and notify here.  These
@@ -135,7 +135,7 @@ abstract class CommonWebSocketVM extends VM {
     if (!_hasInitiatedConnect) {
       _hasInitiatedConnect = true;
       try {
-        _webSocket!.connect(
+        _webSocket.connect(
             target.networkAddress, _onOpen, _onMessage, _onError, _onClose);
       } catch (_, stack) {
         _webSocket = null;
@@ -150,7 +150,7 @@ abstract class CommonWebSocketVM extends VM {
     }
     String serial = (_requestSerial++).toString();
     var request = new _WebSocketRequest(method, params);
-    if ((_webSocket != null) && _webSocket!.isOpen) {
+    if ((_webSocket != null) && _webSocket.isOpen) {
       // Already connected, send request immediately.
       _sendRequest(serial, request);
     } else {
@@ -180,7 +180,7 @@ abstract class CommonWebSocketVM extends VM {
     _notifyConnect();
   }
 
-  Map? _parseJSON(String message) {
+  Map _parseJSON(String message) {
     var map;
     try {
       map = json.decode(message);
@@ -198,7 +198,7 @@ abstract class CommonWebSocketVM extends VM {
   }
 
   void _onBinaryMessage(dynamic data) {
-    _webSocket!.nonStringToByteData(data).then((ByteData bytes) {
+    _webSocket.nonStringToByteData(data).then((ByteData bytes) {
       var metadataOffset = 4;
       var dataOffset = bytes.getUint32(0, Endian.little);
       var metadataLength = dataOffset - metadataOffset;
@@ -292,7 +292,7 @@ abstract class CommonWebSocketVM extends VM {
 
   /// Send all delayed requests.
   void _sendAllDelayedRequests() {
-    assert(_webSocket!.isOpen);
+    assert(_webSocket.isOpen);
     if (_delayedRequests.length == 0) {
       return;
     }
@@ -305,7 +305,7 @@ abstract class CommonWebSocketVM extends VM {
 
   /// Send the request over WebSocket.
   void _sendRequest(String serial, _WebSocketRequest request) {
-    assert(_webSocket!.isOpen);
+    assert(_webSocket.isOpen);
     // Mark request as pending.
     assert(_pendingRequests.containsKey(serial) == false);
     _pendingRequests[serial] = request;
@@ -332,7 +332,7 @@ abstract class CommonWebSocketVM extends VM {
           'GET [${serial}] ${request.method}(${request.params}) from ${target.networkAddress}');
     }
     // Send message.
-    _webSocket!.send(message);
+    _webSocket.send(message);
   }
 
   String toString() => displayName;
diff --git a/runtime/observatory/lib/service_html.dart b/runtime/observatory/lib/service_html.dart
index ad22944927e..ddc42f5d2d3 100644
--- a/runtime/observatory/lib/service_html.dart
+++ b/runtime/observatory/lib/service_html.dart
@@ -14,26 +14,25 @@ import 'package:observatory/service_common.dart';
 export 'package:observatory/service_common.dart';
 
 class _HtmlWebSocket implements CommonWebSocket {
-  WebSocket? _webSocket;
+  WebSocket _webSocket;
 
   void connect(String address, void onOpen(), void onMessage(dynamic data),
       void onError(), void onClose()) {
-    var socket = new WebSocket(address);
-    socket.onClose.listen((CloseEvent) => onClose());
-    socket.onError.listen((Event) => onError());
-    socket.onOpen.listen((Event) => onOpen());
-    socket.onMessage.listen((MessageEvent event) => onMessage(event.data));
-    _webSocket = socket;
+    _webSocket = new WebSocket(address);
+    _webSocket.onClose.listen((CloseEvent) => onClose());
+    _webSocket.onError.listen((Event) => onError());
+    _webSocket.onOpen.listen((Event) => onOpen());
+    _webSocket.onMessage.listen((MessageEvent event) => onMessage(event.data));
   }
 
-  bool get isOpen => _webSocket!.readyState == WebSocket.OPEN;
+  bool get isOpen => _webSocket.readyState == WebSocket.OPEN;
 
   void send(dynamic data) {
-    _webSocket!.send(data);
+    _webSocket.send(data);
   }
 
   void close() {
-    _webSocket!.close();
+    _webSocket.close();
   }
 
   Future<ByteData> nonStringToByteData(dynamic data) {
diff --git a/runtime/observatory/lib/service_io.dart b/runtime/observatory/lib/service_io.dart
index 987f6781817..2ecbbf11391 100644
--- a/runtime/observatory/lib/service_io.dart
+++ b/runtime/observatory/lib/service_io.dart
@@ -15,13 +15,13 @@ import 'package:observatory/service_common.dart';
 export 'package:observatory/service_common.dart';
 
 class _IOWebSocket implements CommonWebSocket {
-  WebSocket? _webSocket;
+  WebSocket _webSocket;
 
   void connect(String address, void onOpen(), void onMessage(dynamic data),
       void onError(), void onClose()) {
     WebSocket.connect(address).then((WebSocket socket) {
       _webSocket = socket;
-      socket.listen(onMessage,
+      _webSocket.listen(onMessage,
           onError: (dynamic) => onError(),
           onDone: onClose,
           cancelOnError: true);
@@ -32,14 +32,16 @@ class _IOWebSocket implements CommonWebSocket {
   }
 
   bool get isOpen =>
-      (_webSocket != null) && (_webSocket!.readyState == WebSocket.open);
+      (_webSocket != null) && (_webSocket.readyState == WebSocket.open);
 
   void send(dynamic data) {
-    _webSocket!.add(data);
+    _webSocket.add(data);
   }
 
   void close() {
-    _webSocket?.close();
+    if (_webSocket != null) {
+      _webSocket.close();
+    }
   }
 
   Future<ByteData> nonStringToByteData(dynamic data) {
diff --git a/runtime/observatory/lib/src/allocation_profile/allocation_profile.dart b/runtime/observatory/lib/src/allocation_profile/allocation_profile.dart
index 0ec7fa15d5c..c1e7aa1e9c9 100644
--- a/runtime/observatory/lib/src/allocation_profile/allocation_profile.dart
+++ b/runtime/observatory/lib/src/allocation_profile/allocation_profile.dart
@@ -6,16 +6,15 @@ part of allocation_profiler;
 
 class AllocationProfile implements M.AllocationProfile {
   static const _lastServiceGC = 'dateLastServiceGC';
-  final DateTime? lastServiceGC;
+  final DateTime lastServiceGC;
   static const _lastAccumulatorReset = 'dateLastAccumulatorReset';
-  final DateTime? lastAccumulatorReset;
+  final DateTime lastAccumulatorReset;
   final S.HeapSpace newSpace;
   final S.HeapSpace oldSpace;
   final S.HeapSpace totalSpace;
   final Iterable<M.ClassHeapStats> members;
 
-  AllocationProfile(S.ServiceMap map,
-      {Map/*<String, List<String>>*/ ? defaults})
+  AllocationProfile(S.ServiceMap map, {Map/*<String, List<String>>*/ defaults})
       : lastAccumulatorReset = _intString2DateTime(map[_lastAccumulatorReset]),
         lastServiceGC = _intString2DateTime(map[_lastServiceGC]),
         oldSpace = new S.HeapSpace()..update(map['_heaps']['old']),
@@ -26,7 +25,7 @@ class AllocationProfile implements M.AllocationProfile {
     totalSpace.add(newSpace);
   }
 
-  static DateTime? _intString2DateTime(String milliseconds) {
+  static DateTime _intString2DateTime(String milliseconds) {
     if ((milliseconds == null) || milliseconds == '') {
       return null;
     }
@@ -39,7 +38,7 @@ class AllocationProfile implements M.AllocationProfile {
   }
 
   static List<M.ClassHeapStats> _convertMembers(Iterable/*<S.ServiceMap>*/ raw,
-      {Map/*<String, List<String>>*/ ? defaults}) {
+      {Map/*<String, List<String>>*/ defaults}) {
     final List<M.ClassHeapStats> members =
         raw.map<ClassHeapStats>(_convertMember).toList();
     if (defaults == null) {
@@ -50,26 +49,26 @@ class AllocationProfile implements M.AllocationProfile {
     final Map<String, List<ClassHeapStats>> accumulators =
         <String, List<ClassHeapStats>>{};
     defaults.forEach((/*String*/ key, /*List<String>*/ values) {
-      final classes = aliases[key]!;
+      final classes = aliases[key];
       accumulators.addAll(new Map.fromIterable(values, value: (_) => classes));
     });
     final List<M.ClassHeapStats> result = <M.ClassHeapStats>[];
     members.forEach((M.ClassHeapStats member) {
-      if (accumulators.containsKey(member.clazz!.id)) {
-        accumulators[member.clazz!.id]!.add(member as ClassHeapStats);
+      if (accumulators.containsKey(member.clazz.id)) {
+        accumulators[member.clazz.id].add(member);
       } else {
-        result.add(member as ClassHeapStats);
+        result.add(member);
       }
     });
     return result
       ..addAll(
-          aliases.keys.map((key) => new ClassesHeapStats(key, aliases[key]!)));
+          aliases.keys.map((key) => new ClassesHeapStats(key, aliases[key])));
   }
 }
 
 class ClassHeapStats implements M.ClassHeapStats {
-  final S.Class? clazz;
-  final String? displayName = null;
+  final S.Class clazz;
+  final String displayName = null;
   final S.Allocations newSpace;
   final S.Allocations oldSpace;
 
@@ -80,8 +79,8 @@ class ClassHeapStats implements M.ClassHeapStats {
 }
 
 class ClassesHeapStats implements M.ClassHeapStats {
-  final S.Class? clazz = null;
-  final String? displayName;
+  final S.Class clazz = null;
+  final String displayName;
   final S.Allocations newSpace;
   final S.Allocations oldSpace;
 
diff --git a/runtime/observatory/lib/src/app/application.dart b/runtime/observatory/lib/src/app/application.dart
index 4311b6bf256..e3c4d7bc016 100644
--- a/runtime/observatory/lib/src/app/application.dart
+++ b/runtime/observatory/lib/src/app/application.dart
@@ -7,30 +7,30 @@ part of app;
 /// The observatory application. Instances of this are created and owned
 /// by the observatory_application custom element.
 class ObservatoryApplication {
-  static late ObservatoryApplication app;
+  static ObservatoryApplication app;
   final RenderingQueue queue = new RenderingQueue();
   final TargetRepository targets = new TargetRepository(isConnectedVMTarget);
   final EventRepository events = new EventRepository();
   final NotificationRepository notifications = new NotificationRepository();
   final _pageRegistry = <Page>[];
-  late LocationManager _locationManager;
+  LocationManager _locationManager;
   LocationManager get locationManager => _locationManager;
-  Page? currentPage;
+  Page currentPage;
   bool _vmConnected = false;
-  VM? _vm;
-  VM get vm => _vm!;
+  VM _vm;
+  VM get vm => _vm;
 
   static bool isConnectedVMTarget(M.Target target) {
     if (app._vm is CommonWebSocketVM) {
       if ((app._vm as CommonWebSocketVM).target == target) {
-        return app._vm!.isConnected;
+        return app._vm.isConnected;
       }
     }
     return false;
   }
 
-  _switchVM(VM? newVM) {
-    final VM? oldVM = _vm;
+  _switchVM(VM newVM) {
+    final VM oldVM = _vm;
 
     Logger.root.info('_switchVM from:${oldVM} to:${newVM}');
 
@@ -89,14 +89,14 @@ class ObservatoryApplication {
     _vm = newVM;
   }
 
-  StreamSubscription? _gcSubscription;
-  StreamSubscription? _loggingSubscription;
+  StreamSubscription _gcSubscription;
+  StreamSubscription _loggingSubscription;
 
   Future startGCEventListener() async {
     if (_gcSubscription != null || _vm == null) {
       return;
     }
-    _gcSubscription = await _vm!.listenEventStream(VM.kGCStream, _onEvent);
+    _gcSubscription = await _vm.listenEventStream(VM.kGCStream, _onEvent);
   }
 
   Future startLoggingEventListener() async {
@@ -104,14 +104,14 @@ class ObservatoryApplication {
       return;
     }
     _loggingSubscription =
-        await _vm!.listenEventStream(Isolate.kLoggingStream, _onEvent);
+        await _vm.listenEventStream(Isolate.kLoggingStream, _onEvent);
   }
 
   Future stopGCEventListener() async {
     if (_gcSubscription == null) {
       return;
     }
-    _gcSubscription!.cancel();
+    _gcSubscription.cancel();
     _gcSubscription = null;
   }
 
@@ -119,15 +119,16 @@ class ObservatoryApplication {
     if (_loggingSubscription == null) {
       return;
     }
-    _loggingSubscription!.cancel();
+    _loggingSubscription.cancel();
     _loggingSubscription = null;
   }
 
   final ObservatoryApplicationElement rootElement;
 
-  ServiceObject? lastErrorOrException;
+  ServiceObject lastErrorOrException;
 
   void _initOnce() {
+    assert(app == null);
     app = this;
     _registerPages();
     Analytics.initialize();
@@ -145,7 +146,7 @@ class ObservatoryApplication {
 
   void _onEvent(ServiceEvent event) {
     assert(event.kind != ServiceEvent.kNone);
-    M.Event? e = createEventFromServiceEvent(event);
+    M.Event e = createEventFromServiceEvent(event);
     if (e != null) {
       events.add(e);
     }
@@ -178,7 +179,7 @@ class ObservatoryApplication {
     _pageRegistry.add(new ErrorPage(this));
   }
 
-  void _visit(Uri uri, Map<String, String> internalArguments) {
+  void _visit(Uri uri, Map internalArguments) {
     if (internalArguments['trace'] != null) {
       var traceArg = internalArguments['trace'];
       if (traceArg == 'on') {
@@ -188,7 +189,7 @@ class ObservatoryApplication {
       }
     }
     if (Tracer.current != null) {
-      Tracer.current!.reset();
+      Tracer.current.reset();
     }
     for (var i = 0; i < _pageRegistry.length; i++) {
       var page = _pageRegistry[i];
@@ -210,7 +211,7 @@ class ObservatoryApplication {
     }
     if (currentPage != null) {
       Logger.root.info('Uninstalling page: $currentPage');
-      currentPage!.onUninstall();
+      currentPage.onUninstall();
       // Clear children.
       rootElement.children.clear();
     }
@@ -221,7 +222,7 @@ class ObservatoryApplication {
       Logger.root.severe('Failed to install page: $e');
     }
     // Add new page.
-    rootElement.children.add(page.element!);
+    rootElement.children.add(page.element);
 
     // Remember page.
     currentPage = page;
@@ -238,22 +239,22 @@ class ObservatoryApplication {
         _switchVM(null);
       } else {
         final bool currentTarget =
-            (_vm as WebSocketVM?)?.target == targets.current;
-        final bool currentTargetConnected = (_vm != null) && _vm!.isConnected;
+            (_vm as WebSocketVM)?.target == targets.current;
+        final bool currentTargetConnected = (_vm != null) && _vm.isConnected;
         if (!currentTarget || !currentTargetConnected) {
-          _switchVM(new WebSocketVM(targets.current!));
-          _vm!.onConnect.then((_) {
+          _switchVM(new WebSocketVM(targets.current));
+          _vm.onConnect.then((_) {
             app.locationManager.go(Uris.vm());
           });
-          _vm!.load();
+          _vm.load();
         } else if (currentTargetConnected) {
           app.locationManager.go(Uris.vm());
         }
       }
     });
 
-    Logger.root.info('Setting initial target to ${targets.current!.name}');
-    _switchVM(new WebSocketVM(targets.current!));
+    Logger.root.info('Setting initial target to ${targets.current.name}');
+    _switchVM(new WebSocketVM(targets.current));
     _initOnce();
 
     // delete pause events.
diff --git a/runtime/observatory/lib/src/app/location_manager.dart b/runtime/observatory/lib/src/app/location_manager.dart
index 2f231d83337..a0e1af123e0 100644
--- a/runtime/observatory/lib/src/app/location_manager.dart
+++ b/runtime/observatory/lib/src/app/location_manager.dart
@@ -11,11 +11,11 @@ class LocationManager {
   /// application URL.
   final Map<String, String> internalArguments = new Map<String, String>();
 
-  Uri? _uri;
+  Uri _uri;
 
   /// [uri] is the application uri. Application uris consist of a path and
   /// the queryParameters map.
-  Uri get uri => _uri!;
+  Uri get uri => _uri;
 
   LocationManager(this._app) {
     window.onPopState.listen(_onBrowserNavigation);
@@ -90,14 +90,13 @@ class LocationManager {
 
   /// Notify the current page that something has changed.
   _visit() {
-    Chain.capture(() => _app._visit(_uri!, internalArguments),
-        onError: (e, st) {
+    Chain.capture(() => _app._visit(_uri, internalArguments), onError: (e, st) {
       if (e is IsolateNotFound) {
         var newPath = ((_app.vm == null || _app.vm.isDisconnected)
             ? '/vm-connect'
             : '/isolate-reconnect');
         var parameters = <String, dynamic>{};
-        parameters.addAll(_uri!.queryParameters);
+        parameters.addAll(_uri.queryParameters);
         parameters['originalUri'] = _uri.toString();
         parameters['isolateId'] = parameters['isolateId'];
         var generatedUri = new Uri(path: newPath, queryParameters: parameters);
@@ -127,7 +126,7 @@ class LocationManager {
   }
 
   makeLinkReplacingParameters(Map updatedParameters) {
-    var parameters = new Map<String, dynamic>.from(_uri!.queryParameters);
+    var parameters = new Map.from(_uri.queryParameters);
     updatedParameters.forEach((k, v) {
       parameters[k] = v;
     });
@@ -142,7 +141,7 @@ class LocationManager {
   }
 
   makeLinkForwardingParameters(String newPath) {
-    var parameters = _uri!.queryParameters;
+    var parameters = _uri.queryParameters;
     var generatedUri = new Uri(path: newPath, queryParameters: parameters);
     return makeLink(generatedUri.toString());
   }
@@ -162,7 +161,7 @@ class LocationManager {
     event.preventDefault();
     // 'currentTarget' is the dom element that would process the event.
     // If we use 'target' we might get an <em> element or somesuch.
-    Element target = event.currentTarget as Element;
-    go(target.attributes['href']!);
+    Element target = event.currentTarget;
+    go(target.attributes['href']);
   }
 }
diff --git a/runtime/observatory/lib/src/app/notification.dart b/runtime/observatory/lib/src/app/notification.dart
index 933cdb9fadc..1f5f9d76f9e 100644
--- a/runtime/observatory/lib/src/app/notification.dart
+++ b/runtime/observatory/lib/src/app/notification.dart
@@ -8,7 +8,7 @@ class ExceptionNotification implements M.ExceptionNotification {
   final exception;
 
   /// [optional]
-  final StackTrace? stacktrace;
+  final StackTrace stacktrace;
   ExceptionNotification(this.exception, {this.stacktrace});
 }
 
diff --git a/runtime/observatory/lib/src/app/page.dart b/runtime/observatory/lib/src/app/page.dart
index 410676dfbc3..5018e021480 100644
--- a/runtime/observatory/lib/src/app/page.dart
+++ b/runtime/observatory/lib/src/app/page.dart
@@ -55,7 +55,7 @@ RetainingPathRepository _retainingPathRepository =
 abstract class Page {
   final ObservatoryApplication app;
   final Map<String, String> internalArguments = <String, String>{};
-  HtmlElement? element;
+  HtmlElement element;
 
   Page(this.app);
 
@@ -70,7 +70,7 @@ abstract class Page {
   }
 
   /// Called when the page should update its state based on [uri].
-  void visit(Uri uri, Map<String, String> internalArguments) {
+  void visit(Uri uri, Map internalArguments) {
     this.internalArguments.clear();
     this.internalArguments.addAll(internalArguments);
     Analytics.reportPageView(uri);
@@ -96,7 +96,7 @@ abstract class MatchingPage extends Page {
 
   Future<Isolate> getIsolate(Uri uri) {
     var isolateId = uri.queryParameters['isolateId'];
-    return app.vm.getIsolate(isolateId!).then((isolate) {
+    return app.vm.getIsolate(isolateId).then((isolate) {
       if (isolate == null) {
         throw new IsolateNotFound(isolateId);
       }
@@ -107,6 +107,7 @@ abstract class MatchingPage extends Page {
   EditorRepository getEditor(Uri uri) {
     final editor = uri.queryParameters['editor'];
     return new EditorRepository(app.vm, editor: editor);
+    return null;
   }
 
   bool canVisit(Uri uri) => uri.path == path;
@@ -119,7 +120,7 @@ class SimplePage extends MatchingPage {
 
   void onInstall() {
     if (element == null) {
-      element = new Element.tag(elementTagName) as HtmlElement;
+      element = new Element.tag(elementTagName);
     }
   }
 }
@@ -169,7 +170,7 @@ class VMPage extends MatchingPage {
       return;
     }
     app.vm.reload().then((serviceObject) {
-      VM vm = serviceObject as VM;
+      VM vm = serviceObject;
       container.children = <Element>[
         new VMViewElement(
                 vm,
@@ -257,7 +258,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new ClassViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -280,7 +281,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new CodeViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -296,7 +297,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new ContextViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -317,7 +318,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new FieldViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -336,7 +337,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new InstanceViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -374,7 +375,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new FunctionViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -393,7 +394,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new ICDataViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -410,7 +411,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new SingleTargetCacheViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -427,7 +428,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new SubtypeTestCacheViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -444,7 +445,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new UnlinkedCallViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -461,7 +462,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new LibraryViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -481,7 +482,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new MegamorphicCacheViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -498,7 +499,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new ObjectPoolViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -515,13 +516,13 @@ class InspectPage extends MatchingPage {
       var pos;
       if (app.locationManager.internalArguments['pos'] != null) {
         try {
-          pos = int.parse(app.locationManager.internalArguments['pos']!);
+          pos = int.parse(app.locationManager.internalArguments['pos']);
         } catch (_) {}
       }
       container.children = <Element>[
         new ScriptViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -539,7 +540,7 @@ class InspectPage extends MatchingPage {
       container.children = <Element>[
         new ObjectViewElement(
                 app.vm,
-                obj.isolate as Isolate,
+                obj.isolate,
                 obj,
                 app.events,
                 app.notifications,
@@ -553,8 +554,8 @@ class InspectPage extends MatchingPage {
       ];
     } else if (obj is Sentinel) {
       container.children = <Element>[
-        new SentinelViewElement(app.vm, obj.isolate as Isolate, obj, app.events,
-                app.notifications,
+        new SentinelViewElement(
+                app.vm, obj.isolate, obj, app.events, app.notifications,
                 queue: app.queue)
             .element
       ];
@@ -911,8 +912,8 @@ class IsolateReconnectPage extends Page {
               app.vm,
               app.events,
               app.notifications,
-              uri.queryParameters['isolateId']!,
-              Uri.parse(uri.queryParameters['originalUri']!))
+              uri.queryParameters['isolateId'],
+              Uri.parse(uri.queryParameters['originalUri']))
           .element
     ];
     assert(element != null);
@@ -927,7 +928,7 @@ class MetricsPage extends MatchingPage {
 
   final DivElement container = new DivElement();
 
-  Isolate? lastIsolate;
+  Isolate lastIsolate;
 
   void _visit(Uri uri) {
     super._visit(uri);
@@ -953,7 +954,7 @@ class MetricsPage extends MatchingPage {
   @override
   void onUninstall() {
     super.onUninstall();
-    _metricRepository.stopSampling(lastIsolate!);
+    _metricRepository.stopSampling(lastIsolate);
     container.children = const [];
   }
 }
diff --git a/runtime/observatory/lib/src/cli/command.dart b/runtime/observatory/lib/src/cli/command.dart
index 7beb26f5a8d..0a0fc425572 100644
--- a/runtime/observatory/lib/src/cli/command.dart
+++ b/runtime/observatory/lib/src/cli/command.dart
@@ -46,8 +46,8 @@ abstract class _CommandBase {
   // A command may optionally have sub-commands.
   List<Command> _children = <Command>[];
 
-  _CommandBase? _parent;
-  int get _depth => (_parent == null ? 0 : _parent!._depth + 1);
+  _CommandBase _parent;
+  int get _depth => (_parent == null ? 0 : _parent._depth + 1);
 
   // Override in subclasses to provide command-specific argument completion.
   //
@@ -115,7 +115,7 @@ abstract class _CommandBase {
 
 // The root of a tree of commands.
 class RootCommand extends _CommandBase {
-  RootCommand(List<Command> children, [List<String>? history])
+  RootCommand(List<Command> children, [List<String> history])
       : this._(children, history ?? ['']);
 
   RootCommand._(List<Command> children, List<String> history)
@@ -161,7 +161,7 @@ class RootCommand extends _CommandBase {
       // If we are showing all possiblities, also include local
       // completions for the parent command.
       return commands[0]
-          ._parent!
+          ._parent
           ._buildCompletions(args, false)
           .then((localCompletions) {
         completions.addAll(localCompletions);
@@ -232,7 +232,7 @@ class RootCommand extends _CommandBase {
     throw 'should-not-execute-the-root-command';
   }
 
-  String toString() => 'RootCommand';
+  toString() => 'RootCommand';
 }
 
 // A node in the command tree.
@@ -240,18 +240,18 @@ abstract class Command extends _CommandBase {
   Command(this.name, List<Command> children) : super(children);
 
   final String name;
-  String? alias;
+  String alias;
 
   String get fullName {
     if (_parent is RootCommand) {
       return name;
     } else {
-      Command parent = _parent as Command;
+      Command parent = _parent;
       return '${parent.fullName} $name';
     }
   }
 
-  String toString() => 'Command(${name})';
+  toString() => 'Command(${name})';
 }
 
 abstract class CommandException implements Exception {}
diff --git a/runtime/observatory/lib/src/debugger/debugger.dart b/runtime/observatory/lib/src/debugger/debugger.dart
index f243e021112..fb297c79982 100644
--- a/runtime/observatory/lib/src/debugger/debugger.dart
+++ b/runtime/observatory/lib/src/debugger/debugger.dart
@@ -8,7 +8,7 @@ part of debugger;
 abstract class Debugger {
   VM get vm;
   Isolate get isolate;
-  M.ObjectRepository? objects;
-  ServiceMap? get stack;
-  int? get currentFrame;
+  M.ObjectRepository objects;
+  ServiceMap get stack;
+  int get currentFrame;
 }
diff --git a/runtime/observatory/lib/src/debugger/debugger_location.dart b/runtime/observatory/lib/src/debugger/debugger_location.dart
index 344a010a516..a186fd4be11 100644
--- a/runtime/observatory/lib/src/debugger/debugger_location.dart
+++ b/runtime/observatory/lib/src/debugger/debugger_location.dart
@@ -50,12 +50,12 @@ class DebuggerLocation {
         new DebuggerLocation.error("Invalid source location '${locDesc}'"));
   }
 
-  static Future<Frame?> _currentFrame(Debugger debugger) async {
-    ServiceMap? stack = debugger.stack;
+  static Future<Frame> _currentFrame(Debugger debugger) async {
+    ServiceMap stack = debugger.stack;
     if (stack == null || stack['frames'].length == 0) {
       return null;
     }
-    return stack['frames'][debugger.currentFrame] as Frame?;
+    return stack['frames'][debugger.currentFrame];
   }
 
   static Future<DebuggerLocation> _currentLocation(Debugger debugger) async {
@@ -64,10 +64,10 @@ class DebuggerLocation {
       return new DebuggerLocation.error(
           'A script must be provided when the stack is empty');
     }
-    Script script = frame.location!.script;
+    Script script = frame.location.script;
     await script.load();
-    var line = script.tokenToLine(frame.location!.tokenPos);
-    var col = script.tokenToCol(frame.location!.tokenPos);
+    var line = script.tokenToLine(frame.location.tokenPos);
+    var col = script.tokenToCol(frame.location.tokenPos);
     return new DebuggerLocation.file(script, line, col);
   }
 
@@ -87,7 +87,7 @@ class DebuggerLocation {
     if (colStr != null) {
       colStr = colStr.substring(1);
     }
-    var line = int.tryParse(lineStr!) ?? -1;
+    var line = int.tryParse(lineStr) ?? -1;
     var col = (colStr != null ? int.tryParse(colStr) ?? -1 : null);
     if (line == -1) {
       return new Future.value(
@@ -121,7 +121,7 @@ class DebuggerLocation {
         return new Future.value(new DebuggerLocation.error(
             'A script must be provided when the stack is empty'));
       }
-      Script script = frame.location!.script;
+      Script script = frame.location.script;
       await script.load();
       return new DebuggerLocation.file(script, line, col);
     }
@@ -139,7 +139,7 @@ class DebuggerLocation {
       var matches = <Script>{};
       for (var lib in isolate.libraries) {
         for (var script in lib.scripts) {
-          final String haystack = useUri ? script.uri : script.name!;
+          final String haystack = useUri ? script.uri : script.name;
           if (allowPrefix) {
             if (haystack.startsWith(name)) {
               matches.add(script);
@@ -162,7 +162,7 @@ class DebuggerLocation {
       assert(lib.loaded);
       for (var function in lib.functions) {
         if (allowPrefix) {
-          if (function.name!.startsWith(name)) {
+          if (function.name.startsWith(name)) {
             matches.add(function);
           }
         } else {
@@ -194,7 +194,7 @@ class DebuggerLocation {
     for (var lib in isolate.libraries) {
       for (var cls in lib.classes) {
         if (allowPrefix) {
-          if (cls.name!.startsWith(name)) {
+          if (cls.name.startsWith(name)) {
             matches.add(cls);
           }
         } else {
@@ -207,7 +207,7 @@ class DebuggerLocation {
     return matches;
   }
 
-  static ServiceFunction? _getConstructor(Class cls, String name) {
+  static ServiceFunction _getConstructor(Class cls, String name) {
     for (var function in cls.functions) {
       assert(cls.loaded);
       if (name == function.name) {
@@ -222,7 +222,7 @@ class DebuggerLocation {
   static Future<DebuggerLocation> _parseFunction(
       Debugger debugger, Match match) {
     Isolate isolate = debugger.isolate;
-    var base = match.group(1)!;
+    var base = match.group(1);
     var qualifier = match.group(2);
     assert(base != null);
 
@@ -234,7 +234,7 @@ class DebuggerLocation {
 
         for (var cls in classes) {
           // Look for a self-named constructor.
-          var constructor = _getConstructor(cls, cls.name!);
+          var constructor = _getConstructor(cls, cls.name);
           if (constructor != null) {
             functions.add(constructor);
           }
@@ -300,8 +300,9 @@ class DebuggerLocation {
   static Future<List<String>> _completeFunction(
       Debugger debugger, Match match) {
     Isolate isolate = debugger.isolate;
-    var base = match.group(1) ?? '';
+    var base = match.group(1);
     var qualifier = match.group(2);
+    base = (base == null ? '' : base);
 
     if (qualifier == null) {
       return _lookupClass(isolate, base, allowPrefix: true).then((classes) {
@@ -309,12 +310,12 @@ class DebuggerLocation {
 
         // Complete top-level function names.
         var functions = _lookupFunction(isolate, base, allowPrefix: true);
-        var funcNames = functions.map((f) => f.name!).toList();
+        var funcNames = functions.map((f) => f.name).toList();
         funcNames.sort();
         completions.addAll(funcNames);
 
         // Complete class names.
-        var classNames = classes.map((f) => f.name!).toList();
+        var classNames = classes.map((f) => f.name).toList();
         classNames.sort();
         completions.addAll(classNames);
 
@@ -326,12 +327,12 @@ class DebuggerLocation {
         for (var cls in classes) {
           for (var function in cls.functions) {
             if (function.kind == M.FunctionKind.constructor) {
-              if (function.name!.startsWith(match.group(0)!)) {
-                completions.add(function.name!);
+              if (function.name.startsWith(match.group(0))) {
+                completions.add(function.name);
               }
             } else {
-              if (function.qualifiedName!.startsWith(match.group(0)!)) {
-                completions.add(function.qualifiedName!);
+              if (function.qualifiedName.startsWith(match.group(0))) {
+                completions.add(function.qualifiedName);
               }
             }
           }
@@ -353,29 +354,34 @@ class DebuggerLocation {
     var lineStr;
     var lineStrComplete = false;
     var colStr;
-    if (_startsWithDigit(match.group(1)!)) {
+    if (_startsWithDigit(match.group(1))) {
       // CASE 1: We have matched a prefix of (lineStr:)(colStr)
       var frame = await _currentFrame(debugger);
       if (frame == null) {
         return [];
       }
-      scriptName = frame.location!.script.name;
+      scriptName = frame.location.script.name;
       scriptNameComplete = true;
-      lineStr = match.group(1) ?? '';
+      lineStr = match.group(1);
+      lineStr = (lineStr == null ? '' : lineStr);
       if (lineStr.endsWith(':')) {
         lineStr = lineStr.substring(0, lineStr.length - 1);
         lineStrComplete = true;
       }
-      colStr = match.group(2) ?? '';
+      colStr = match.group(2);
+      colStr = (colStr == null ? '' : colStr);
     } else {
       // CASE 2: We have matched a prefix of (scriptName:)(lineStr)(:colStr)
-      scriptName = match.group(1) ?? '';
+      scriptName = match.group(1);
+      scriptName = (scriptName == null ? '' : scriptName);
       if (scriptName.endsWith(':')) {
         scriptName = scriptName.substring(0, scriptName.length - 1);
         scriptNameComplete = true;
       }
-      lineStr = match.group(2) ?? '';
-      colStr = match.group(3) ?? '';
+      lineStr = match.group(2);
+      lineStr = (lineStr == null ? '' : lineStr);
+      colStr = match.group(3);
+      colStr = (colStr == null ? '' : colStr);
       if (colStr.startsWith(':')) {
         lineStrComplete = true;
         colStr = colStr.substring(1);
@@ -388,7 +394,7 @@ class DebuggerLocation {
           await _lookupScript(debugger.isolate, scriptName, allowPrefix: true);
       var completions = <String>[];
       for (var script in scripts) {
-        completions.add(script.name! + ':');
+        completions.add(script.name + ':');
       }
       completions.sort();
       return completions;
@@ -405,8 +411,8 @@ class DebuggerLocation {
         // Complete the line.
         var sharedPrefix = '${script.name}:';
         var completions = <String>[];
-        var report = await script.isolate!.getSourceReport(
-            [Isolate.kPossibleBreakpointsReport], script) as ServiceMap;
+        var report = await script.isolate
+            .getSourceReport([Isolate.kPossibleBreakpointsReport], script);
         Set<int> possibleBpts = getPossibleBreakpointLines(report, script);
         for (var line in possibleBpts) {
           var currentLineStr = line.toString();
@@ -419,7 +425,7 @@ class DebuggerLocation {
       } else {
         // Complete the column.
         int lineNum = int.parse(lineStr);
-        var scriptLine = script.getLine(lineNum)!;
+        var scriptLine = script.getLine(lineNum);
         var sharedPrefix = '${script.name}:${lineStr}:';
         var completions = <String>[];
         int maxCol = scriptLine.text.trimRight().runes.length;
@@ -437,20 +443,20 @@ class DebuggerLocation {
   String toString() {
     if (valid) {
       if (function != null) {
-        return '${function!.qualifiedName}';
+        return '${function.qualifiedName}';
       } else if (col != null) {
-        return '${script!.name}:${line}:${col}';
+        return '${script.name}:${line}:${col}';
       } else {
-        return '${script!.name}:${line}';
+        return '${script.name}:${line}';
       }
     }
     return 'invalid source location (${errorMessage})';
   }
 
-  Script? script;
-  int? line;
-  int? col;
-  ServiceFunction? function;
-  String? errorMessage;
+  Script script;
+  int line;
+  int col;
+  ServiceFunction function;
+  String errorMessage;
   bool get valid => (errorMessage == null);
 }
diff --git a/runtime/observatory/lib/src/elements/allocation_profile.dart b/runtime/observatory/lib/src/elements/allocation_profile.dart
index be21bcfc862..7770d155aea 100644
--- a/runtime/observatory/lib/src/elements/allocation_profile.dart
+++ b/runtime/observatory/lib/src/elements/allocation_profile.dart
@@ -48,20 +48,20 @@ class AllocationProfileElement extends CustomElement implements Renderable {
         VirtualCollectionElement.tag
       ]);
 
-  late RenderingScheduler<AllocationProfileElement> _r;
+  RenderingScheduler<AllocationProfileElement> _r;
 
   Stream<RenderedEvent<AllocationProfileElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.AllocationProfileRepository _repository;
-  M.AllocationProfile? _profile;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.AllocationProfileRepository _repository;
+  M.AllocationProfile _profile;
   bool _autoRefresh = false;
   bool _isCompacted = false;
-  late StreamSubscription _gcSubscription;
+  StreamSubscription _gcSubscription;
   _SortingField _sortingField = _SortingField.size;
   _SortingDirection _sortingDirection = _SortingDirection.descending;
 
@@ -75,7 +75,7 @@ class AllocationProfileElement extends CustomElement implements Renderable {
       M.EventRepository events,
       M.NotificationRepository notifications,
       M.AllocationProfileRepository repository,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -174,9 +174,9 @@ class AllocationProfileElement extends CustomElement implements Renderable {
                             ..text = 'last forced GC at',
                           new DivElement()
                             ..classes = ['memberValue']
-                            ..text = _profile!.lastServiceGC == null
+                            ..text = _profile.lastServiceGC == null
                                 ? '---'
-                                : '${_profile!.lastServiceGC}',
+                                : '${_profile.lastServiceGC}',
                         ],
                     ],
                   new HRElement(),
@@ -190,14 +190,14 @@ class AllocationProfileElement extends CustomElement implements Renderable {
                   ? [
                       new HeadingElement.h2()
                         ..text = 'New Generation '
-                            '(${_usedCaption(_profile!.newSpace)})',
+                            '(${_usedCaption(_profile.newSpace)})',
                     ]
                   : [
                       new HeadingElement.h2()..text = 'New Generation',
                       new BRElement(),
                       new DivElement()
                         ..classes = ['memberList']
-                        ..children = _createSpaceMembers(_profile!.newSpace),
+                        ..children = _createSpaceMembers(_profile.newSpace),
                     ],
             new DivElement()
               ..classes = ['heap-space', 'left']
@@ -205,14 +205,14 @@ class AllocationProfileElement extends CustomElement implements Renderable {
                   ? [
                       new HeadingElement.h2()
                         ..text = 'Old Generation '
-                            '(${_usedCaption(_profile!.oldSpace)})',
+                            '(${_usedCaption(_profile.oldSpace)})',
                     ]
                   : [
                       new HeadingElement.h2()..text = 'Old Generation',
                       new BRElement(),
                       new DivElement()
                         ..classes = ['memberList']
-                        ..children = _createSpaceMembers(_profile!.oldSpace),
+                        ..children = _createSpaceMembers(_profile.oldSpace),
                     ],
             new DivElement()
               ..classes = ['heap-space', 'left']
@@ -220,22 +220,22 @@ class AllocationProfileElement extends CustomElement implements Renderable {
                   ? [
                       new HeadingElement.h2()
                         ..text = 'Total '
-                            '(${_usedCaption(_profile!.totalSpace)})',
+                            '(${_usedCaption(_profile.totalSpace)})',
                     ]
                   : [
                       new HeadingElement.h2()..text = 'Total',
                       new BRElement(),
                       new DivElement()
                         ..classes = ['memberList']
-                        ..children = _createSpaceMembers(_profile!.totalSpace),
+                        ..children = _createSpaceMembers(_profile.totalSpace),
                     ],
             new ButtonElement()
               ..classes = ['compact']
               ..text = _isCompacted ? 'expand ▼' : 'compact ▲'
               ..onClick.listen((_) {
-                _isCompacted = !_isCompacted;
-                _r.dirty();
-              }),
+                    _isCompacted = !_isCompacted;
+                          _r.dirty();
+               }),
             new HRElement()
           ],
         new DivElement()
@@ -245,7 +245,7 @@ class AllocationProfileElement extends CustomElement implements Renderable {
                     _createCollectionLine, _updateCollectionLine,
                     createHeader: _createCollectionHeader,
                     search: _search,
-                    items: _profile!.members.toList()..sort(_createSorter()),
+                    items: _profile.members.toList()..sort(_createSorter()),
                     queue: _r.queue)
                 .element
           ]
@@ -293,7 +293,7 @@ class AllocationProfileElement extends CustomElement implements Renderable {
         getter = _getInstances;
         break;
       case _SortingField.className:
-        getter = (M.ClassHeapStats s) => s.clazz!.name;
+        getter = (M.ClassHeapStats s) => s.clazz.name;
         break;
     }
     switch (_sortingDirection) {
@@ -376,26 +376,35 @@ class AllocationProfileElement extends CustomElement implements Renderable {
                 _SortingField.newInternalSize, _SortingDirection.descending),
             _createHeaderButton(const ['bytes'], 'External',
                 _SortingField.newExternalSize, _SortingDirection.descending),
-            _createHeaderButton(const ['bytes'], 'Size', _SortingField.newSize,
+            _createHeaderButton(const ['bytes'], 'Size',
+                _SortingField.newSize, _SortingDirection.descending),
+            _createHeaderButton(
+                const ['instances'], 'Instances',
+                _SortingField.newInstances,
                 _SortingDirection.descending),
-            _createHeaderButton(const ['instances'], 'Instances',
-                _SortingField.newInstances, _SortingDirection.descending),
+
             _createHeaderButton(const ['bytes'], 'Internal',
                 _SortingField.oldInternalSize, _SortingDirection.descending),
             _createHeaderButton(const ['bytes'], 'External',
                 _SortingField.oldExternalSize, _SortingDirection.descending),
-            _createHeaderButton(const ['bytes'], 'Size', _SortingField.oldSize,
+            _createHeaderButton(const ['bytes'], 'Size',
+                _SortingField.oldSize, _SortingDirection.descending),
+            _createHeaderButton(
+                const ['instances'], 'Instances',
+                _SortingField.oldInstances,
                 _SortingDirection.descending),
-            _createHeaderButton(const ['instances'], 'Instances',
-                _SortingField.oldInstances, _SortingDirection.descending),
+
             _createHeaderButton(const ['bytes'], 'Internal',
                 _SortingField.internalSize, _SortingDirection.descending),
             _createHeaderButton(const ['bytes'], 'External',
                 _SortingField.externalSize, _SortingDirection.descending),
-            _createHeaderButton(const ['bytes'], 'Size', _SortingField.size,
+            _createHeaderButton(const ['bytes'], 'Size',
+                _SortingField.size, _SortingDirection.descending),
+            _createHeaderButton(
+                const ['instances'], 'Instances',
+                _SortingField.instances,
                 _SortingDirection.descending),
-            _createHeaderButton(const ['instances'], 'Instances',
-                _SortingField.instances, _SortingDirection.descending),
+
             _createHeaderButton(const ['name'], 'Class',
                 _SortingField.className, _SortingDirection.ascending)
           ],
@@ -443,14 +452,14 @@ class AllocationProfileElement extends CustomElement implements Renderable {
     e.children[9].text = Utils.formatSize(_getExternalSize(item));
     e.children[10].text = Utils.formatSize(_getSize(item));
     e.children[11].text = '${_getInstances(item)}';
-    e.children[12] = new ClassRefElement(_isolate, item.clazz!, queue: _r.queue)
+    e.children[12] = new ClassRefElement(_isolate, item.clazz, queue: _r.queue)
         .element
       ..classes = ['name'];
   }
 
   bool _search(Pattern pattern, itemDynamic) {
     M.ClassHeapStats item = itemDynamic;
-    return item.clazz!.name!.contains(pattern);
+    return item.clazz.name.contains(pattern);
   }
 
   static String _usedCaption(M.HeapSpace space) =>
@@ -537,11 +546,11 @@ class AllocationProfileElement extends CustomElement implements Renderable {
           'Class'
         ].join(',') +
         '\n';
-    AnchorElement tl = document.createElement('a') as AnchorElement;
+    AnchorElement tl = document.createElement('a');
     tl
       ..attributes['href'] = 'data:text/plain;charset=utf-8,' +
           Uri.encodeComponent(header +
-              (_profile!.members.toList()..sort(_createSorter()))
+              (_profile.members.toList()..sort(_createSorter()))
                   .map(_csvOut)
                   .join('\n'))
       ..attributes['download'] = 'heap-profile.csv'
@@ -562,23 +571,32 @@ class AllocationProfileElement extends CustomElement implements Renderable {
       _getExternalSize(s),
       _getSize(s),
       _getInstances(s),
-      s.clazz!.name
+      s.clazz.name
     ].join(',');
   }
 
-  static int _getNewInstances(M.ClassHeapStats s) => s.newSpace.instances;
-  static int _getNewInternalSize(M.ClassHeapStats s) => s.newSpace.internalSize;
-  static int _getNewExternalSize(M.ClassHeapStats s) => s.newSpace.externalSize;
-  static int _getNewSize(M.ClassHeapStats s) => s.newSpace.size;
-  static int _getOldInstances(M.ClassHeapStats s) => s.oldSpace.instances;
-  static int _getOldInternalSize(M.ClassHeapStats s) => s.oldSpace.internalSize;
-  static int _getOldExternalSize(M.ClassHeapStats s) => s.oldSpace.externalSize;
-  static int _getOldSize(M.ClassHeapStats s) => s.oldSpace.size;
+  static int _getNewInstances(M.ClassHeapStats s) =>
+      s.newSpace.instances;
+  static int _getNewInternalSize(M.ClassHeapStats s) =>
+      s.newSpace.internalSize;
+  static int _getNewExternalSize(M.ClassHeapStats s) =>
+      s.newSpace.externalSize;
+  static int _getNewSize(M.ClassHeapStats s) =>
+      s.newSpace.size;
+  static int _getOldInstances(M.ClassHeapStats s) =>
+      s.oldSpace.instances;
+  static int _getOldInternalSize(M.ClassHeapStats s) =>
+      s.oldSpace.internalSize;
+  static int _getOldExternalSize(M.ClassHeapStats s) =>
+      s.oldSpace.externalSize;
+  static int _getOldSize(M.ClassHeapStats s) =>
+      s.oldSpace.size;
   static int _getInstances(M.ClassHeapStats s) =>
       s.newSpace.instances + s.oldSpace.instances;
   static int _getInternalSize(M.ClassHeapStats s) =>
       s.newSpace.internalSize + s.oldSpace.internalSize;
   static int _getExternalSize(M.ClassHeapStats s) =>
       s.newSpace.externalSize + s.oldSpace.externalSize;
-  static int _getSize(M.ClassHeapStats s) => s.newSpace.size + s.oldSpace.size;
+  static int _getSize(M.ClassHeapStats s) =>
+      s.newSpace.size + s.oldSpace.size;
 }
diff --git a/runtime/observatory/lib/src/elements/class_allocation_profile.dart b/runtime/observatory/lib/src/elements/class_allocation_profile.dart
index d1225429eb0..f9d3e2efa7a 100644
--- a/runtime/observatory/lib/src/elements/class_allocation_profile.dart
+++ b/runtime/observatory/lib/src/elements/class_allocation_profile.dart
@@ -21,27 +21,27 @@ class ClassAllocationProfileElement extends CustomElement
         CpuProfileVirtualTreeElement.tag,
       ]);
 
-  late RenderingScheduler<ClassAllocationProfileElement> _r;
+  RenderingScheduler<ClassAllocationProfileElement> _r;
 
   Stream<RenderedEvent<ClassAllocationProfileElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.Class _cls;
-  late M.ClassSampleProfileRepository _profiles;
-  late Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
-  M.SampleProfileLoadingProgress? _progress;
-  late M.SampleProfileTag _tag = M.SampleProfileTag.none;
-  late ProfileTreeMode _mode = ProfileTreeMode.function;
-  late M.ProfileTreeDirection _direction = M.ProfileTreeDirection.exclusive;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.Class _cls;
+  M.ClassSampleProfileRepository _profiles;
+  Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
+  M.SampleProfileLoadingProgress _progress;
+  M.SampleProfileTag _tag = M.SampleProfileTag.none;
+  ProfileTreeMode _mode = ProfileTreeMode.function;
+  M.ProfileTreeDirection _direction = M.ProfileTreeDirection.exclusive;
 
   M.IsolateRef get isolate => _isolate;
   M.Class get cls => _cls;
 
   factory ClassAllocationProfileElement(M.VM vm, M.IsolateRef isolate,
       M.Class cls, M.ClassSampleProfileRepository profiles,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(cls != null);
@@ -79,7 +79,7 @@ class ClassAllocationProfileElement extends CustomElement
       return;
     }
     final content = <HtmlElement>[
-      (new SampleBufferControlElement(_vm, _progress!, _progressStream,
+      (new SampleBufferControlElement(_vm, _progress, _progressStream,
               selectedTag: _tag, queue: _r.queue)
             ..onTagChange.listen((e) {
               _tag = e.element.selectedTag;
@@ -87,8 +87,8 @@ class ClassAllocationProfileElement extends CustomElement
             }))
           .element
     ];
-    if (_progress!.status == M.SampleProfileLoadingStatus.loaded) {
-      late CpuProfileVirtualTreeElement tree;
+    if (_progress.status == M.SampleProfileLoadingStatus.loaded) {
+      CpuProfileVirtualTreeElement tree;
       content.addAll([
         new BRElement(),
         (new StackTraceTreeConfigElement(
@@ -104,7 +104,7 @@ class ClassAllocationProfileElement extends CustomElement
               }))
             .element,
         new BRElement(),
-        (tree = new CpuProfileVirtualTreeElement(_isolate, _progress!.profile,
+        (tree = new CpuProfileVirtualTreeElement(_isolate, _progress.profile,
                 queue: _r.queue))
             .element
       ]);
@@ -119,7 +119,7 @@ class ClassAllocationProfileElement extends CustomElement
     _r.dirty();
     _progress = (await _progressStream.first).progress;
     _r.dirty();
-    if (M.isSampleProcessRunning(_progress!.status)) {
+    if (M.isSampleProcessRunning(_progress.status)) {
       _progress = (await _progressStream.last).progress;
       _r.dirty();
     }
diff --git a/runtime/observatory/lib/src/elements/class_instances.dart b/runtime/observatory/lib/src/elements/class_instances.dart
index 666ed49a915..b81d859d019 100644
--- a/runtime/observatory/lib/src/elements/class_instances.dart
+++ b/runtime/observatory/lib/src/elements/class_instances.dart
@@ -22,19 +22,19 @@ class ClassInstancesElement extends CustomElement implements Renderable {
     RetainingPathElement.tag,
   ]);
 
-  late RenderingScheduler<ClassInstancesElement> _r;
+  RenderingScheduler<ClassInstancesElement> _r;
 
   Stream<RenderedEvent<ClassInstancesElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.Class _cls;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.StronglyReachableInstancesRepository _stronglyReachableInstances;
-  late M.ObjectRepository _objects;
-  M.Guarded<M.Instance>? _retainedSize = null;
+  M.IsolateRef _isolate;
+  M.Class _cls;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.StronglyReachableInstancesRepository _stronglyReachableInstances;
+  M.ObjectRepository _objects;
+  M.Guarded<M.Instance> _retainedSize = null;
   bool _loadingRetainedBytes = false;
-  M.Guarded<M.Instance>? _reachableSize = null;
+  M.Guarded<M.Instance> _reachableSize = null;
   bool _loadingReachableBytes = false;
 
   M.IsolateRef get isolate => _isolate;
@@ -47,7 +47,7 @@ class ClassInstancesElement extends CustomElement implements Renderable {
       M.ReachableSizeRepository reachableSizes,
       M.StronglyReachableInstancesRepository stronglyReachableInstances,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(cls != null);
     assert(retainedSizes != null);
@@ -80,15 +80,15 @@ class ClassInstancesElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  StronglyReachableInstancesElement? _strong;
+  StronglyReachableInstancesElement _strong;
 
   void render() {
     _strong = _strong ??
         new StronglyReachableInstancesElement(
             _isolate, _cls, _stronglyReachableInstances, _objects,
             queue: _r.queue);
-    final instanceCount = _cls.newSpace!.instances + _cls.oldSpace!.instances;
-    final size = Utils.formatSize(_cls.newSpace!.size + _cls.oldSpace!.size);
+    final instanceCount = _cls.newSpace.instances + _cls.oldSpace.instances;
+    final size = Utils.formatSize(_cls.newSpace.size + _cls.oldSpace.size);
     children = <Element>[
       new DivElement()
         ..classes = ['memberList']
@@ -111,7 +111,7 @@ class ClassInstancesElement extends CustomElement implements Renderable {
                 ..text = 'strongly reachable ',
               new DivElement()
                 ..classes = ['memberValue']
-                ..children = <Element>[_strong!.element]
+                ..children = <Element>[_strong.element]
             ],
           new DivElement()
             ..classes = ['memberItem']
@@ -144,14 +144,14 @@ class ClassInstancesElement extends CustomElement implements Renderable {
   List<Element> _createReachableSizeValue() {
     final content = <Element>[];
     if (_reachableSize != null) {
-      if (_reachableSize!.isSentinel) {
-        content.add(new SentinelValueElement(_reachableSize!.asSentinel!,
-                queue: _r.queue)
-            .element);
+      if (_reachableSize.isSentinel) {
+        content.add(
+            new SentinelValueElement(_reachableSize.asSentinel, queue: _r.queue)
+                .element);
       } else {
         content.add(new SpanElement()
           ..text = Utils.formatSize(
-              int.parse(_reachableSize!.asValue!.valueAsString!)));
+              int.parse(_reachableSize.asValue.valueAsString)));
       }
     } else {
       content.add(new SpanElement()..text = '...');
@@ -163,7 +163,7 @@ class ClassInstancesElement extends CustomElement implements Renderable {
     button.onClick.listen((_) async {
       button.disabled = true;
       _loadingReachableBytes = true;
-      _reachableSize = await _reachableSizes.get(_isolate, _cls.id!);
+      _reachableSize = await _reachableSizes.get(_isolate, _cls.id);
       _r.dirty();
     });
     content.add(button);
@@ -173,14 +173,14 @@ class ClassInstancesElement extends CustomElement implements Renderable {
   List<Element> _createRetainedSizeValue() {
     final content = <Element>[];
     if (_retainedSize != null) {
-      if (_retainedSize!.isSentinel) {
-        content.add(new SentinelValueElement(_retainedSize!.asSentinel!,
-                queue: _r.queue)
-            .element);
+      if (_retainedSize.isSentinel) {
+        content.add(
+            new SentinelValueElement(_retainedSize.asSentinel, queue: _r.queue)
+                .element);
       } else {
         content.add(new SpanElement()
-          ..text = Utils.formatSize(
-              int.parse(_retainedSize!.asValue!.valueAsString!)));
+          ..text =
+              Utils.formatSize(int.parse(_retainedSize.asValue.valueAsString)));
       }
     } else {
       content.add(new SpanElement()..text = '...');
@@ -192,7 +192,7 @@ class ClassInstancesElement extends CustomElement implements Renderable {
     button.onClick.listen((_) async {
       button.disabled = true;
       _loadingRetainedBytes = true;
-      _retainedSize = await _retainedSizes.get(_isolate, _cls.id!);
+      _retainedSize = await _retainedSizes.get(_isolate, _cls.id);
       _r.dirty();
     });
     content.add(button);
diff --git a/runtime/observatory/lib/src/elements/class_ref.dart b/runtime/observatory/lib/src/elements/class_ref.dart
index 6141d51d04e..409dddb9361 100644
--- a/runtime/observatory/lib/src/elements/class_ref.dart
+++ b/runtime/observatory/lib/src/elements/class_ref.dart
@@ -12,18 +12,18 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class ClassRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<ClassRefElement>('class-ref');
 
-  late RenderingScheduler<ClassRefElement> _r;
+  RenderingScheduler<ClassRefElement> _r;
 
   Stream<RenderedEvent<ClassRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ClassRef _class;
+  M.IsolateRef _isolate;
+  M.ClassRef _class;
 
   M.IsolateRef get isolate => _isolate;
   M.ClassRef get cls => _class;
 
   factory ClassRefElement(M.IsolateRef isolate, M.ClassRef cls,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(cls != null);
     ClassRefElement e = new ClassRefElement.created();
     e._r = new RenderingScheduler<ClassRefElement>(e, queue: queue);
diff --git a/runtime/observatory/lib/src/elements/class_tree.dart b/runtime/observatory/lib/src/elements/class_tree.dart
index 0136a44e4f0..30c51f855de 100644
--- a/runtime/observatory/lib/src/elements/class_tree.dart
+++ b/runtime/observatory/lib/src/elements/class_tree.dart
@@ -29,26 +29,26 @@ class ClassTreeElement extends CustomElement implements Renderable {
     VirtualTreeElement.tag
   ]);
 
-  late RenderingScheduler<ClassTreeElement> _r;
+  RenderingScheduler<ClassTreeElement> _r;
 
   Stream<RenderedEvent<ClassTreeElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.ClassRepository _classes;
-  M.Class? _object;
+  M.VMRef _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.ClassRepository _classes;
+  M.Class _object;
   final _subclasses = <String, Iterable<M.Class>>{};
-  final _mixins = <String, List<M.Instance>?>{};
+  final _mixins = <String, List<M.Instance>>{};
 
   factory ClassTreeElement(
-      M.VM vm,
+      M.VMRef vm,
       M.IsolateRef isolate,
       M.EventRepository events,
       M.NotificationRepository notifications,
       M.ClassRepository classes,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -80,7 +80,7 @@ class ClassTreeElement extends CustomElement implements Renderable {
     _r.disable(notify: true);
   }
 
-  VirtualTreeElement? _tree;
+  VirtualTreeElement _tree;
 
   void render() {
     children = <Element>[
@@ -108,8 +108,8 @@ class ClassTreeElement extends CustomElement implements Renderable {
   Element _createTree() {
     _tree = new VirtualTreeElement(_create, _update, _children,
         items: [_object], search: _search, queue: _r.queue);
-    _tree!.expand(_object, autoExpandSingleChildNodes: true);
-    return _tree!.element;
+    _tree.expand(_object, autoExpandSingleChildNodes: true);
+    return _tree.element;
   }
 
   Future _refresh() async {
@@ -121,26 +121,25 @@ class ClassTreeElement extends CustomElement implements Renderable {
   }
 
   Future<M.Class> _register(M.Class cls) async {
-    _subclasses[cls.id!] = await Future.wait(
-        (await Future.wait(cls.subclasses!.map(_getActualChildrens)))
+    _subclasses[cls.id] = await Future.wait(
+        (await Future.wait(cls.subclasses.map(_getActualChildrens)))
             .expand((f) => f)
             .map(_register));
     return cls;
   }
 
   Future<Iterable<M.Class>> _getActualChildrens(M.ClassRef ref) async {
-    var cls = await _classes.get(_isolate, ref.id!);
-    if (cls.isPatch!) {
+    var cls = await _classes.get(_isolate, ref.id);
+    if (cls.isPatch) {
       return const [];
     }
     if (cls.mixin == null) {
       return [cls];
     }
-    return (await Future.wait(cls.subclasses!.map(_getActualChildrens)))
+    return (await Future.wait(cls.subclasses.map(_getActualChildrens)))
         .expand((f) => f)
           ..forEach((subcls) {
-            _mixins[subcls.id!] = (_mixins[subcls.id!] ?? [])
-              ..add(cls.mixin as M.Instance);
+            _mixins[subcls.id] = (_mixins[subcls.id] ?? [])..add(cls.mixin);
           });
   }
 
@@ -158,23 +157,23 @@ class ClassTreeElement extends CustomElement implements Renderable {
 
   void _update(HtmlElement el, classDynamic, int index) {
     M.Class cls = classDynamic;
-    virtualTreeUpdateLines(el.children[0] as SpanElement, index);
-    if (cls.subclasses!.isEmpty) {
+    virtualTreeUpdateLines(el.children[0], index);
+    if (cls.subclasses.isEmpty) {
       el.children[1].text = '';
     } else {
-      el.children[1].text = _tree!.isExpanded(cls) ? '▼' : '►';
+      el.children[1].text = _tree.isExpanded(cls) ? '▼' : '►';
     }
     el.children[2].children = <Element>[
       new ClassRefElement(_isolate, cls, queue: _r.queue).element
     ];
     if (_mixins[cls.id] != null) {
-      el.children[2].children.addAll(_createMixins(_mixins[cls.id]!));
+      el.children[2].children.addAll(_createMixins(_mixins[cls.id]));
     }
   }
 
   bool _search(Pattern pattern, classDynamic) {
     M.Class cls = classDynamic;
-    return cls.name!.contains(pattern);
+    return cls.name.contains(pattern);
   }
 
   List<Element> _createMixins(List<M.Instance> types) {
@@ -182,8 +181,8 @@ class ClassTreeElement extends CustomElement implements Renderable {
         .expand((type) => <Element>[
               new SpanElement()..text = ', ',
               type.typeClass == null
-                  ? (new SpanElement()..text = type.name!.split('<').first)
-                  : new ClassRefElement(_isolate, type.typeClass!,
+                  ? (new SpanElement()..text = type.name.split('<').first)
+                  : new ClassRefElement(_isolate, type.typeClass,
                           queue: _r.queue)
                       .element
             ])
@@ -194,6 +193,6 @@ class ClassTreeElement extends CustomElement implements Renderable {
 
   Iterable<M.Class> _children(classDynamic) {
     M.Class cls = classDynamic;
-    return _subclasses[cls.id]!;
+    return _subclasses[cls.id];
   }
 }
diff --git a/runtime/observatory/lib/src/elements/class_view.dart b/runtime/observatory/lib/src/elements/class_view.dart
index 121056007a6..0349bb90082 100644
--- a/runtime/observatory/lib/src/elements/class_view.dart
+++ b/runtime/observatory/lib/src/elements/class_view.dart
@@ -56,27 +56,27 @@ class ClassViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<ClassViewElement> _r;
+  RenderingScheduler<ClassViewElement> _r;
 
   Stream<RenderedEvent<ClassViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Class _cls;
-  late M.ClassRepository _classes;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.StronglyReachableInstancesRepository _stronglyReachableInstances;
-  late M.FieldRepository _fields;
-  late M.ScriptRepository _scripts;
-  late M.ObjectRepository _objects;
-  late M.EvalRepository _eval;
-  late M.ClassSampleProfileRepository _profiles;
-  Iterable<M.Field>? _classFields;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Class _cls;
+  M.ClassRepository _classes;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.StronglyReachableInstancesRepository _stronglyReachableInstances;
+  M.FieldRepository _fields;
+  M.ScriptRepository _scripts;
+  M.ObjectRepository _objects;
+  M.EvalRepository _eval;
+  M.ClassSampleProfileRepository _profiles;
+  Iterable<M.Field> _classFields;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -100,7 +100,7 @@ class ClassViewElement extends CustomElement implements Renderable {
       M.EvalRepository eval,
       M.StronglyReachableInstancesRepository stronglyReachable,
       M.ClassSampleProfileRepository profiles,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -154,8 +154,8 @@ class ClassViewElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  ObjectCommonElement? _common;
-  ClassInstancesElement? _classInstances;
+  ObjectCommonElement _common;
+  ClassInstancesElement _classInstances;
   bool _loadProfile = false;
 
   void render() {
@@ -168,10 +168,10 @@ class ClassViewElement extends CustomElement implements Renderable {
             _reachableSizes, _stronglyReachableInstances, _objects,
             queue: _r.queue);
     var header = '';
-    if (_cls.isAbstract!) {
+    if (_cls.isAbstract) {
       header += 'abstract ';
     }
-    if (_cls.isPatch!) {
+    if (_cls.isPatch) {
       header += 'patch ';
     }
     children = <Element>[
@@ -205,7 +205,7 @@ class ClassViewElement extends CustomElement implements Renderable {
         ..children = <Element>[
           new HeadingElement.h2()..text = '$header class ${_cls.name}',
           new HRElement(),
-          _common!.element,
+          _common.element,
           new BRElement(),
           new DivElement()
             ..classes = ['memberList']
@@ -215,7 +215,7 @@ class ClassViewElement extends CustomElement implements Renderable {
                 ? const []
                 : [
                     new HRElement(),
-                    new ErrorRefElement(_cls.error!, queue: _r.queue).element
+                    new ErrorRefElement(_cls.error, queue: _r.queue).element
                   ],
           new HRElement(),
           new EvalBoxElement(_isolate, _cls, _objects, _eval, queue: _r.queue)
@@ -227,7 +227,7 @@ class ClassViewElement extends CustomElement implements Renderable {
             ..children = _createElements(),
           new HRElement(),
           new HeadingElement.h2()..text = 'Instances',
-          new DivElement()..children = [_classInstances!.element],
+          new DivElement()..children = [_classInstances.element],
           new HRElement(),
           new HeadingElement.h2()..text = 'Allocations',
           new DivElement()
@@ -238,7 +238,7 @@ class ClassViewElement extends CustomElement implements Renderable {
                 ..text = 'Tracing allocations?	',
               new DivElement()
                 ..classes = ['memberValue']
-                ..children = _cls.traceAllocations!
+                ..children = _cls.traceAllocations
                     ? [
                         new SpanElement()..text = 'Yes ',
                         new ButtonElement()
@@ -274,7 +274,7 @@ class ClassViewElement extends CustomElement implements Renderable {
             ..children = _cls.location != null
                 ? [
                     new HRElement(),
-                    new SourceInsetElement(_isolate, _cls.location!, _scripts,
+                    new SourceInsetElement(_isolate, _cls.location, _scripts,
                             _objects, _events,
                             queue: _r.queue)
                         .element
@@ -286,8 +286,8 @@ class ClassViewElement extends CustomElement implements Renderable {
     ];
   }
 
-  bool? _fieldsExpanded;
-  bool? _functionsExpanded;
+  bool _fieldsExpanded;
+  bool _functionsExpanded;
 
   List<Element> _createMembers() {
     final members = <Element>[];
@@ -301,7 +301,7 @@ class ClassViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = <Element>[
-              new LibraryRefElement(_isolate, _cls.library!, queue: _r.queue)
+              new LibraryRefElement(_isolate, _cls.library, queue: _r.queue)
                   .element
             ]
         ]);
@@ -316,7 +316,7 @@ class ClassViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = <Element>[
-              new SourceLinkElement(_isolate, _cls.location!, _scripts,
+              new SourceLinkElement(_isolate, _cls.location, _scripts,
                       queue: _r.queue)
                   .element
             ]
@@ -332,7 +332,7 @@ class ClassViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = <Element>[
-              new ClassRefElement(_isolate, _cls.superclass!, queue: _r.queue)
+              new ClassRefElement(_isolate, _cls.superclass, queue: _r.queue)
                   .element
             ]
         ]);
@@ -347,7 +347,7 @@ class ClassViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = <Element>[
-              new InstanceRefElement(_isolate, _cls.superType!, _objects,
+              new InstanceRefElement(_isolate, _cls.superType, _objects,
                       queue: _r.queue)
                   .element
             ]
@@ -363,13 +363,13 @@ class ClassViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = <Element>[
-              new InstanceRefElement(_isolate, _cls.mixin!, _objects,
+              new InstanceRefElement(_isolate, _cls.mixin, _objects,
                       queue: _r.queue)
                   .element
             ]
         ]);
     }
-    if (_cls.subclasses!.length > 0) {
+    if (_cls.subclasses.length > 0) {
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
@@ -378,7 +378,7 @@ class ClassViewElement extends CustomElement implements Renderable {
             ..text = 'extended by',
           new DivElement()
             ..classes = ['memberValue']
-            ..children = (_cls.subclasses!
+            ..children = (_cls.subclasses
                 .expand((subcls) => <Element>[
                       new ClassRefElement(_isolate, subcls, queue: _r.queue)
                           .element,
@@ -391,7 +391,7 @@ class ClassViewElement extends CustomElement implements Renderable {
 
     members.add(new BRElement());
 
-    if (_cls.interfaces!.length > 0) {
+    if (_cls.interfaces.length > 0) {
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
@@ -400,7 +400,7 @@ class ClassViewElement extends CustomElement implements Renderable {
             ..text = 'implements',
           new DivElement()
             ..classes = ['memberValue']
-            ..children = (_cls.interfaces!
+            ..children = (_cls.interfaces
                 .expand((interf) => <Element>[
                       new InstanceRefElement(_isolate, interf, _objects,
                               queue: _r.queue)
@@ -428,8 +428,8 @@ class ClassViewElement extends CustomElement implements Renderable {
 
   List<Element> _createElements() {
     final members = <Element>[];
-    if (_classFields != null && _classFields!.isNotEmpty) {
-      final fields = _classFields!.toList();
+    if (_classFields != null && _classFields.isNotEmpty) {
+      final fields = _classFields.toList();
       _fieldsExpanded = _fieldsExpanded ?? (fields.length <= 8);
       members.add(new DivElement()
         ..classes = ['memberItem']
@@ -440,7 +440,7 @@ class ClassViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = <Element>[
-              (new CurlyBlockElement(expanded: _fieldsExpanded!)
+              (new CurlyBlockElement(expanded: _fieldsExpanded)
                     ..onToggle
                         .listen((e) => _fieldsExpanded = e.control.expanded)
                     ..content = <Element>[
@@ -474,8 +474,8 @@ class ClassViewElement extends CustomElement implements Renderable {
         ]);
     }
 
-    if (_cls.functions!.isNotEmpty) {
-      final functions = _cls.functions!.toList();
+    if (_cls.functions.isNotEmpty) {
+      final functions = _cls.functions.toList();
       _functionsExpanded = _functionsExpanded ?? (functions.length <= 8);
       members.add(new DivElement()
         ..classes = ['memberItem']
@@ -486,7 +486,7 @@ class ClassViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = <Element>[
-              (new CurlyBlockElement(expanded: _functionsExpanded!)
+              (new CurlyBlockElement(expanded: _functionsExpanded)
                     ..onToggle
                         .listen((e) => _functionsExpanded = e.control.expanded)
                     ..content = (functions
@@ -505,14 +505,14 @@ class ClassViewElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _cls = await _classes.get(_isolate, _cls.id!);
+    _cls = await _classes.get(_isolate, _cls.id);
     await _loadAdditionalData();
     _r.dirty();
   }
 
   Future _loadAdditionalData() async {
-    _classFields = await Future.wait(
-        _cls.fields!.map((f) => _fields.get(_isolate, f.id!)));
+    _classFields =
+        await Future.wait(_cls.fields.map((f) => _fields.get(_isolate, f.id)));
     _r.dirty();
   }
 }
diff --git a/runtime/observatory/lib/src/elements/code_ref.dart b/runtime/observatory/lib/src/elements/code_ref.dart
index c76f676b7ee..f6c7f0324ee 100644
--- a/runtime/observatory/lib/src/elements/code_ref.dart
+++ b/runtime/observatory/lib/src/elements/code_ref.dart
@@ -15,18 +15,18 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class CodeRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<CodeRefElement>('code-ref');
 
-  late RenderingScheduler<CodeRefElement> _r;
+  RenderingScheduler<CodeRefElement> _r;
 
   Stream<RenderedEvent<CodeRefElement>> get onRendered => _r.onRendered;
 
-  M.IsolateRef? _isolate;
-  late M.CodeRef _code;
+  M.IsolateRef _isolate;
+  M.CodeRef _code;
 
-  M.IsolateRef get isolate => _isolate!;
+  M.IsolateRef get isolate => _isolate;
   M.CodeRef get code => _code;
 
-  factory CodeRefElement(M.IsolateRef? isolate, M.CodeRef code,
-      {RenderingQueue? queue}) {
+  factory CodeRefElement(M.IsolateRef isolate, M.CodeRef code,
+      {RenderingQueue queue}) {
     assert(code != null);
     CodeRefElement e = new CodeRefElement.created();
     e._r = new RenderingScheduler<CodeRefElement>(e, queue: queue);
@@ -55,7 +55,7 @@ class CodeRefElement extends CustomElement implements Renderable {
       new AnchorElement(
           href: ((M.isSyntheticCode(_code.kind)) || (_isolate == null))
               ? null
-              : Uris.inspect(_isolate!, object: _code))
+              : Uris.inspect(_isolate, object: _code))
         ..text = _code.name
     ];
   }
diff --git a/runtime/observatory/lib/src/elements/code_view.dart b/runtime/observatory/lib/src/elements/code_view.dart
index 4493338d59b..c5061ad0094 100644
--- a/runtime/observatory/lib/src/elements/code_view.dart
+++ b/runtime/observatory/lib/src/elements/code_view.dart
@@ -51,22 +51,22 @@ class CodeViewElement extends CustomElement implements Renderable {
     ObjectPoolRefElement.tag,
   ]);
 
-  late RenderingScheduler<CodeViewElement> _r;
+  RenderingScheduler<CodeViewElement> _r;
 
   Stream<RenderedEvent<CodeViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Code _code;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
-  late DisassemblyTable disassemblyTable;
-  late InlineTable inlineTable;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Code _code;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
+  DisassemblyTable disassemblyTable;
+  InlineTable inlineTable;
 
   static const kDisassemblyColumnIndex = 3;
 
@@ -86,7 +86,7 @@ class CodeViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -143,15 +143,15 @@ class CodeViewElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  TableElement? _disassemblyTable;
-  TableElement? _inlineRangeTable;
-  Element? _disassemblyTableBody;
-  Element? _inlineRangeTableBody;
+  TableElement _disassemblyTable;
+  TableElement _inlineRangeTable;
+  Element _disassemblyTableBody;
+  Element _inlineRangeTableBody;
 
   void render() {
     if (_inlineRangeTable == null) {
       _inlineRangeTable = new TableElement()..classes = ['table'];
-      _inlineRangeTable!.createTHead().children = <Element>[
+      _inlineRangeTable.createTHead().children = <Element>[
         new TableRowElement()
           ..children = <Element>[
             document.createElement('th')
@@ -166,12 +166,12 @@ class CodeViewElement extends CustomElement implements Renderable {
             document.createElement('th')..text = 'Functions',
           ]
       ];
-      _inlineRangeTableBody = _inlineRangeTable!.createTBody();
-      _inlineRangeTableBody!.classes = ['monospace'];
+      _inlineRangeTableBody = _inlineRangeTable.createTBody();
+      _inlineRangeTableBody.classes = ['monospace'];
     }
     if (_disassemblyTable == null) {
       _disassemblyTable = new TableElement()..classes = ['table'];
-      _disassemblyTable!.createTHead().children = <Element>[
+      _disassemblyTable.createTHead().children = <Element>[
         new TableRowElement()
           ..children = <Element>[
             document.createElement('th')
@@ -193,17 +193,17 @@ class CodeViewElement extends CustomElement implements Renderable {
               ..text = 'Object',
           ]
       ];
-      _disassemblyTableBody = _disassemblyTable!.createTBody();
-      _disassemblyTableBody!.classes = ['monospace'];
+      _disassemblyTableBody = _disassemblyTable.createTBody();
+      _disassemblyTableBody.classes = ['monospace'];
     }
-    final inlinedFunctions = _code.inlinedFunctions!.toList();
+    final inlinedFunctions = _code.inlinedFunctions.toList();
     final S.Code code = _code as S.Code;
     children = <Element>[
       navBar(<Element>[
         new NavTopMenuElement(queue: _r.queue).element,
         new NavVMMenuElement(_vm, _events, queue: _r.queue).element,
         new NavIsolateMenuElement(_isolate, _events, queue: _r.queue).element,
-        navMenu(_code.name!),
+        navMenu(_code.name),
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
@@ -222,7 +222,7 @@ class CodeViewElement extends CustomElement implements Renderable {
         ..classes = ['content-centered-big']
         ..children = <Element>[
           new HeadingElement.h1()
-            ..text = (M.isDartCode(_code.kind) && _code.isOptimized!)
+            ..text = (M.isDartCode(_code.kind) && _code.isOptimized)
                 ? 'Optimized code for ${_code.name}'
                 : 'Code for ${_code.name}',
           new HRElement(),
@@ -254,7 +254,7 @@ class CodeViewElement extends CustomElement implements Renderable {
                           ..text = 'Optimized',
                         new DivElement()
                           ..classes = ['memberValue']
-                          ..text = _code.isOptimized! ? 'Yes' : 'No'
+                          ..text = _code.isOptimized ? 'Yes' : 'No'
                       ],
               new DivElement()
                 ..classes = ['memberItem']
@@ -265,7 +265,7 @@ class CodeViewElement extends CustomElement implements Renderable {
                   new DivElement()
                     ..classes = ['memberValue']
                     ..children = <Element>[
-                      new FunctionRefElement(_isolate, _code.function!,
+                      new FunctionRefElement(_isolate, _code.function,
                               queue: _r.queue)
                           .element
                     ]
@@ -280,7 +280,7 @@ class CodeViewElement extends CustomElement implements Renderable {
                           ..text = 'Inclusive',
                         new DivElement()
                           ..classes = ['memberValue']
-                          ..text = '${code.profile!.formattedInclusiveTicks}'
+                          ..text = '${code.profile.formattedInclusiveTicks}'
                       ],
               new DivElement()
                 ..classes = ['memberItem']
@@ -292,7 +292,7 @@ class CodeViewElement extends CustomElement implements Renderable {
                           ..text = 'Exclusive',
                         new DivElement()
                           ..classes = ['memberValue']
-                          ..text = '${code.profile!.formattedExclusiveTicks}'
+                          ..text = '${code.profile.formattedExclusiveTicks}'
                       ],
               new DivElement()
                 ..classes = ['memberItem']
@@ -303,7 +303,7 @@ class CodeViewElement extends CustomElement implements Renderable {
                   new DivElement()
                     ..classes = ['memberValue']
                     ..children = <Element>[
-                      new ObjectPoolRefElement(_isolate, _code.objectPool!,
+                      new ObjectPoolRefElement(_isolate, _code.objectPool,
                               queue: _r.queue)
                           .element
                     ]
@@ -334,9 +334,9 @@ class CodeViewElement extends CustomElement implements Renderable {
                       ]
             ],
           new HRElement(),
-          _inlineRangeTable!,
+          _inlineRangeTable,
           new HRElement(),
-          _disassemblyTable!
+          _disassemblyTable
         ],
     ];
     _updateDisassembly();
@@ -351,10 +351,11 @@ class CodeViewElement extends CustomElement implements Renderable {
 
   Future _refreshTicks() async {
     S.Code code = _code as S.Code;
-    final isolate = code.isolate!;
-    var response = await isolate.invokeRpc('_getCpuProfile', {'tags': 'None'});
+    final isolate = code.isolate;
+    S.ServiceMap response =
+        await isolate.invokeRpc('_getCpuProfile', {'tags': 'None'});
     final cpuProfile = new SampleProfile();
-    await cpuProfile.load(isolate, response as S.ServiceMap);
+    await cpuProfile.load(isolate, response);
     _r.dirty();
   }
 
@@ -376,7 +377,7 @@ class CodeViewElement extends CustomElement implements Renderable {
     if (code.profile == null) {
       return '';
     }
-    var intervalTick = code.profile!.intervalTicks[interval.start];
+    var intervalTick = code.profile.intervalTicks[interval.start];
     if (intervalTick == null) {
       return '';
     }
@@ -385,7 +386,7 @@ class CodeViewElement extends CustomElement implements Renderable {
       return '';
     }
     var pcent = Utils.formatPercent(
-        intervalTick.inclusiveTicks, code.profile!.profile.sampleCount);
+        intervalTick.inclusiveTicks, code.profile.profile.sampleCount);
     return '$pcent (${intervalTick.inclusiveTicks})';
   }
 
@@ -394,22 +395,21 @@ class CodeViewElement extends CustomElement implements Renderable {
     if (code.profile == null) {
       return '';
     }
-    var intervalTick = code.profile!.intervalTicks[interval.start];
+    var intervalTick = code.profile.intervalTicks[interval.start];
     if (intervalTick == null) {
       return '';
     }
     var pcent = Utils.formatPercent(
-        intervalTick.exclusiveTicks, code.profile!.profile.sampleCount);
+        intervalTick.exclusiveTicks, code.profile.profile.sampleCount);
     return '$pcent (${intervalTick.exclusiveTicks})';
   }
 
   String _formattedInclusive(S.CodeInstruction instruction) {
     S.Code code = _code as S.Code;
-    var profile = code.profile;
-    if (profile == null) {
+    if (code.profile == null) {
       return '';
     }
-    var tick = profile.addressTicks[instruction.address];
+    var tick = code.profile.addressTicks[instruction.address];
     if (tick == null) {
       return '';
     }
@@ -417,23 +417,22 @@ class CodeViewElement extends CustomElement implements Renderable {
     if (tick.inclusiveTicks == tick.exclusiveTicks) {
       return '';
     }
-    var pcent =
-        Utils.formatPercent(tick.inclusiveTicks, profile.profile.sampleCount);
+    var pcent = Utils.formatPercent(
+        tick.inclusiveTicks, code.profile.profile.sampleCount);
     return '$pcent (${tick.inclusiveTicks})';
   }
 
   String _formattedExclusive(S.CodeInstruction instruction) {
     S.Code code = _code as S.Code;
-    var profile = code.profile;
-    if (profile == null) {
+    if (code.profile == null) {
       return '';
     }
-    var tick = profile.addressTicks[instruction.address];
+    var tick = code.profile.addressTicks[instruction.address];
     if (tick == null) {
       return '';
     }
-    var pcent =
-        Utils.formatPercent(tick.exclusiveTicks, profile.profile.sampleCount);
+    var pcent = Utils.formatPercent(
+        tick.exclusiveTicks, code.profile.profile.sampleCount);
     return '$pcent (${tick.exclusiveTicks})';
   }
 
@@ -456,7 +455,7 @@ class CodeViewElement extends CustomElement implements Renderable {
   }
 
   void _addDisassemblyDOMRow() {
-    var tableBody = _disassemblyTableBody!;
+    var tableBody = _disassemblyTableBody;
     assert(tableBody != null);
     var tr = new TableRowElement();
 
@@ -507,7 +506,7 @@ class CodeViewElement extends CustomElement implements Renderable {
   }
 
   void _updateDisassemblyDOMTable() {
-    var tableBody = _disassemblyTableBody!;
+    var tableBody = _disassemblyTableBody;
     assert(tableBody != null);
     // Resize DOM table.
     if (tableBody.children.length > disassemblyTable.sortedRows.length) {
@@ -532,7 +531,7 @@ class CodeViewElement extends CustomElement implements Renderable {
     var i = 0;
     for (var tr in tableBody.children) {
       var rowIndex = disassemblyTable.sortedRows[i];
-      _fillDisassemblyDOMRow(tr as TableRowElement, rowIndex);
+      _fillDisassemblyDOMRow(tr, rowIndex);
       i++;
     }
   }
@@ -557,7 +556,7 @@ class CodeViewElement extends CustomElement implements Renderable {
   }
 
   void _addInlineDOMRow() {
-    var tableBody = _inlineRangeTableBody!;
+    var tableBody = _inlineRangeTableBody;
     assert(tableBody != null);
     var tr = new TableRowElement();
 
@@ -610,7 +609,7 @@ class CodeViewElement extends CustomElement implements Renderable {
   }
 
   void _updateInlineDOMTable() {
-    var tableBody = _inlineRangeTableBody!;
+    var tableBody = _inlineRangeTableBody;
     // Resize DOM table.
     if (tableBody.children.length > inlineTable.sortedRows.length) {
       // Shrink the table.
@@ -630,7 +629,7 @@ class CodeViewElement extends CustomElement implements Renderable {
     for (var i = 0; i < inlineTable.sortedRows.length; i++) {
       var rowIndex = inlineTable.sortedRows[i];
       var tr = tableBody.children[i];
-      _fillInlineDOMRow(tr as TableRowElement, rowIndex);
+      _fillInlineDOMRow(tr, rowIndex);
     }
   }
 
@@ -639,7 +638,7 @@ class CodeViewElement extends CustomElement implements Renderable {
     _updateInlineDOMTable();
   }
 
-  static String _codeKindToString(M.CodeKind? kind) {
+  static String _codeKindToString(M.CodeKind kind) {
     switch (kind) {
       case M.CodeKind.dart:
         return 'dart';
diff --git a/runtime/observatory/lib/src/elements/containers/search_bar.dart b/runtime/observatory/lib/src/elements/containers/search_bar.dart
index 8921cfcf377..593956ab318 100644
--- a/runtime/observatory/lib/src/elements/containers/search_bar.dart
+++ b/runtime/observatory/lib/src/elements/containers/search_bar.dart
@@ -19,7 +19,7 @@ typedef Iterable<dynamic> SearchBarSearchCallback(Pattern pattern);
 class SearchBarElement extends CustomElement implements Renderable {
   static const tag = const Tag<SearchBarElement>('search-bar');
 
-  late RenderingScheduler<SearchBarElement> _r;
+  RenderingScheduler<SearchBarElement> _r;
 
   StreamController<SearchResultSelected> _onSearchResultSelected =
       new StreamController<SearchResultSelected>.broadcast();
@@ -28,11 +28,11 @@ class SearchBarElement extends CustomElement implements Renderable {
   Stream<SearchResultSelected> get onSearchResultSelected =>
       _onSearchResultSelected.stream;
 
-  late StreamSubscription _onKeyDownSubscription;
+  StreamSubscription _onKeyDownSubscription;
 
-  Element? _workspace;
-  late SearchBarSearchCallback _search;
-  late bool _isOpen;
+  Element _workspace;
+  SearchBarSearchCallback _search;
+  bool _isOpen;
   bool _focusRequested = false;
   String _lastValue = '';
   List _results = const [];
@@ -43,7 +43,7 @@ class SearchBarElement extends CustomElement implements Renderable {
 
   set isOpen(bool value) {
     if (!value) {
-      _input!.value = '';
+      _input.value = '';
       _lastValue = '';
       if (_results.isNotEmpty) {
         _results = const [];
@@ -55,7 +55,7 @@ class SearchBarElement extends CustomElement implements Renderable {
   }
 
   factory SearchBarElement(SearchBarSearchCallback search,
-      {bool isOpen: false, Element? workspace, RenderingQueue? queue}) {
+      {bool isOpen: false, Element workspace, RenderingQueue queue}) {
     assert(search != null);
     assert(isOpen != null);
     SearchBarElement e = new SearchBarElement.created();
@@ -96,15 +96,15 @@ class SearchBarElement extends CustomElement implements Renderable {
     _onKeyDownSubscription.cancel();
   }
 
-  TextInputElement? _input;
-  SpanElement? _resultsArea;
+  TextInputElement _input;
+  SpanElement _resultsArea;
 
   void render() {
     if (_input == null) {
       _input = new TextInputElement()
         ..onKeyPress.listen((e) {
           if (e.keyCode == KeyCode.ENTER) {
-            if (_input!.value == '') {
+            if (_input.value == '') {
               _lastValue = '';
               if (_results.isNotEmpty) {
                 _results = const [];
@@ -112,9 +112,9 @@ class SearchBarElement extends CustomElement implements Renderable {
                 _triggerSearchResultSelected();
                 _r.dirty();
               }
-            } else if (_input!.value != _lastValue) {
-              _lastValue = _input!.value;
-              _results = _doSearch(_input!.value);
+            } else if (_input.value != _lastValue) {
+              _lastValue = _input.value;
+              _results = _doSearch(_input.value);
               _current = 0;
               _triggerSearchResultSelected();
               _r.dirty();
@@ -129,8 +129,8 @@ class SearchBarElement extends CustomElement implements Renderable {
         });
       _resultsArea = new SpanElement();
       children = <Element>[
-        _input!,
-        _resultsArea!,
+        _input,
+        _resultsArea,
         new ButtonElement()
           ..text = '❌'
           ..onClick.listen((_) {
@@ -138,7 +138,7 @@ class SearchBarElement extends CustomElement implements Renderable {
           })
       ];
     }
-    _resultsArea!.nodes = [
+    _resultsArea.nodes = [
       new ButtonElement()
         ..text = '▲'
         ..disabled = _results.isEmpty
@@ -152,7 +152,7 @@ class SearchBarElement extends CustomElement implements Renderable {
     ];
     style.visibility = isOpen ? null : 'collapse';
     if (_focusRequested) {
-      _input!.focus();
+      _input.focus();
       _focusRequested = false;
     }
   }
@@ -202,6 +202,6 @@ class _CaseInsensitivePatternString implements Pattern {
   Iterable<Match> allMatches(String string, [int start = 0]) =>
       _pattern.allMatches(string.toLowerCase(), start);
 
-  Match? matchAsPrefix(String string, [int start = 0]) =>
+  Match matchAsPrefix(String string, [int start = 0]) =>
       _pattern.matchAsPrefix(string.toLowerCase(), start);
 }
diff --git a/runtime/observatory/lib/src/elements/containers/virtual_collection.dart b/runtime/observatory/lib/src/elements/containers/virtual_collection.dart
index a21f4b963d6..1446716a315 100644
--- a/runtime/observatory/lib/src/elements/containers/virtual_collection.dart
+++ b/runtime/observatory/lib/src/elements/containers/virtual_collection.dart
@@ -19,23 +19,23 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
   static const tag = const Tag<VirtualCollectionElement>('virtual-collection',
       dependencies: const [SearchBarElement.tag]);
 
-  late RenderingScheduler<VirtualCollectionElement> _r;
+  RenderingScheduler<VirtualCollectionElement> _r;
 
   Stream<RenderedEvent<VirtualCollectionElement>> get onRendered =>
       _r.onRendered;
 
-  late VirtualCollectionCreateCallback _create;
-  VirtualCollectionHeaderCallback? _createHeader;
-  late VirtualCollectionUpdateCallback _update;
-  VirtualCollectionSearchCallback? _search;
-  double? _itemHeight;
-  int? _top;
-  double? _height;
-  List? _items;
-  late StreamSubscription _onScrollSubscription;
-  late StreamSubscription _onResizeSubscription;
+  VirtualCollectionCreateCallback _create;
+  VirtualCollectionHeaderCallback _createHeader;
+  VirtualCollectionUpdateCallback _update;
+  VirtualCollectionSearchCallback _search;
+  double _itemHeight;
+  int _top;
+  double _height;
+  List _items;
+  StreamSubscription _onScrollSubscription;
+  StreamSubscription _onResizeSubscription;
 
-  List? get items => _items;
+  List get items => _items;
 
   set items(Iterable value) {
     _items = new List.unmodifiable(value);
@@ -47,9 +47,9 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
   factory VirtualCollectionElement(VirtualCollectionCreateCallback create,
       VirtualCollectionUpdateCallback update,
       {Iterable items: const [],
-      VirtualCollectionHeaderCallback? createHeader,
-      VirtualCollectionSearchCallback? search,
-      RenderingQueue? queue}) {
+      VirtualCollectionHeaderCallback createHeader,
+      VirtualCollectionSearchCallback search,
+      RenderingQueue queue}) {
     assert(create != null);
     assert(update != null);
     assert(items != null);
@@ -84,8 +84,8 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
     _onResizeSubscription.cancel();
   }
 
-  DivElement? _header;
-  SearchBarElement? _searcher;
+  DivElement _header;
+  SearchBarElement _searcher;
   final DivElement _viewport = new DivElement()
     ..classes = ['viewport', 'container'];
   final DivElement _spacer = new DivElement()..classes = ['spacer'];
@@ -106,11 +106,10 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
     if (el_index < 0) {
       return null;
     }
-    final item_index = _top! +
-        el_index -
-        safeFloor(_buffer.children.length * _inverse_preload);
-    if (0 <= item_index && item_index < items!.length) {
-      return _items![item_index];
+    final item_index =
+        _top + el_index - safeFloor(_buffer.children.length * _inverse_preload);
+    if (0 <= item_index && item_index < items.length) {
+      return _items[item_index];
     }
     return null;
   }
@@ -152,39 +151,38 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
               ..onSearchResultSelected.listen((e) {
                 takeIntoView(e.item);
               });
-        children.insert(0, _searcher!.element);
+        children.insert(0, _searcher.element);
       }
       if (_createHeader != null) {
         _header = new DivElement()
           ..classes = ['header', 'container']
-          ..children = _createHeader!();
-        children.insert(0, _header!);
-        final rect = _header!.getBoundingClientRect();
-        _header!.classes.add('attached');
+          ..children = _createHeader();
+        children.insert(0, _header);
+        final rect = _header.getBoundingClientRect();
+        _header.classes.add('attached');
         _viewport.style.top = '${rect.height}px';
-        final width = _header!.children.fold(0.0, _foldWidth);
+        final width = _header.children.fold(0.0, _foldWidth);
         _buffer.style.minWidth = '${width}px';
       }
-      _itemHeight =
-          _buffer.children[0].getBoundingClientRect().height as double;
-      _height = getBoundingClientRect().height as double;
+      _itemHeight = _buffer.children[0].getBoundingClientRect().height;
+      _height = getBoundingClientRect().height;
     }
 
     if (_takeIntoView != null) {
-      final index = items!.indexOf(_takeIntoView);
+      final index = items.indexOf(_takeIntoView);
       if (index >= 0) {
-        final minScrollTop = _itemHeight! * (index + 1) - _height!;
-        final maxScrollTop = _itemHeight! * index;
+        final minScrollTop = _itemHeight * (index + 1) - _height;
+        final maxScrollTop = _itemHeight * index;
         _viewport.scrollTop =
             safeFloor((maxScrollTop - minScrollTop) / 2 + minScrollTop);
       }
       _takeIntoView = null;
     }
 
-    final top = safeFloor(_viewport.scrollTop / _itemHeight!);
+    final top = safeFloor(_viewport.scrollTop / _itemHeight);
 
-    _spacer.style.height = '${_itemHeight! * (_items!.length)}px';
-    final tail_length = safeCeil(_height! / _itemHeight! / _preload);
+    _spacer.style.height = '${_itemHeight * (_items.length)}px';
+    final tail_length = safeCeil(_height / _itemHeight / _preload);
     final length = tail_length * 2 + tail_length * _preload;
 
     if (_buffer.children.length < length) {
@@ -196,13 +194,13 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
       _top = null; // force update;
     }
 
-    if ((_top == null) || ((top - _top!).abs() >= tail_length)) {
-      _buffer.style.top = '${_itemHeight! * (top - tail_length)}px';
+    if ((_top == null) || ((top - _top).abs() >= tail_length)) {
+      _buffer.style.top = '${_itemHeight * (top - tail_length)}px';
       int i = top - tail_length;
-      for (final e in _buffer.children) {
-        if (0 <= i && i < _items!.length) {
+      for (final HtmlElement e in _buffer.children) {
+        if (0 <= i && i < _items.length) {
           e.style.display = null;
-          _update(e as HtmlElement, _items![i], i);
+          _update(e, _items[i], i);
         } else {
           e.style.display = 'hidden';
         }
@@ -212,11 +210,11 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
     }
 
     if (_searcher != null) {
-      final current = _searcher!.current;
-      int i = _top! - tail_length;
-      for (final e in _buffer.children) {
-        if (0 <= i && i < _items!.length) {
-          if (_items![i] == current) {
+      final current = _searcher.current;
+      int i = _top - tail_length;
+      for (final HtmlElement e in _buffer.children) {
+        if (0 <= i && i < _items.length) {
+          if (_items[i] == current) {
             e.classes.add('marked');
           } else {
             e.classes.remove('marked');
@@ -229,14 +227,14 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
   }
 
   double _foldWidth(double value, Element child) {
-    return math.max(value, child.getBoundingClientRect().width as double);
+    return math.max(value, child.getBoundingClientRect().width);
   }
 
   void _updateHeader() {
     if (_header != null) {
-      _header!.style.left = '${-_viewport.scrollLeft}px';
+      _header.style.left = '${-_viewport.scrollLeft}px';
       final width = _buffer.getBoundingClientRect().width;
-      _header!.children.last.style.width = '${width}px';
+      _header.children.last.style.width = '${width}px';
     }
   }
 
@@ -247,8 +245,8 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
   }
 
   void _onResize(_) {
-    final newHeight = getBoundingClientRect().height as double;
-    if (newHeight > _height!) {
+    final newHeight = getBoundingClientRect().height;
+    if (newHeight > _height) {
       _height = newHeight;
       _r.dirty();
     } else {
@@ -259,6 +257,6 @@ class VirtualCollectionElement extends CustomElement implements Renderable {
   }
 
   Iterable<dynamic> _doSearch(Pattern search) {
-    return _items!.where((item) => _search!(search, item));
+    return _items.where((item) => _search(search, item));
   }
 }
diff --git a/runtime/observatory/lib/src/elements/containers/virtual_tree.dart b/runtime/observatory/lib/src/elements/containers/virtual_tree.dart
index 14e016b3cdc..09690d6f2fa 100644
--- a/runtime/observatory/lib/src/elements/containers/virtual_tree.dart
+++ b/runtime/observatory/lib/src/elements/containers/virtual_tree.dart
@@ -30,13 +30,13 @@ class VirtualTreeElement extends CustomElement implements Renderable {
   static const tag = const Tag<VirtualTreeElement>('virtual-tree',
       dependencies: const [VirtualCollectionElement.tag]);
 
-  late RenderingScheduler<VirtualTreeElement> _r;
+  RenderingScheduler<VirtualTreeElement> _r;
 
   Stream<RenderedEvent<VirtualTreeElement>> get onRendered => _r.onRendered;
 
-  late VritualTreeGetChildrenCallback _children;
-  late List _items;
-  late List _depths;
+  VritualTreeGetChildrenCallback _children;
+  List _items;
+  List _depths;
   final Set _expanded = new Set();
 
   List get items => _items;
@@ -50,8 +50,8 @@ class VirtualTreeElement extends CustomElement implements Renderable {
   factory VirtualTreeElement(VirtualTreeCreateCallback create,
       VirtualTreeUpdateCallback update, VritualTreeGetChildrenCallback children,
       {Iterable items: const [],
-      VirtualTreeSearchCallback? search,
-      RenderingQueue? queue}) {
+      VirtualTreeSearchCallback search,
+      RenderingQueue queue}) {
     assert(create != null);
     assert(update != null);
     assert(children != null);
@@ -64,7 +64,7 @@ class VirtualTreeElement extends CustomElement implements Renderable {
       return element = create((
           {bool autoToggleSingleChildNodes: false,
           bool autoToggleWholeTree: false}) {
-        var item = e._collection!.getItemFromElement(element);
+        var item = e._collection.getItemFromElement(element);
         if (e.isExpanded(item)) {
           e.collapse(item,
               autoCollapseWholeTree: autoToggleWholeTree,
@@ -147,11 +147,11 @@ class VirtualTreeElement extends CustomElement implements Renderable {
     children = const [];
   }
 
-  VirtualCollectionElement? _collection;
+  VirtualCollectionElement _collection;
 
   void render() {
     if (children.length == 0) {
-      children = <Element>[_collection!.element];
+      children = <Element>[_collection.element];
     }
 
     final items = [];
@@ -179,8 +179,8 @@ class VirtualTreeElement extends CustomElement implements Renderable {
     }
 
     _depths = depths;
-    _collection!.items = items;
+    _collection.items = items;
 
-    _r.waitFor([_collection!.onRendered.first]);
+    _r.waitFor([_collection.onRendered.first]);
   }
 }
diff --git a/runtime/observatory/lib/src/elements/context_ref.dart b/runtime/observatory/lib/src/elements/context_ref.dart
index 0a004680472..f88bf007612 100644
--- a/runtime/observatory/lib/src/elements/context_ref.dart
+++ b/runtime/observatory/lib/src/elements/context_ref.dart
@@ -15,15 +15,15 @@ class ContextRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<ContextRefElement>('context-ref',
       dependencies: const [CurlyBlockElement.tag]);
 
-  late RenderingScheduler<ContextRefElement> _r;
+  RenderingScheduler<ContextRefElement> _r;
 
   Stream<RenderedEvent<ContextRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ContextRef _context;
-  late M.ObjectRepository _objects;
-  M.Context? _loadedContext;
-  late bool _expandable;
+  M.IsolateRef _isolate;
+  M.ContextRef _context;
+  M.ObjectRepository _objects;
+  M.Context _loadedContext;
+  bool _expandable;
   bool _expanded = false;
 
   M.IsolateRef get isolate => _isolate;
@@ -31,7 +31,7 @@ class ContextRefElement extends CustomElement implements Renderable {
 
   factory ContextRefElement(
       M.IsolateRef isolate, M.ContextRef context, M.ObjectRepository objects,
-      {RenderingQueue? queue, bool expandable: true}) {
+      {RenderingQueue queue, bool expandable: true}) {
     assert(isolate != null);
     assert(context != null);
     assert(objects != null);
@@ -60,7 +60,7 @@ class ContextRefElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _loadedContext = await _objects.get(_isolate, _context.id!) as M.Context;
+    _loadedContext = await _objects.get(_isolate, _context.id);
     _r.dirty();
   }
 
@@ -102,7 +102,7 @@ class ContextRefElement extends CustomElement implements Renderable {
       return [new SpanElement()..text = 'Loading...'];
     }
     var members = <Element>[];
-    if (_loadedContext!.parentContext != null) {
+    if (_loadedContext.parentContext != null) {
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
@@ -113,14 +113,14 @@ class ContextRefElement extends CustomElement implements Renderable {
             ..classes = ['memberName']
             ..children = <Element>[
               new ContextRefElement(
-                      _isolate, _loadedContext!.parentContext!, _objects,
+                      _isolate, _loadedContext.parentContext, _objects,
                       queue: _r.queue)
                   .element
             ]
         ]);
     }
-    if (_loadedContext!.variables!.isNotEmpty) {
-      var variables = _loadedContext!.variables!.toList();
+    if (_loadedContext.variables.isNotEmpty) {
+      var variables = _loadedContext.variables.toList();
       for (var index = 0; index < variables.length; index++) {
         var variable = variables[index];
         members.add(new DivElement()
diff --git a/runtime/observatory/lib/src/elements/context_view.dart b/runtime/observatory/lib/src/elements/context_view.dart
index d53009e2c11..b85b31508e5 100644
--- a/runtime/observatory/lib/src/elements/context_view.dart
+++ b/runtime/observatory/lib/src/elements/context_view.dart
@@ -36,21 +36,21 @@ class ContextViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<ContextViewElement> _r;
+  RenderingScheduler<ContextViewElement> _r;
 
   Stream<RenderedEvent<ContextViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Context _context;
-  late M.ContextRepository _contexts;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Context _context;
+  M.ContextRepository _contexts;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -69,7 +69,7 @@ class ContextViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -118,13 +118,13 @@ class ContextViewElement extends CustomElement implements Renderable {
         new NavTopMenuElement(queue: _r.queue).element,
         new NavVMMenuElement(_vm, _events, queue: _r.queue).element,
         new NavIsolateMenuElement(_isolate, _events, queue: _r.queue).element,
-        new NavClassMenuElement(_isolate, _context.clazz!, queue: _r.queue)
+        new NavClassMenuElement(_isolate, _context.clazz, queue: _r.queue)
             .element,
         navMenu('instance'),
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
-                _context = await _contexts.get(_isolate, _context.id!);
+                _context = await _contexts.get(_isolate, _context.id);
                 _r.dirty();
               }))
             .element,
@@ -160,7 +160,7 @@ class ContextViewElement extends CustomElement implements Renderable {
                       ..classes = ['memberName']
                       ..children = <Element>[
                         new ContextRefElement(
-                                _isolate, _context.parentContext!, _objects,
+                                _isolate, _context.parentContext, _objects,
                                 queue: _r.queue)
                             .element
                       ]
@@ -170,7 +170,7 @@ class ContextViewElement extends CustomElement implements Renderable {
       ]);
     }
     content.add(new HRElement());
-    if (_context.variables!.isNotEmpty) {
+    if (_context.variables.isNotEmpty) {
       int index = 0;
       content.addAll([
         new DivElement()
@@ -181,7 +181,7 @@ class ContextViewElement extends CustomElement implements Renderable {
                   ..content = <Element>[
                     new DivElement()
                       ..classes = ['memberList']
-                      ..children = _context.variables!
+                      ..children = _context.variables
                           .map<Element>((variable) => new DivElement()
                             ..classes = ['memberItem']
                             ..children = <Element>[
diff --git a/runtime/observatory/lib/src/elements/cpu_profile.dart b/runtime/observatory/lib/src/elements/cpu_profile.dart
index 4cd262228db..f6fafd0fc34 100644
--- a/runtime/observatory/lib/src/elements/cpu_profile.dart
+++ b/runtime/observatory/lib/src/elements/cpu_profile.dart
@@ -34,21 +34,21 @@ class CpuProfileElement extends CustomElement implements Renderable {
     CpuProfileVirtualTreeElement.tag,
   ]);
 
-  late RenderingScheduler<CpuProfileElement> _r;
+  RenderingScheduler<CpuProfileElement> _r;
 
   Stream<RenderedEvent<CpuProfileElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.IsolateSampleProfileRepository _profiles;
-  late Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
-  M.SampleProfileLoadingProgress? _progress;
-  late M.SampleProfileTag _tag = M.SampleProfileTag.none;
-  late ProfileTreeMode _mode = ProfileTreeMode.function;
-  late M.ProfileTreeDirection _direction = M.ProfileTreeDirection.exclusive;
-  late String _filter = '';
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.IsolateSampleProfileRepository _profiles;
+  Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
+  M.SampleProfileLoadingProgress _progress;
+  M.SampleProfileTag _tag = M.SampleProfileTag.none;
+  ProfileTreeMode _mode = ProfileTreeMode.function;
+  M.ProfileTreeDirection _direction = M.ProfileTreeDirection.exclusive;
+  String _filter = '';
 
   M.IsolateRef get isolate => _isolate;
   M.NotificationRepository get notifications => _notifications;
@@ -61,7 +61,7 @@ class CpuProfileElement extends CustomElement implements Renderable {
       M.EventRepository events,
       M.NotificationRepository notifications,
       M.IsolateSampleProfileRepository profiles,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -112,16 +112,15 @@ class CpuProfileElement extends CustomElement implements Renderable {
       children = content;
       return;
     }
-    content.add((new SampleBufferControlElement(
-            _vm, _progress!, _progressStream,
+    content.add((new SampleBufferControlElement(_vm, _progress, _progressStream,
             selectedTag: _tag, queue: _r.queue)
           ..onTagChange.listen((e) {
             _tag = e.element.selectedTag;
             _request();
           }))
         .element);
-    if (_progress!.status == M.SampleProfileLoadingStatus.loaded) {
-      late CpuProfileVirtualTreeElement tree;
+    if (_progress.status == M.SampleProfileLoadingStatus.loaded) {
+      CpuProfileVirtualTreeElement tree;
       content.addAll([
         new BRElement(),
         (new StackTraceTreeConfigElement(
@@ -147,7 +146,7 @@ class CpuProfileElement extends CustomElement implements Renderable {
               }))
             .element,
         new BRElement(),
-        (tree = new CpuProfileVirtualTreeElement(_isolate, _progress!.profile,
+        (tree = new CpuProfileVirtualTreeElement(_isolate, _progress.profile,
                 queue: _r.queue))
             .element
       ]);
@@ -162,7 +161,7 @@ class CpuProfileElement extends CustomElement implements Renderable {
     _r.dirty();
     _progress = (await _progressStream.first).progress;
     _r.dirty();
-    if (M.isSampleProcessRunning(_progress!.status)) {
+    if (M.isSampleProcessRunning(_progress.status)) {
       _progress = (await _progressStream.last).progress;
       _r.dirty();
     }
diff --git a/runtime/observatory/lib/src/elements/cpu_profile/virtual_tree.dart b/runtime/observatory/lib/src/elements/cpu_profile/virtual_tree.dart
index 9e4752e9c5e..a01d842c6b5 100644
--- a/runtime/observatory/lib/src/elements/cpu_profile/virtual_tree.dart
+++ b/runtime/observatory/lib/src/elements/cpu_profile/virtual_tree.dart
@@ -22,24 +22,24 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
   static const tag =
       const Tag<CpuProfileVirtualTreeElement>('cpu-profile-virtual-tree');
 
-  late RenderingScheduler<CpuProfileVirtualTreeElement> _r;
+  RenderingScheduler<CpuProfileVirtualTreeElement> _r;
 
   Stream<RenderedEvent<CpuProfileVirtualTreeElement>> get onRendered =>
       _r.onRendered;
 
-  late M.ProfileTreeDirection _direction;
-  late ProfileTreeMode _mode;
-  late M.SampleProfileType _type;
-  late M.IsolateRef _isolate;
-  late M.SampleProfile _profile;
-  Iterable<M.CallTreeNodeFilter>? _filters;
+  M.ProfileTreeDirection _direction;
+  ProfileTreeMode _mode;
+  M.SampleProfileType _type;
+  M.IsolateRef _isolate;
+  M.SampleProfile _profile;
+  Iterable<M.CallTreeNodeFilter> _filters;
 
   M.ProfileTreeDirection get direction => _direction;
   ProfileTreeMode get mode => _mode;
   M.SampleProfileType get type => _type;
   M.IsolateRef get isolate => _isolate;
   M.SampleProfile get profile => _profile;
-  Iterable<M.CallTreeNodeFilter>? get filters => _filters;
+  Iterable<M.CallTreeNodeFilter> get filters => _filters;
 
   set direction(M.ProfileTreeDirection value) =>
       _direction = _r.checkAndReact(_direction, value);
@@ -49,18 +49,18 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
     _r.dirty();
   }
 
-  factory CpuProfileVirtualTreeElement(Object? owner, M.SampleProfile profile,
+  factory CpuProfileVirtualTreeElement(Object owner, M.SampleProfile profile,
       {ProfileTreeMode mode: ProfileTreeMode.function,
       M.SampleProfileType type: M.SampleProfileType.cpu,
       M.ProfileTreeDirection direction: M.ProfileTreeDirection.exclusive,
-      RenderingQueue? queue}) {
+      RenderingQueue queue}) {
     assert(profile != null);
     assert(mode != null);
     assert(direction != null);
     CpuProfileVirtualTreeElement e = new CpuProfileVirtualTreeElement.created();
     e._r =
         new RenderingScheduler<CpuProfileVirtualTreeElement>(e, queue: queue);
-    e._isolate = owner as M.Isolate;
+    e._isolate = owner;
     e._profile = profile;
     e._mode = mode;
     e._type = type;
@@ -83,7 +83,7 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  VirtualTreeElement? _tree;
+  VirtualTreeElement _tree;
 
   void render() {
     var tree;
@@ -123,7 +123,7 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
         throw new Exception('Unknown SampleProfileType: $type');
     }
     if (filters != null) {
-      tree = filters!.fold(tree, (dynamic tree, filter) {
+      tree = filters.fold(tree, (tree, filter) {
         return tree?.filtered(filter);
       });
     }
@@ -141,9 +141,9 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
       ];
       return;
     } else if (tree.root.children.length == 1) {
-      _tree!.expand(tree.root.children.first, autoExpandSingleChildNodes: true);
+      _tree.expand(tree.root.children.first, autoExpandSingleChildNodes: true);
     }
-    children = <Element>[_tree!.element];
+    children = <Element>[_tree.element];
   }
 
   static HtmlElement _createCpuRow(toggle) {
@@ -205,13 +205,13 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
     _updateLines(element.children[2].children, depth);
     if (item.children.isNotEmpty) {
       element.children[3].text =
-          _tree!.isExpanded(item) ? _expandedIcon : _collapsedIcon;
+          _tree.isExpanded(item) ? _expandedIcon : _collapsedIcon;
     } else {
       element.children[3].text = '';
     }
     element.children[4].text = Utils.formatPercentNormalized(item.percentage);
     element.children[5] = (new FunctionRefElement(
-            _isolate, item.profileFunction.function!,
+            _isolate, item.profileFunction.function,
             queue: _r.queue)
           ..classes = ['name'])
         .element;
@@ -230,13 +230,13 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
     _updateLines(element.children[2].children, depth);
     if (item.children.isNotEmpty) {
       element.children[3].text =
-          _tree!.isExpanded(item) ? _expandedIcon : _collapsedIcon;
+          _tree.isExpanded(item) ? _expandedIcon : _collapsedIcon;
     } else {
       element.children[3].text = '';
     }
     element.children[4].text = Utils.formatPercentNormalized(item.percentage);
     element.children[5] = (new FunctionRefElement(
-            null, item.profileFunction.function!,
+            null, item.profileFunction.function,
             queue: _r.queue)
           ..classes = ['name'])
         .element;
@@ -245,7 +245,7 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
   bool _searchFunction(Pattern pattern, itemDynamic) {
     M.FunctionCallTreeNode item = itemDynamic;
     return M
-        .getFunctionFullName(item.profileFunction.function!)
+        .getFunctionFullName(item.profileFunction.function)
         .contains(pattern);
   }
 
@@ -258,13 +258,13 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
     _updateLines(element.children[2].children, depth);
     if (item.children.isNotEmpty) {
       element.children[3].text =
-          _tree!.isExpanded(item) ? _expandedIcon : _collapsedIcon;
+          _tree.isExpanded(item) ? _expandedIcon : _collapsedIcon;
     } else {
       element.children[3].text = '';
     }
     element.children[4].text = Utils.formatPercentNormalized(item.percentage);
     element.children[5] =
-        (new CodeRefElement(_isolate, item.profileCode.code!, queue: _r.queue)
+        (new CodeRefElement(_isolate, item.profileCode.code, queue: _r.queue)
               ..classes = ['name'])
             .element;
   }
@@ -282,20 +282,20 @@ class CpuProfileVirtualTreeElement extends CustomElement implements Renderable {
     _updateLines(element.children[2].children, depth);
     if (item.children.isNotEmpty) {
       element.children[3].text =
-          _tree!.isExpanded(item) ? _expandedIcon : _collapsedIcon;
+          _tree.isExpanded(item) ? _expandedIcon : _collapsedIcon;
     } else {
       element.children[3].text = '';
     }
     element.children[4].text = Utils.formatPercentNormalized(item.percentage);
     element.children[5] =
-        (new CodeRefElement(null, item.profileCode.code!, queue: _r.queue)
+        (new CodeRefElement(null, item.profileCode.code, queue: _r.queue)
               ..classes = ['name'])
             .element;
   }
 
   bool _searchCode(Pattern pattern, itemDynamic) {
     M.CodeCallTreeNode item = itemDynamic;
-    return item.profileCode.code!.name!.contains(pattern);
+    return item.profileCode.code.name.contains(pattern);
   }
 
   static _updateLines(List<Element> lines, int n) {
diff --git a/runtime/observatory/lib/src/elements/cpu_profile_table.dart b/runtime/observatory/lib/src/elements/cpu_profile_table.dart
index a151b7206ea..e225e482d43 100644
--- a/runtime/observatory/lib/src/elements/cpu_profile_table.dart
+++ b/runtime/observatory/lib/src/elements/cpu_profile_table.dart
@@ -44,17 +44,17 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
         VirtualCollectionElement.tag
       ]);
 
-  late RenderingScheduler<CpuProfileTableElement> _r;
+  RenderingScheduler<CpuProfileTableElement> _r;
 
   Stream<RenderedEvent<CpuProfileTableElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.IsolateSampleProfileRepository _profiles;
-  late Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
-  M.SampleProfileLoadingProgress? _progress;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.IsolateSampleProfileRepository _profiles;
+  Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
+  M.SampleProfileLoadingProgress _progress;
   final _sortingField = <_Table, _SortingField>{
     _Table.functions: _SortingField.exclusive,
     _Table.caller: _SortingField.caller,
@@ -78,7 +78,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
       M.EventRepository events,
       M.NotificationRepository notifications,
       M.IsolateSampleProfileRepository profiles,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -129,10 +129,10 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
       children = content;
       return;
     }
-    content.add(new SampleBufferControlElement(_vm, _progress!, _progressStream,
+    content.add(new SampleBufferControlElement(_vm, _progress, _progressStream,
             showTag: false, queue: _r.queue)
         .element);
-    if (_progress!.status == M.SampleProfileLoadingStatus.loaded) {
+    if (_progress.status == M.SampleProfileLoadingStatus.loaded) {
       content.add(new BRElement());
       content.addAll(_createTables());
       content.add(new BRElement());
@@ -141,10 +141,10 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
     children = content;
   }
 
-  M.ProfileFunction? _selected;
-  VirtualCollectionElement? _functions;
-  VirtualCollectionElement? _callers;
-  VirtualCollectionElement? _callees;
+  M.ProfileFunction _selected;
+  VirtualCollectionElement _functions;
+  VirtualCollectionElement _callers;
+  VirtualCollectionElement _callees;
 
   List<Element> _createTables() {
     _functions = _functions ??
@@ -153,11 +153,11 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
             search: _searchFunction,
             queue: _r.queue);
     // If there's no samples, don't populate the function list.
-    _functions!.items = (_progress!.profile.sampleCount != 0)
-        ? _progress!.profile.functions.toList()
+    _functions.items = (_progress.profile.sampleCount != 0)
+        ? _progress.profile.functions.toList()
         : []
       ..sort(_createSorter(_Table.functions));
-    _functions!.takeIntoView(_selected);
+    _functions.takeIntoView(_selected);
     _callers = _callers ??
         new VirtualCollectionElement(_createCaller, _updateCaller,
             createHeader: _createCallerHeader,
@@ -169,13 +169,13 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
             search: _searchFunction,
             queue: _r.queue);
     if (_selected != null) {
-      _callers!.items = _selected!.callers.keys.toList()
+      _callers.items = _selected.callers.keys.toList()
         ..sort(_createSorter(_Table.caller));
-      _callees!.items = _selected!.callees.keys.toList()
+      _callees.items = _selected.callees.keys.toList()
         ..sort(_createSorter(_Table.callee));
     } else {
-      _callers!.items = const [];
-      _callees!.items = const [];
+      _callers.items = const [];
+      _callees.items = const [];
     }
     return <Element>[
       new DivElement()
@@ -183,25 +183,25 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
         ..children = <Element>[
           new DivElement()
             ..classes = ['profile-trees-all']
-            ..children = <Element>[_functions!.element],
+            ..children = <Element>[_functions.element],
           new DivElement()
             ..classes = ['profile-trees-current']
             ..children = <Element>[
               new DivElement()
                 ..classes = ['profile-trees-caller']
-                ..children = <Element>[_callers!.element],
+                ..children = <Element>[_callers.element],
               new DivElement()
                 ..classes = ['profile-trees-selected']
                 ..children = _selected == null
                     ? [new SpanElement()..text = 'No element selected']
                     : [
-                        new FunctionRefElement(_isolate, _selected!.function!,
+                        new FunctionRefElement(_isolate, _selected.function,
                                 queue: _r.queue)
                             .element
                       ],
               new DivElement()
                 ..classes = ['profile-trees-callee']
-                ..children = <Element>[_callees!.element]
+                ..children = <Element>[_callees.element]
             ]
         ]
     ];
@@ -223,7 +223,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
       if (e.target is AnchorElement) {
         return;
       }
-      _selected = _functions!.getItemFromElement(element);
+      _selected = _functions.getItemFromElement(element);
       _r.dirty();
     });
     return element;
@@ -238,7 +238,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
     }
     e.children[0].text = Utils.formatPercentNormalized(_getExclusiveT(item));
     e.children[1].text = Utils.formatPercentNormalized(_getInclusiveT(item));
-    e.children[2].text = M.getFunctionFullName(item.function!);
+    e.children[2].text = M.getFunctionFullName(item.function);
   }
 
   List<HtmlElement> _createFunctionHeader() => [
@@ -264,7 +264,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
 
   bool _searchFunction(Pattern pattern, itemDynamic) {
     M.ProfileFunction item = itemDynamic;
-    return M.getFunctionFullName(item.function!).contains(pattern);
+    return M.getFunctionFullName(item.function).contains(pattern);
   }
 
   void _setSorting(
@@ -298,7 +298,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
       if (e.target is AnchorElement) {
         return;
       }
-      _selected = _callees!.getItemFromElement(element);
+      _selected = _callees.getItemFromElement(element);
       _r.dirty();
     });
     return element;
@@ -337,7 +337,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
       if (e.target is AnchorElement) {
         return;
       }
-      _selected = _callers!.getItemFromElement(element);
+      _selected = _callers.getItemFromElement(element);
       _r.dirty();
     });
     return element;
@@ -375,7 +375,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
         ..onClick.listen((_) => _setSorting(table, field, direction));
 
   List<Element> _createTree() {
-    late CpuProfileVirtualTreeElement tree;
+    CpuProfileVirtualTreeElement tree;
     return [
       (new StackTraceTreeConfigElement(
               showMode: false,
@@ -397,7 +397,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
             }))
           .element,
       new BRElement(),
-      (tree = new CpuProfileVirtualTreeElement(_isolate, _progress!.profile,
+      (tree = new CpuProfileVirtualTreeElement(_isolate, _progress.profile,
               mode: ProfileTreeMode.function,
               direction: M.ProfileTreeDirection.exclusive,
               queue: _r.queue)
@@ -425,7 +425,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
     _r.dirty();
     _progress = (await _progressStream.first).progress;
     _r.dirty();
-    if (M.isSampleProcessRunning(_progress!.status)) {
+    if (M.isSampleProcessRunning(_progress.status)) {
       _progress = (await _progressStream.last).progress;
       _r.dirty();
     }
@@ -459,7 +459,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
         getter = _getCallerT;
         break;
       case _SortingField.method:
-        getter = (M.ProfileFunction s) => M.getFunctionFullName(s.function!);
+        getter = (M.ProfileFunction s) => M.getFunctionFullName(s.function);
         break;
     }
     switch (_sortingDirection[table]) {
@@ -481,9 +481,7 @@ class CpuProfileTableElement extends CustomElement implements Renderable {
   static double _getInclusiveT(M.ProfileFunction f) =>
       f.normalizedInclusiveTicks;
   double _getCalleeT(M.ProfileFunction f) =>
-      _selected!.callees[f]! /
-      _selected!.callees.values.reduce((a, b) => a + b);
+      _selected.callees[f] / _selected.callees.values.reduce((a, b) => a + b);
   double _getCallerT(M.ProfileFunction f) =>
-      _selected!.callers[f]! /
-      _selected!.callers.values.reduce((a, b) => a + b);
+      _selected.callers[f] / _selected.callers.values.reduce((a, b) => a + b);
 }
diff --git a/runtime/observatory/lib/src/elements/curly_block.dart b/runtime/observatory/lib/src/elements/curly_block.dart
index abd8a3a6c4b..7bb7084ad42 100644
--- a/runtime/observatory/lib/src/elements/curly_block.dart
+++ b/runtime/observatory/lib/src/elements/curly_block.dart
@@ -18,15 +18,15 @@ class CurlyBlockToggleEvent {
 class CurlyBlockElement extends CustomElement implements Renderable {
   static const tag = const Tag<CurlyBlockElement>('curly-block');
 
-  late RenderingScheduler<CurlyBlockElement> _r;
+  RenderingScheduler<CurlyBlockElement> _r;
 
   final StreamController<CurlyBlockToggleEvent> _onToggle =
       new StreamController<CurlyBlockToggleEvent>.broadcast();
   Stream<CurlyBlockToggleEvent> get onToggle => _onToggle.stream;
   Stream<RenderedEvent<CurlyBlockElement>> get onRendered => _r.onRendered;
 
-  late bool _expanded;
-  late bool _disabled;
+  bool _expanded;
+  bool _disabled;
   Iterable<Element> _content = const [];
 
   bool get expanded => _expanded;
@@ -45,7 +45,7 @@ class CurlyBlockElement extends CustomElement implements Renderable {
   }
 
   factory CurlyBlockElement(
-      {bool expanded: false, bool disabled: false, RenderingQueue? queue}) {
+      {bool expanded: false, bool disabled: false, RenderingQueue queue}) {
     assert(expanded != null);
     assert(disabled != null);
     CurlyBlockElement e = new CurlyBlockElement.created();
diff --git a/runtime/observatory/lib/src/elements/debugger.dart b/runtime/observatory/lib/src/elements/debugger.dart
index e79bcfff5d3..12b5821eb2f 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -36,9 +36,8 @@ import 'package:observatory/src/elements/source_link.dart';
 abstract class DebuggerCommand extends Command {
   ObservatoryDebugger debugger;
 
-  DebuggerCommand(Debugger debugger, name, List<Command> children)
-      : debugger = debugger as ObservatoryDebugger,
-        super(name, children);
+  DebuggerCommand(this.debugger, name, List<Command> children)
+      : super(name, children);
 
   String get helpShort;
   String get helpLong;
@@ -47,7 +46,7 @@ abstract class DebuggerCommand extends Command {
 // TODO(turnidge): Rewrite HelpCommand so that it is a general utility
 // provided by the cli library.
 class HelpCommand extends DebuggerCommand {
-  HelpCommand(ObservatoryDebugger debugger)
+  HelpCommand(Debugger debugger)
       : super(debugger, 'help', <Command>[
           new HelpHotkeysCommand(debugger),
         ]);
@@ -67,8 +66,7 @@ class HelpCommand extends DebuggerCommand {
       var commands = debugger.cmd.matchCommand(<String>[], false);
       commands.sort((a, b) => a.name.compareTo(b.name));
       con.print('List of commands:\n');
-      for (Command c in commands) {
-        DebuggerCommand command = c as DebuggerCommand;
+      for (DebuggerCommand command in commands) {
         con.print('${_nameAndAlias(command).padRight(12)} '
             '- ${command.helpShort}');
       }
@@ -91,8 +89,7 @@ class HelpCommand extends DebuggerCommand {
         return new Future.value(null);
       }
       con.print('');
-      for (Command c in commands) {
-        DebuggerCommand command = c as DebuggerCommand;
+      for (DebuggerCommand command in commands) {
         con.printBold(_nameAndAlias(command));
         con.print(command.helpLong);
 
@@ -105,8 +102,7 @@ class HelpCommand extends DebuggerCommand {
         if (subCommands.isNotEmpty) {
           subCommands.sort((a, b) => a.name.compareTo(b.name));
           con.print('Subcommands:\n');
-          for (Command c in subCommands) {
-            DebuggerCommand subCommand = c as DebuggerCommand;
+          for (DebuggerCommand subCommand in subCommands) {
             con.print('    ${subCommand.fullName.padRight(16)} '
                 '- ${subCommand.helpShort}');
           }
@@ -181,15 +177,14 @@ class PrintCommand extends DebuggerCommand {
     }
     var expression = args.join('');
     var response =
-        await debugger.isolate.evalFrame(debugger.currentFrame!, expression);
+        await debugger.isolate.evalFrame(debugger.currentFrame, expression);
     if (response is S.DartError) {
-      debugger.console.print(response.message!);
+      debugger.console.print(response.message);
     } else if (response is S.Sentinel) {
       debugger.console.print(response.valueAsString);
     } else {
       debugger.console.print('= ', newline: false);
-      debugger.console.printRef(
-          debugger.isolate, response as S.Instance, debugger.objects!);
+      debugger.console.printRef(debugger.isolate, response, debugger.objects);
     }
   }
 
@@ -219,7 +214,7 @@ class DownCommand extends DebuggerCommand {
     try {
       debugger.downFrame(count);
       debugger.console.print('frame = ${debugger.currentFrame}');
-    } on dynamic catch (e) {
+    } catch (e) {
       debugger.console
           .print('frame must be in range [${e.start}..${e.end - 1}]');
     }
@@ -256,7 +251,7 @@ class UpCommand extends DebuggerCommand {
       debugger.console.print('frame = ${debugger.currentFrame}');
     } on RangeError catch (e) {
       debugger.console
-          .print('frame must be in range [${e.start}..${e.end! - 1}]');
+          .print('frame must be in range [${e.start}..${e.end - 1}]');
     }
     return new Future.value(null);
   }
@@ -293,7 +288,7 @@ class FrameCommand extends DebuggerCommand {
       debugger.console.print('frame = ${debugger.currentFrame}');
     } on RangeError catch (e) {
       debugger.console
-          .print('frame must be in range [${e.start}..${e.end! - 1}]');
+          .print('frame must be in range [${e.start}..${e.end - 1}]');
     }
     return new Future.value(null);
   }
@@ -425,7 +420,7 @@ class RewindCommand extends DebuggerCommand {
 
   Future run(List<String> args) async {
     try {
-      int? count = 1;
+      int count = 1;
       if (args.length == 1) {
         count = int.tryParse(args[0]);
         if (count == null || count < 1 || count >= debugger.stackDepth) {
@@ -438,10 +433,10 @@ class RewindCommand extends DebuggerCommand {
         debugger.console.print('rewind expects 0 or 1 argument');
         return;
       }
-      await debugger.rewind(count!);
+      await debugger.rewind(count);
     } on S.ServerRpcException catch (e) {
       if (e.code == S.ServerRpcException.kCannotResume) {
-        debugger.console.printRed(e.data!['details']);
+        debugger.console.printRed(e.data['details']);
       } else {
         rethrow;
       }
@@ -479,7 +474,7 @@ class ReloadCommand extends DebuggerCommand {
     } on S.ServerRpcException catch (e) {
       if (e.code == S.ServerRpcException.kIsolateReloadBarred ||
           e.code == S.ServerRpcException.kIsolateIsReloading) {
-        debugger.console.printRed(e.data!['details']);
+        debugger.console.printRed(e.data['details']);
       } else {
         rethrow;
       }
@@ -532,7 +527,7 @@ class LogCommand extends DebuggerCommand {
     return new Future.value(null);
   }
 
-  Level? _findLevel(String levelName) {
+  Level _findLevel(String levelName) {
     levelName = levelName.toUpperCase();
     for (var level in Level.LEVELS) {
       if (level.name == levelName) {
@@ -674,7 +669,7 @@ class SetCommand extends DebuggerCommand {
   Future run(List<String> args) async {
     if (args.length == 0) {
       for (var name in _options.keys) {
-        dynamic getHandler = _options[name]![2];
+        dynamic getHandler = _options[name][2];
         var value = await getHandler(debugger, name);
         debugger.console.print("${name} = ${value}");
       }
@@ -727,7 +722,7 @@ class SetCommand extends DebuggerCommand {
       var prefix = args[1];
       var optionInfo = _options[name];
       if (optionInfo != null) {
-        dynamic validValues = optionInfo[0];
+        var validValues = optionInfo[0];
         for (var value in validValues) {
           if (value.startsWith(prefix)) {
             result.add('${args[0]}${value} ');
@@ -766,7 +761,7 @@ class BreakCommand extends DebuggerCommand {
     if (loc.valid) {
       if (loc.function != null) {
         try {
-          await debugger.isolate.addBreakpointAtEntry(loc.function!);
+          await debugger.isolate.addBreakpointAtEntry(loc.function);
         } on S.ServerRpcException catch (e) {
           if (e.code == S.ServerRpcException.kCannotAddBreakpoint) {
             debugger.console.print('Unable to set breakpoint at ${loc}');
@@ -776,15 +771,15 @@ class BreakCommand extends DebuggerCommand {
         }
       } else {
         assert(loc.script != null);
-        var script = loc.script!;
+        var script = loc.script;
         await script.load();
-        if (loc.line! < 1 || loc.line! > script.lines.length) {
+        if (loc.line < 1 || loc.line > script.lines.length) {
           debugger.console.print(
               'line number must be in range [1..${script.lines.length}]');
           return;
         }
         try {
-          await debugger.isolate.addBreakpoint(script, loc.line!, loc.col);
+          await debugger.isolate.addBreakpoint(script, loc.line, loc.col);
         } on S.ServerRpcException catch (e) {
           if (e.code == S.ServerRpcException.kCannotAddBreakpoint) {
             debugger.console.print('Unable to set breakpoint at ${loc}');
@@ -794,7 +789,7 @@ class BreakCommand extends DebuggerCommand {
         }
       }
     } else {
-      debugger.console.print(loc.errorMessage!);
+      debugger.console.print(loc.errorMessage);
     }
   }
 
@@ -848,7 +843,7 @@ class ClearCommand extends DebuggerCommand {
     var arg = (args.length == 0 ? '' : args[0]);
     var loc = await DebuggerLocation.parse(debugger, arg);
     if (!loc.valid) {
-      debugger.console.print(loc.errorMessage!);
+      debugger.console.print(loc.errorMessage);
       return;
     }
     if (loc.function != null) {
@@ -857,20 +852,20 @@ class ClearCommand extends DebuggerCommand {
       return;
     }
 
-    var script = loc.script!;
-    if (loc.line! < 1 || loc.line! > script.lines.length) {
+    var script = loc.script;
+    if (loc.line < 1 || loc.line > script.lines.length) {
       debugger.console
           .print('line number must be in range [1..${script.lines.length}]');
       return;
     }
-    var lineInfo = script.getLine(loc.line!)!;
+    var lineInfo = script.getLine(loc.line);
     var bpts = lineInfo.breakpoints;
     var foundBreakpoint = false;
     if (bpts != null) {
       var bptList = bpts.toList();
       for (var bpt in bptList) {
         if (loc.col == null ||
-            loc.col == script.tokenToCol(bpt.location!.tokenPos)) {
+            loc.col == script.tokenToCol(bpt.location.tokenPos)) {
           foundBreakpoint = true;
           var result = await debugger.isolate.removeBreakpoint(bpt);
           if (result is S.DartError) {
@@ -1066,7 +1061,7 @@ class IsolateCommand extends DebuggerCommand {
       }
     }
     for (var isolate in debugger.vm.isolates) {
-      if (isolate.name!.startsWith(args[0])) {
+      if (isolate.name.startsWith(args[0])) {
         result.add('${isolate.name} ');
       }
     }
@@ -1113,7 +1108,7 @@ class IsolateListCommand extends DebuggerCommand {
     const maxRunStateLen = 7;
     var maxNameLen = 'NAME'.length;
     for (var isolate in debugger.vm.isolates) {
-      maxNameLen = max(maxNameLen, isolate.name!.length);
+      maxNameLen = max(maxNameLen, isolate.name.length);
     }
     debugger.console.print("${'ID'.padLeft(maxIdLen, ' ')} "
         "${'ORIGIN'.padLeft(maxIdLen, ' ')} "
@@ -1125,7 +1120,7 @@ class IsolateListCommand extends DebuggerCommand {
       debugger.console
           .print("${isolate.number.toString().padLeft(maxIdLen, ' ')} "
               "${isolate.originNumber.toString().padLeft(maxIdLen, ' ')} "
-              "${isolate.name!.padRight(maxNameLen, ' ')} "
+              "${isolate.name.padRight(maxNameLen, ' ')} "
               "${_isolateRunState(isolate).padRight(maxRunStateLen, ' ')} "
               "${current}");
     }
@@ -1230,7 +1225,7 @@ class VmListCommand extends DebuggerCommand {
 
     for (var vm in vmList) {
       maxAddrLen = max(maxAddrLen, vm.target.networkAddress.length);
-      maxNameLen = max(maxNameLen, vm.name!.length);
+      maxNameLen = max(maxNameLen, vm.name.length);
     }
 
     debugger.console.print("${'ADDRESS'.padRight(maxAddrLen, ' ')} "
@@ -1240,7 +1235,7 @@ class VmListCommand extends DebuggerCommand {
       String current = (vm == debugger.vm ? '*' : '');
       debugger.console
           .print("${vm.target.networkAddress.padRight(maxAddrLen, ' ')} "
-              "${vm.name!.padRight(maxNameLen, ' ')} "
+              "${vm.name.padRight(maxNameLen, ' ')} "
               "${current}");
     }
   }
@@ -1297,19 +1292,19 @@ class _ConsoleStreamPrinter {
 
   _ConsoleStreamPrinter(this._debugger);
   Level _minimumLogLevel = Level.OFF;
-  String? _savedStream;
-  String? _savedIsolate;
-  String? _savedLine;
+  String _savedStream;
+  String _savedIsolate;
+  String _savedLine;
   List<String> _buffer = [];
 
   void onEvent(String streamName, S.ServiceEvent event) {
     if (event.kind == S.ServiceEvent.kLogging) {
       // Check if we should print this log message.
-      if (event.logRecord!['level'].value < _minimumLogLevel.value) {
+      if (event.logRecord['level'].value < _minimumLogLevel.value) {
         return;
       }
     }
-    String isolateName = event.isolate!.name!;
+    String isolateName = event.isolate.name;
     // If we get a line from a different isolate/stream, flush
     // any pending output, even if it is not newline-terminated.
     if ((_savedIsolate != null && isolateName != _savedIsolate) ||
@@ -1319,14 +1314,14 @@ class _ConsoleStreamPrinter {
     String data;
     bool hasNewline;
     if (event.kind == S.ServiceEvent.kLogging) {
-      data = event.logRecord!["message"].valueAsString;
+      data = event.logRecord["message"].valueAsString;
       hasNewline = true;
     } else {
-      data = event.bytesAsString!;
+      data = event.bytesAsString;
       hasNewline = data.endsWith('\n');
     }
     if (_savedLine != null) {
-      data = _savedLine! + data;
+      data = _savedLine + data;
       _savedIsolate = null;
       _savedStream = null;
       _savedLine = null;
@@ -1350,7 +1345,7 @@ class _ConsoleStreamPrinter {
   void flush() {
     // If there is any saved output, flush it now.
     if (_savedLine != null) {
-      _buffer.add(_format(_savedIsolate!, _savedStream!, _savedLine!));
+      _buffer.add(_format(_savedIsolate, _savedStream, _savedLine));
       _savedIsolate = null;
       _savedStream = null;
       _savedLine = null;
@@ -1369,18 +1364,18 @@ class _ConsoleStreamPrinter {
 // Tracks the state for an isolate debugging session.
 class ObservatoryDebugger extends Debugger {
   final SettingsGroup settings = new SettingsGroup('debugger');
-  late RootCommand cmd;
-  late DebuggerPageElement page;
-  late DebuggerConsoleElement console;
-  late DebuggerInputElement input;
-  late DebuggerStackElement stackElement;
-  S.ServiceMap? stack;
+  RootCommand cmd;
+  DebuggerPageElement page;
+  DebuggerConsoleElement console;
+  DebuggerInputElement input;
+  DebuggerStackElement stackElement;
+  S.ServiceMap stack;
   final S.Isolate isolate;
   String breakOnException = "none"; // Last known setting.
 
-  int? get currentFrame => _currentFrame;
+  int get currentFrame => _currentFrame;
 
-  void set currentFrame(int? value) {
+  void set currentFrame(int value) {
     if (value != null && (value < 0 || value >= stackDepth)) {
       throw new RangeError.range(value, 0, stackDepth);
     }
@@ -1390,7 +1385,7 @@ class ObservatoryDebugger extends Debugger {
     }
   }
 
-  int? _currentFrame = null;
+  int _currentFrame = null;
 
   bool get upIsDown => _upIsDown;
   void set upIsDown(bool value) {
@@ -1398,7 +1393,7 @@ class ObservatoryDebugger extends Debugger {
     _upIsDown = value;
   }
 
-  bool _upIsDown = false;
+  bool _upIsDown;
 
   bool get causalAsyncStacks => _causalAsyncStacks;
   void set causalAsyncStacks(bool value) {
@@ -1406,7 +1401,7 @@ class ObservatoryDebugger extends Debugger {
     _causalAsyncStacks = value;
   }
 
-  bool _causalAsyncStacks = true;
+  bool _causalAsyncStacks;
 
   bool get awaiterStacks => _awaiterStacks;
   void set awaiterStacks(bool value) {
@@ -1414,7 +1409,7 @@ class ObservatoryDebugger extends Debugger {
     _causalAsyncStacks = value;
   }
 
-  bool _awaiterStacks = true;
+  bool _awaiterStacks;
 
   static const String kAwaiterStackFrames = 'awaiterFrames';
   static const String kAsyncCausalStackFrames = 'asyncCausalFrames';
@@ -1422,50 +1417,50 @@ class ObservatoryDebugger extends Debugger {
 
   void upFrame(int count) {
     if (_upIsDown) {
-      currentFrame = currentFrame! + count;
+      currentFrame += count;
     } else {
-      currentFrame = currentFrame! - count;
+      currentFrame -= count;
     }
   }
 
   void downFrame(int count) {
     if (_upIsDown) {
-      currentFrame = currentFrame! - count;
+      currentFrame -= count;
     } else {
-      currentFrame = currentFrame! + count;
+      currentFrame += count;
     }
   }
 
   int get stackDepth {
     if (awaiterStacks) {
-      var awaiterStackFrames = stack![kAwaiterStackFrames];
+      var awaiterStackFrames = stack[kAwaiterStackFrames];
       if (awaiterStackFrames != null) {
         return awaiterStackFrames.length;
       }
     }
     if (causalAsyncStacks) {
-      var asyncCausalStackFrames = stack![kAsyncCausalStackFrames];
+      var asyncCausalStackFrames = stack[kAsyncCausalStackFrames];
       if (asyncCausalStackFrames != null) {
         return asyncCausalStackFrames.length;
       }
     }
-    return stack![kStackFrames].length;
+    return stack[kStackFrames].length;
   }
 
   List get stackFrames {
     if (awaiterStacks) {
-      var awaiterStackFrames = stack![kAwaiterStackFrames];
+      var awaiterStackFrames = stack[kAwaiterStackFrames];
       if (awaiterStackFrames != null) {
         return awaiterStackFrames;
       }
     }
     if (causalAsyncStacks) {
-      var asyncCausalStackFrames = stack![kAsyncCausalStackFrames];
+      var asyncCausalStackFrames = stack[kAsyncCausalStackFrames];
       if (asyncCausalStackFrames != null) {
         return asyncCausalStackFrames;
       }
     }
-    return stack![kStackFrames] ?? [];
+    return stack[kStackFrames] ?? [];
   }
 
   static final _history = [''];
@@ -1502,7 +1497,7 @@ class ObservatoryDebugger extends Debugger {
   }
 
   void _loadSettings() {
-    _upIsDown = settings.get('up-is-down') ?? false;
+    _upIsDown = settings.get('up-is-down');
     _causalAsyncStacks = settings.get('causal-async-stacks') ?? true;
     _awaiterStacks = settings.get('awaiter-stacks') ?? true;
   }
@@ -1523,11 +1518,11 @@ class ObservatoryDebugger extends Debugger {
 
     if ((breakOnException != isolate.exceptionsPauseInfo) &&
         (isolate.exceptionsPauseInfo != null)) {
-      breakOnException = isolate.exceptionsPauseInfo!;
+      breakOnException = isolate.exceptionsPauseInfo;
     }
 
     isolate.reload().then((serviceObject) {
-      S.Isolate response = serviceObject as S.Isolate;
+      S.Isolate response = serviceObject;
       if (response.isSentinel) {
         // The isolate has gone away.  The IsolateExit event will
         // clear the isolate for the debugger page.
@@ -1578,7 +1573,7 @@ class ObservatoryDebugger extends Debugger {
     });
   }
 
-  Future<S.ServiceMap?> _refreshStack(M.DebugEvent? pauseEvent) {
+  Future<S.ServiceMap> _refreshStack(M.DebugEvent pauseEvent) {
     return isolate.getStack().then((result) {
       if (result.isSentinel) {
         // The isolate has gone away.  The IsolateExit event will
@@ -1586,8 +1581,8 @@ class ObservatoryDebugger extends Debugger {
         return;
       }
       stack = result;
-      stackElement.updateStack(stack!, pauseEvent);
-      if (stack!['frames'].length > 0) {
+      stackElement.updateStack(stack, pauseEvent);
+      if (stack['frames'].length > 0) {
         currentFrame = 0;
       } else {
         currentFrame = null;
@@ -1605,18 +1600,18 @@ class ObservatoryDebugger extends Debugger {
     } else if (isolate.running) {
       console.print("Isolate is running (type 'pause' to interrupt)");
     } else if (isolate.pauseEvent != null) {
-      _reportPause(isolate.pauseEvent!);
+      _reportPause(isolate.pauseEvent);
     } else {
       console.print('Isolate is in unknown state');
     }
     warnOutOfDate();
   }
 
-  void _reportIsolateError(S.Isolate? isolate, M.DebugEvent event) {
+  void _reportIsolateError(S.Isolate isolate, M.DebugEvent event) {
     if (isolate == null) {
       return;
     }
-    S.DartError? error = isolate.error;
+    S.DartError error = isolate.error;
     if (error == null) {
       return;
     }
@@ -1626,11 +1621,11 @@ class ObservatoryDebugger extends Debugger {
     } else {
       console.printBold('Isolate has exited due to an unhandled exception:');
     }
-    console.print(error.message!);
+    console.print(error.message);
     console.newline();
     if (event is M.PauseExceptionEvent &&
-        (error.exception!.isStackOverflowError ||
-            error.exception!.isOutOfMemoryError)) {
+        (error.exception.isStackOverflowError ||
+            error.exception.isOutOfMemoryError)) {
       console.printBold(
           'When an unhandled stack overflow or OOM exception occurs, the VM '
           'has run out of memory and cannot keep the stack alive while '
@@ -1657,14 +1652,14 @@ class ObservatoryDebugger extends Debugger {
       console.print("Paused at an unhandled exception "
           "(type 'continue' or [F7] to exit the isolate')");
       _reportIsolateError(isolate, event);
-    } else if (stack!['frames'].length > 0) {
-      S.Frame frame = stack!['frames'][0];
-      var script = frame.location!.script;
+    } else if (stack['frames'].length > 0) {
+      S.Frame frame = stack['frames'][0];
+      var script = frame.location.script;
       script.load().then((_) {
-        var line = script.tokenToLine(frame.location!.tokenPos);
-        var col = script.tokenToCol(frame.location!.tokenPos);
+        var line = script.tokenToLine(frame.location.tokenPos);
+        var col = script.tokenToCol(frame.location.tokenPos);
         if ((event is M.PauseBreakpointEvent) && (event.breakpoint != null)) {
-          var bpId = event.breakpoint!.number;
+          var bpId = event.breakpoint.number;
           console.print('Paused at breakpoint ${bpId} at '
               '${script.name}:${line}:${col}');
         } else if ((event is M.PauseExceptionEvent) &&
@@ -1674,7 +1669,7 @@ class ObservatoryDebugger extends Debugger {
           // This seems to be missing if we are paused-at-exception after
           // paused-at-isolate-exit. Maybe we shutdown part of the debugger too
           // soon?
-          console.printRef(isolate, event.exception as S.Instance, objects!);
+          console.printRef(isolate, event.exception, objects);
         } else {
           console.print('Paused at ${script.name}:${line}:${col}');
         }
@@ -1701,12 +1696,12 @@ class ObservatoryDebugger extends Debugger {
       default:
         break;
     }
-    var script = bpt!.location!.script;
+    var script = bpt.location.script;
     await script.load();
 
     var bpId = bpt.number;
-    var locString = await bpt.location!.toUserString();
-    if (bpt.resolved!) {
+    var locString = await bpt.location.toUserString();
+    if (bpt.resolved) {
       console.print('Breakpoint ${bpId} ${verb} at ${locString}');
     } else {
       console.print('Future breakpoint ${bpId} ${verb} at ${locString}');
@@ -1716,20 +1711,20 @@ class ObservatoryDebugger extends Debugger {
   void onEvent(S.ServiceEvent event) {
     switch (event.kind) {
       case S.ServiceEvent.kVMUpdate:
-        S.VM vm = event.owner as S.VM;
+        S.VM vm = event.owner;
         console.print("VM ${vm.displayName} renamed to '${vm.name}'");
         break;
 
       case S.ServiceEvent.kIsolateStart:
         {
-          S.Isolate iso = event.owner as S.Isolate;
+          S.Isolate iso = event.owner;
           console.print("Isolate ${iso.number} '${iso.name}' has been created");
         }
         break;
 
       case S.ServiceEvent.kIsolateExit:
         {
-          S.Isolate iso = event.owner as S.Isolate;
+          S.Isolate iso = event.owner;
           if (iso == isolate) {
             console.print("The current isolate ${iso.number} '${iso.name}' "
                 "has exited");
@@ -1748,13 +1743,13 @@ class ObservatoryDebugger extends Debugger {
 
       case S.ServiceEvent.kDebuggerSettingsUpdate:
         if (breakOnException != event.exceptions) {
-          breakOnException = event.exceptions!;
+          breakOnException = event.exceptions;
           console.print("Now pausing for exceptions: $breakOnException");
         }
         break;
 
       case S.ServiceEvent.kIsolateUpdate:
-        S.Isolate iso = event.owner as S.Isolate;
+        S.Isolate iso = event.owner;
         console.print("Isolate ${iso.number} renamed to '${iso.name}'");
         break;
 
@@ -1773,7 +1768,7 @@ class ObservatoryDebugger extends Debugger {
       case S.ServiceEvent.kPauseInterrupted:
       case S.ServiceEvent.kPauseException:
         if (event.owner == isolate) {
-          var e = createEventFromServiceEvent(event) as M.DebugEvent;
+          var e = createEventFromServiceEvent(event);
           _refreshStack(e).then((_) async {
             flushStdio();
             if (isolate != null) {
@@ -1807,7 +1802,7 @@ class ObservatoryDebugger extends Debugger {
         break;
 
       case S.ServiceEvent.kLogging:
-        _consolePrinter.onEvent(event.logRecord!['level'].name, event);
+        _consolePrinter.onEvent(event.logRecord['level'].name, event);
         break;
 
       default:
@@ -1816,7 +1811,7 @@ class ObservatoryDebugger extends Debugger {
     }
   }
 
-  late _ConsoleStreamPrinter _consolePrinter;
+  _ConsoleStreamPrinter _consolePrinter;
 
   void flushStdio() {
     _consolePrinter.flush();
@@ -1870,11 +1865,11 @@ class ObservatoryDebugger extends Debugger {
   }
 
   // TODO(turnidge): Implement real command line history.
-  String? lastCommand;
+  String lastCommand;
 
   Future run(String command) {
     if (command == '' && lastCommand != null) {
-      command = lastCommand!;
+      command = lastCommand;
     }
     console.printBold('\$ $command');
     return cmd.runCommand(command).then((_) {
@@ -1926,7 +1921,7 @@ class ObservatoryDebugger extends Debugger {
     var script = loc.script;
     var line = loc.line;
     if (script != null && line != null) {
-      var bpts = script.getLine(line)!.breakpoints;
+      var bpts = script.getLine(line).breakpoints;
       if (bpts == null || bpts.isEmpty) {
         // Set a new breakpoint.
         // TODO(turnidge): Set this breakpoint at current column.
@@ -1945,8 +1940,7 @@ class ObservatoryDebugger extends Debugger {
 
   Future smartNext() async {
     if (isolatePaused()) {
-      M.AsyncSuspensionEvent event =
-          isolate.pauseEvent as M.AsyncSuspensionEvent;
+      M.AsyncSuspensionEvent event = isolate.pauseEvent;
       if (event.atAsyncSuspension) {
         return asyncNext();
       } else {
@@ -1959,8 +1953,7 @@ class ObservatoryDebugger extends Debugger {
 
   Future asyncNext() async {
     if (isolatePaused()) {
-      M.AsyncSuspensionEvent event =
-          isolate.pauseEvent as M.AsyncSuspensionEvent;
+      M.AsyncSuspensionEvent event = isolate.pauseEvent;
       if (!event.atAsyncSuspension) {
         console.print("No async continuation at this location");
       } else {
@@ -2028,11 +2021,11 @@ class DebuggerPageElement extends CustomElement implements Renderable {
     NavNotifyElement.tag,
   ]);
 
-  late S.Isolate _isolate;
-  late ObservatoryDebugger _debugger;
-  late M.ObjectRepository _objects;
-  late M.ScriptRepository _scripts;
-  late M.EventRepository _events;
+  S.Isolate _isolate;
+  ObservatoryDebugger _debugger;
+  M.ObjectRepository _objects;
+  M.ScriptRepository _scripts;
+  M.EventRepository _events;
 
   factory DebuggerPageElement(S.Isolate isolate, M.ObjectRepository objects,
       M.ScriptRepository scripts, M.EventRepository events) {
@@ -2054,16 +2047,16 @@ class DebuggerPageElement extends CustomElement implements Renderable {
 
   DebuggerPageElement.created() : super.created(tag);
 
-  Future<StreamSubscription>? _vmSubscriptionFuture;
-  Future<StreamSubscription>? _isolateSubscriptionFuture;
-  Future<StreamSubscription>? _debugSubscriptionFuture;
-  Future<StreamSubscription>? _stdoutSubscriptionFuture;
-  Future<StreamSubscription>? _stderrSubscriptionFuture;
-  Future<StreamSubscription>? _logSubscriptionFuture;
+  Future<StreamSubscription> _vmSubscriptionFuture;
+  Future<StreamSubscription> _isolateSubscriptionFuture;
+  Future<StreamSubscription> _debugSubscriptionFuture;
+  Future<StreamSubscription> _stdoutSubscriptionFuture;
+  Future<StreamSubscription> _stderrSubscriptionFuture;
+  Future<StreamSubscription> _logSubscriptionFuture;
 
   ObservatoryApplication get app => ObservatoryApplication.app;
 
-  Timer? _timer;
+  Timer _timer;
 
   static final consoleElement = new DebuggerConsoleElement();
 
@@ -2126,7 +2119,7 @@ class DebuggerPageElement extends CustomElement implements Renderable {
         app.vm.listenEventStream(S.VM.kStdoutStream, _debugger.onStdout);
     if (_stdoutSubscriptionFuture != null) {
       // TODO(turnidge): How do we want to handle this in general?
-      _stdoutSubscriptionFuture!.catchError((e, st) {
+      _stdoutSubscriptionFuture.catchError((e, st) {
         Logger.root.info('Failed to subscribe to stdout: $e\n$st\n');
         _stdoutSubscriptionFuture = null;
       });
@@ -2135,7 +2128,7 @@ class DebuggerPageElement extends CustomElement implements Renderable {
         app.vm.listenEventStream(S.VM.kStderrStream, _debugger.onStderr);
     if (_stderrSubscriptionFuture != null) {
       // TODO(turnidge): How do we want to handle this in general?
-      _stderrSubscriptionFuture!.catchError((e, st) {
+      _stderrSubscriptionFuture.catchError((e, st) {
         Logger.root.info('Failed to subscribe to stderr: $e\n$st\n');
         _stderrSubscriptionFuture = null;
       });
@@ -2165,19 +2158,19 @@ class DebuggerPageElement extends CustomElement implements Renderable {
 
   @override
   void detached() {
-    _timer!.cancel();
+    _timer.cancel();
     children = const [];
-    S.cancelFutureSubscription(_vmSubscriptionFuture!);
+    S.cancelFutureSubscription(_vmSubscriptionFuture);
     _vmSubscriptionFuture = null;
-    S.cancelFutureSubscription(_isolateSubscriptionFuture!);
+    S.cancelFutureSubscription(_isolateSubscriptionFuture);
     _isolateSubscriptionFuture = null;
-    S.cancelFutureSubscription(_debugSubscriptionFuture!);
+    S.cancelFutureSubscription(_debugSubscriptionFuture);
     _debugSubscriptionFuture = null;
-    S.cancelFutureSubscription(_stdoutSubscriptionFuture!);
+    S.cancelFutureSubscription(_stdoutSubscriptionFuture);
     _stdoutSubscriptionFuture = null;
-    S.cancelFutureSubscription(_stderrSubscriptionFuture!);
+    S.cancelFutureSubscription(_stderrSubscriptionFuture);
     _stderrSubscriptionFuture = null;
-    S.cancelFutureSubscription(_logSubscriptionFuture!);
+    S.cancelFutureSubscription(_logSubscriptionFuture);
     _logSubscriptionFuture = null;
     super.detached();
   }
@@ -2186,12 +2179,12 @@ class DebuggerPageElement extends CustomElement implements Renderable {
 class DebuggerStackElement extends CustomElement implements Renderable {
   static const tag = const Tag<DebuggerStackElement>('debugger-stack');
 
-  late S.Isolate _isolate;
-  late M.ObjectRepository _objects;
-  late M.ScriptRepository _scripts;
-  late M.EventRepository _events;
-  late Element _scroller;
-  late DivElement _isSampled;
+  S.Isolate _isolate;
+  M.ObjectRepository _objects;
+  M.ScriptRepository _scripts;
+  M.EventRepository _events;
+  Element _scroller;
+  DivElement _isSampled;
   bool get isSampled => !_isSampled.classes.contains('hidden');
   set isSampled(bool value) {
     if (value != isSampled) {
@@ -2199,7 +2192,7 @@ class DebuggerStackElement extends CustomElement implements Renderable {
     }
   }
 
-  late DivElement _hasStack;
+  DivElement _hasStack;
   bool get hasStack => _hasStack.classes.contains('hidden');
   set hasStack(bool value) {
     if (value != hasStack) {
@@ -2207,7 +2200,7 @@ class DebuggerStackElement extends CustomElement implements Renderable {
     }
   }
 
-  late DivElement _hasMessages;
+  DivElement _hasMessages;
   bool get hasMessages => _hasMessages.classes.contains('hidden');
   set hasMessages(bool value) {
     if (value != hasMessages) {
@@ -2215,10 +2208,10 @@ class DebuggerStackElement extends CustomElement implements Renderable {
     }
   }
 
-  UListElement? _frameList;
-  UListElement? _messageList;
-  int? currentFrame;
-  late ObservatoryDebugger _debugger;
+  UListElement _frameList;
+  UListElement _messageList;
+  int currentFrame;
+  ObservatoryDebugger _debugger;
 
   factory DebuggerStackElement(
       S.Isolate isolate,
@@ -2326,7 +2319,7 @@ class DebuggerStackElement extends CustomElement implements Renderable {
   ObservatoryApplication get app => ObservatoryApplication.app;
 
   void updateStackFrames(S.ServiceMap newStack) {
-    List frameElements = _frameList!.children;
+    List frameElements = _frameList.children;
     List newFrames;
     if (_debugger.awaiterStacks &&
         (newStack[ObservatoryDebugger.kAwaiterStackFrames] != null)) {
@@ -2344,8 +2337,7 @@ class DebuggerStackElement extends CustomElement implements Renderable {
     int newPos = newFrames.length - 1;
     while (oldPos >= 0 && newPos >= 0) {
       DebuggerFrameElement dbgFrameElement =
-          CustomElement.reverse(frameElements[oldPos].children[0])
-              as DebuggerFrameElement;
+          CustomElement.reverse(frameElements[oldPos].children[0]);
       if (!dbgFrameElement.matchFrame(newFrames[newPos])) {
         // The rest of the frame elements no longer match.  Remove them.
         for (int i = 0; i <= oldPos; i++) {
@@ -2381,8 +2373,7 @@ class DebuggerStackElement extends CustomElement implements Renderable {
     if (frameElements.isNotEmpty) {
       for (int i = newCount; i < frameElements.length; i++) {
         DebuggerFrameElement dbgFrameElement =
-            CustomElement.reverse(frameElements[i].children[0])
-                as DebuggerFrameElement;
+            CustomElement.reverse(frameElements[i].children[0]);
         dbgFrameElement.updateFrame(newFrames[i]);
       }
     }
@@ -2391,7 +2382,7 @@ class DebuggerStackElement extends CustomElement implements Renderable {
   }
 
   void updateStackMessages(S.ServiceMap newStack) {
-    List messageElements = _messageList!.children;
+    List messageElements = _messageList.children;
     List newMessages = newStack['messages'];
 
     // Remove any extra message elements.
@@ -2416,8 +2407,7 @@ class DebuggerStackElement extends CustomElement implements Renderable {
       // Update old messages.
       for (int i = 0; i < newStartingIndex; i++) {
         DebuggerMessageElement e =
-            CustomElement.reverse(messageElements[i].children[0])
-                as DebuggerMessageElement;
+            CustomElement.reverse(messageElements[i].children[0]);
         e.updateMessage(newMessages[i]);
       }
     }
@@ -2425,19 +2415,18 @@ class DebuggerStackElement extends CustomElement implements Renderable {
     hasMessages = messageElements.isNotEmpty;
   }
 
-  void updateStack(S.ServiceMap newStack, M.DebugEvent? pauseEvent) {
+  void updateStack(S.ServiceMap newStack, M.DebugEvent pauseEvent) {
     updateStackFrames(newStack);
     updateStackMessages(newStack);
     isSampled = pauseEvent == null;
   }
 
-  void setCurrentFrame(int? value) {
+  void setCurrentFrame(int value) {
     currentFrame = value;
-    List frameElements = _frameList!.children;
+    List frameElements = _frameList.children;
     for (var frameElement in frameElements) {
       DebuggerFrameElement dbgFrameElement =
-          CustomElement.reverse(frameElement.children[0])
-              as DebuggerFrameElement;
+          CustomElement.reverse(frameElement.children[0]);
       if (dbgFrameElement.frame.index == currentFrame) {
         dbgFrameElement.setCurrent(true);
       } else {
@@ -2452,18 +2441,18 @@ class DebuggerStackElement extends CustomElement implements Renderable {
 class DebuggerFrameElement extends CustomElement implements Renderable {
   static const tag = const Tag<DebuggerFrameElement>('debugger-frame');
 
-  late RenderingScheduler<DebuggerFrameElement> _r;
+  RenderingScheduler<DebuggerFrameElement> _r;
 
   Stream<RenderedEvent<DebuggerFrameElement>> get onRendered => _r.onRendered;
 
-  late Element _scroller;
-  late DivElement _varsDiv;
-  late M.Isolate _isolate;
-  late S.Frame _frame;
+  Element _scroller;
+  DivElement _varsDiv;
+  M.Isolate _isolate;
+  S.Frame _frame;
   S.Frame get frame => _frame;
-  late M.ObjectRepository _objects;
-  late M.ScriptRepository _scripts;
-  late M.EventRepository _events;
+  M.ObjectRepository _objects;
+  M.ScriptRepository _scripts;
+  M.EventRepository _events;
 
   // Is this the current frame?
   bool _current = false;
@@ -2475,7 +2464,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
 
   void setCurrent(bool value) {
     Future load = (_frame.function != null)
-        ? _frame.function!.load()
+        ? _frame.function.load()
         : new Future.value(null);
     load.then((func) {
       _current = value;
@@ -2499,7 +2488,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
       M.ObjectRepository objects,
       M.ScriptRepository scripts,
       M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(frame != null);
     assert(scroller != null);
@@ -2536,12 +2525,12 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
     }
     if (_frame.kind == M.FrameKind.asyncSuspensionMarker) {
       final content = <Element>[
-        new SpanElement()..children = _createMarkerHeader(_frame.marker!)
+        new SpanElement()..children = _createMarkerHeader(_frame.marker)
       ];
       children = content;
       return;
     }
-    late ButtonElement expandButton;
+    ButtonElement expandButton;
     final content = <Element>[
       expandButton = new ButtonElement()
         ..children = _createHeader()
@@ -2555,14 +2544,14 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
         })
     ];
     if (_expanded) {
-      final homeMethod = _frame.function!.homeMethod;
-      String? homeMethodName;
-      if ((homeMethod.dartOwner is S.Class) && homeMethod.isStatic!) {
+      final homeMethod = _frame.function.homeMethod;
+      String homeMethodName;
+      if ((homeMethod.dartOwner is S.Class) && homeMethod.isStatic) {
         homeMethodName = '<class>';
       } else if (homeMethod.dartOwner is S.Library) {
         homeMethodName = '<library>';
       }
-      late ButtonElement collapseButton;
+      ButtonElement collapseButton;
       content.addAll([
         new DivElement()
           ..classes = ['frameDetails']
@@ -2577,11 +2566,11 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
                       : [
                           (new SourceInsetElement(
                                   _isolate,
-                                  _frame.function!.location!,
+                                  _frame.function.location,
                                   _scripts,
                                   _objects,
                                   _events,
-                                  currentPos: _frame.location!.tokenPos,
+                                  currentPos: _frame.location.tokenPos,
                                   variables: _frame.variables,
                                   inDebuggerContext: true,
                                   queue: _r.queue))
@@ -2635,7 +2624,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
                     await _toggleExpand();
                     collapseButton.disabled = false;
                   })
-                  ..children = <Element>[iconExpandLess.clone(true) as Element]
+                  ..children = <Element>[iconExpandLess.clone(true)]
               ]
           ]
       ]);
@@ -2673,7 +2662,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
             ..children = _frame.function == null
                 ? const []
                 : [
-                    new FunctionRefElement(_isolate, _frame.function!,
+                    new FunctionRefElement(_isolate, _frame.function,
                             queue: _r.queue)
                         .element
                   ],
@@ -2683,7 +2672,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
                 ? const []
                 : [
                     new SourceLinkElement(
-                            _isolate, _frame.function!.location!, _scripts,
+                            _isolate, _frame.function.location, _scripts,
                             queue: _r.queue)
                         .element
                   ],
@@ -2693,7 +2682,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
     if (!_expanded) {
       content.add(new DivElement()
         ..classes = ['frameExpander']
-        ..children = <Element>[iconExpandMore.clone(true) as Element]);
+        ..children = <Element>[iconExpandMore.clone(true)]);
     }
     return [
       new DivElement()
@@ -2703,7 +2692,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
   }
 
   String makeExpandKey(String key) {
-    return '${_frame.function!.qualifiedName}/${key}';
+    return '${_frame.function.qualifiedName}/${key}';
   }
 
   bool matchFrame(S.Frame newFrame) {
@@ -2713,8 +2702,8 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
     if (newFrame.function == null) {
       return frame.function == null;
     }
-    return (newFrame.function!.id == _frame.function!.id &&
-        newFrame.location!.script.id == frame.location!.script.id);
+    return (newFrame.function.id == _frame.function.id &&
+        newFrame.location.script.id == frame.location.script.id);
   }
 
   void updateFrame(S.Frame newFrame) {
@@ -2722,15 +2711,15 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
     _frame = newFrame;
   }
 
-  S.Script get script => _frame.location!.script;
+  S.Script get script => _frame.location.script;
 
   int _varsTop(DivElement varsDiv) {
     const minTop = 0;
     if (varsDiv == null) {
       return minTop;
     }
-    final num paddingTop = document.body!.contentEdge.top;
-    final Rectangle parent = varsDiv.parent!.getBoundingClientRect();
+    final num paddingTop = document.body.contentEdge.top;
+    final Rectangle parent = varsDiv.parent.getBoundingClientRect();
     final int varsHeight = varsDiv.clientHeight;
     final int maxTop = (parent.height - varsHeight).toInt();
     final int adjustedTop = (paddingTop - parent.top).toInt();
@@ -2760,8 +2749,8 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
     _r.dirty();
   }
 
-  StreamSubscription? _scrollSubscription;
-  StreamSubscription? _resizeSubscription;
+  StreamSubscription _scrollSubscription;
+  StreamSubscription _resizeSubscription;
 
   void _subscribeToScroll() {
     if (_scroller != null) {
@@ -2776,11 +2765,11 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
 
   void _unsubscribeToScroll() {
     if (_scrollSubscription != null) {
-      _scrollSubscription!.cancel();
+      _scrollSubscription.cancel();
       _scrollSubscription = null;
     }
     if (_resizeSubscription != null) {
-      _resizeSubscription!.cancel();
+      _resizeSubscription.cancel();
       _resizeSubscription = null;
     }
   }
@@ -2801,7 +2790,7 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
   }
 
   Future _toggleExpand() async {
-    await _frame.function!.load();
+    await _frame.function.load();
     _pinned = !_pinned;
     if (_pinned) {
       _expand();
@@ -2814,16 +2803,16 @@ class DebuggerFrameElement extends CustomElement implements Renderable {
 class DebuggerMessageElement extends CustomElement implements Renderable {
   static const tag = const Tag<DebuggerMessageElement>('debugger-message');
 
-  late RenderingScheduler<DebuggerMessageElement> _r;
+  RenderingScheduler<DebuggerMessageElement> _r;
 
   Stream<RenderedEvent<DebuggerMessageElement>> get onRendered => _r.onRendered;
 
-  late S.Isolate _isolate;
-  late S.ServiceMessage _message;
-  late S.ServiceObject _preview;
-  late M.ObjectRepository _objects;
-  late M.ScriptRepository _scripts;
-  late M.EventRepository _events;
+  S.Isolate _isolate;
+  S.ServiceMessage _message;
+  S.ServiceObject _preview;
+  M.ObjectRepository _objects;
+  M.ScriptRepository _scripts;
+  M.EventRepository _events;
 
   // Is this the current message?
   bool _current = false;
@@ -2839,7 +2828,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
       M.ObjectRepository objects,
       M.ScriptRepository scripts,
       M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(message != null);
     assert(objects != null);
@@ -2867,7 +2856,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
     } else {
       classes.remove('current');
     }
-    late ButtonElement expandButton;
+    ButtonElement expandButton;
     final content = <Element>[
       expandButton = new ButtonElement()
         ..children = _createHeader()
@@ -2881,8 +2870,8 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
         })
     ];
     if (_expanded) {
-      late ButtonElement collapseButton;
-      late ButtonElement previewButton;
+      ButtonElement collapseButton;
+      ButtonElement previewButton;
       content.addAll([
         new DivElement()
           ..classes = ['messageDetails']
@@ -2897,7 +2886,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
                       : [
                           new SourceInsetElement(
                                   _isolate,
-                                  _message.handler!.location!,
+                                  _message.handler.location,
                                   _scripts,
                                   _objects,
                                   _events,
@@ -2938,7 +2927,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
                     await _toggleExpand();
                     collapseButton.disabled = false;
                   })
-                  ..children = <Element>[iconExpandLess.clone(true) as Element]
+                  ..children = <Element>[iconExpandLess.clone(true)]
               ]
           ]
       ]);
@@ -2964,7 +2953,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
             ..children = _message.handler == null
                 ? const []
                 : [
-                    new FunctionRefElement(_isolate, _message.handler!,
+                    new FunctionRefElement(_isolate, _message.handler,
                             queue: _r.queue)
                         .element
                   ],
@@ -2973,8 +2962,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
             ..children = _message.location == null
                 ? const []
                 : [
-                    new SourceLinkElement(
-                            _isolate, _message.location!, _scripts,
+                    new SourceLinkElement(_isolate, _message.location, _scripts,
                             queue: _r.queue)
                         .element
                   ],
@@ -2984,7 +2972,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
     if (!_expanded) {
       content.add(new DivElement()
         ..classes = ['messageExpander']
-        ..children = <Element>[iconExpandMore.clone(true) as Element]);
+        ..children = <Element>[iconExpandMore.clone(true)]);
     }
     return [
       new DivElement()
@@ -3028,9 +3016,7 @@ class DebuggerMessageElement extends CustomElement implements Renderable {
   }
 
   Future<S.ServiceObject> previewMessage(_) {
-    return _message.isolate!
-        .getObject(_message.messageObjectId!)
-        .then((result) {
+    return _message.isolate.getObject(_message.messageObjectId).then((result) {
       _preview = result;
       return result;
     });
@@ -3049,7 +3035,7 @@ class DebuggerConsoleElement extends CustomElement implements Renderable {
   DebuggerConsoleElement.created() : super.created(tag);
 
   /// Is [container] scrolled to the within [threshold] pixels of the bottom?
-  static bool _isScrolledToBottom(DivElement? container, [int threshold = 2]) {
+  static bool _isScrolledToBottom(DivElement container, [int threshold = 2]) {
     if (container == null) {
       return false;
     }
@@ -3063,7 +3049,7 @@ class DebuggerConsoleElement extends CustomElement implements Renderable {
   }
 
   /// Scroll [container] so the bottom content is visible.
-  static _scrollToBottom(DivElement? container) {
+  static _scrollToBottom(DivElement container) {
     if (container == null) {
       return;
     }
@@ -3072,10 +3058,10 @@ class DebuggerConsoleElement extends CustomElement implements Renderable {
   }
 
   void _append(HtmlElement span) {
-    bool autoScroll = _isScrolledToBottom(parent as DivElement?);
+    bool autoScroll = _isScrolledToBottom(parent);
     children.add(span);
     if (autoScroll) {
-      _scrollToBottom(parent as DivElement?);
+      _scrollToBottom(parent);
     }
   }
 
@@ -3110,7 +3096,7 @@ class DebuggerConsoleElement extends CustomElement implements Renderable {
   }
 
   void printStdio(List<String> lines) {
-    bool autoScroll = _isScrolledToBottom(parent as DivElement?);
+    bool autoScroll = _isScrolledToBottom(parent);
     for (var line in lines) {
       var span = new SpanElement();
       span.classes.add('green');
@@ -3119,7 +3105,7 @@ class DebuggerConsoleElement extends CustomElement implements Renderable {
       children.add(span);
     }
     if (autoScroll) {
-      _scrollToBottom(parent as DivElement?);
+      _scrollToBottom(parent);
     }
   }
 
@@ -3150,15 +3136,15 @@ class DebuggerConsoleElement extends CustomElement implements Renderable {
 class DebuggerInputElement extends CustomElement implements Renderable {
   static const tag = const Tag<DebuggerInputElement>('debugger-input');
 
-  late S.Isolate _isolate;
-  late ObservatoryDebugger _debugger;
+  S.Isolate _isolate;
+  ObservatoryDebugger _debugger;
   bool _busy = false;
   final _modalPromptDiv = new DivElement()..classes = ['modalPrompt', 'hidden'];
   final _textBox = new TextInputElement()
     ..classes = ['textBox']
     ..autofocus = true;
-  String? get modalPrompt => _modalPromptDiv.text;
-  set modalPrompt(String? value) {
+  String get modalPrompt => _modalPromptDiv.text;
+  set modalPrompt(String value) {
     if (_modalPromptDiv.text == '') {
       _modalPromptDiv.classes.remove('hidden');
     }
@@ -3204,7 +3190,7 @@ class DebuggerInputElement extends CustomElement implements Renderable {
     switch (e.keyCode) {
       case KeyCode.TAB:
         e.preventDefault();
-        int cursorPos = _textBox.selectionStart!;
+        int cursorPos = _textBox.selectionStart;
         _debugger.complete(text.substring(0, cursorPos)).then((completion) {
           text = completion + text.substring(cursorPos);
           // TODO(turnidge): Move the cursor to the end of the
diff --git a/runtime/observatory/lib/src/elements/error_ref.dart b/runtime/observatory/lib/src/elements/error_ref.dart
index bc49760a666..2e250968ff5 100644
--- a/runtime/observatory/lib/src/elements/error_ref.dart
+++ b/runtime/observatory/lib/src/elements/error_ref.dart
@@ -13,15 +13,15 @@ import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 class ErrorRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<ErrorRefElement>('error-ref');
 
-  late RenderingScheduler<ErrorRefElement> _r;
+  RenderingScheduler<ErrorRefElement> _r;
 
   Stream<RenderedEvent<ErrorRefElement>> get onRendered => _r.onRendered;
 
-  late ErrorRef _error;
+  ErrorRef _error;
 
   ErrorRef get error => _error;
 
-  factory ErrorRefElement(ErrorRef error, {RenderingQueue? queue}) {
+  factory ErrorRefElement(ErrorRef error, {RenderingQueue queue}) {
     assert(error != null);
     ErrorRefElement e = new ErrorRefElement.created();
     e._r = new RenderingScheduler<ErrorRefElement>(e, queue: queue);
diff --git a/runtime/observatory/lib/src/elements/error_view.dart b/runtime/observatory/lib/src/elements/error_view.dart
index ea4ad8ed782..71114f428c0 100644
--- a/runtime/observatory/lib/src/elements/error_view.dart
+++ b/runtime/observatory/lib/src/elements/error_view.dart
@@ -22,18 +22,18 @@ class ErrorViewElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<ErrorViewElement> _r;
+  RenderingScheduler<ErrorViewElement> _r;
 
   Stream<RenderedEvent<ErrorViewElement>> get onRendered => _r.onRendered;
 
-  late M.Error _error;
-  late M.NotificationRepository _notifications;
+  M.Error _error;
+  M.NotificationRepository _notifications;
 
   M.Error get error => _error;
 
   factory ErrorViewElement(
       M.NotificationRepository notifications, M.Error error,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(error != null);
     assert(notifications != null);
     ErrorViewElement e = new ErrorViewElement.created();
@@ -78,7 +78,7 @@ class ErrorViewElement extends CustomElement implements Renderable {
     ];
   }
 
-  static String _kindToString(M.ErrorKind? kind) {
+  static String _kindToString(M.ErrorKind kind) {
     switch (kind) {
       case M.ErrorKind.unhandledException:
         return 'Unhandled Exception';
diff --git a/runtime/observatory/lib/src/elements/eval_box.dart b/runtime/observatory/lib/src/elements/eval_box.dart
index 26d0ceb0a3b..db8ce82f6c4 100644
--- a/runtime/observatory/lib/src/elements/eval_box.dart
+++ b/runtime/observatory/lib/src/elements/eval_box.dart
@@ -16,18 +16,18 @@ class EvalBoxElement extends CustomElement implements Renderable {
   static const tag = const Tag<EvalBoxElement>('eval-box',
       dependencies: const [InstanceRefElement.tag]);
 
-  late RenderingScheduler<EvalBoxElement> _r;
+  RenderingScheduler<EvalBoxElement> _r;
 
   Stream<RenderedEvent<EvalBoxElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ObjectRef _context;
-  late M.ObjectRepository _objects;
-  late M.EvalRepository _eval;
+  M.IsolateRef _isolate;
+  M.ObjectRef _context;
+  M.ObjectRepository _objects;
+  M.EvalRepository _eval;
   final _results = <_ExpressionDescription>[];
-  String? _expression = '';
-  late bool _multiline;
-  late Iterable<String> _quickExpressions;
+  String _expression = '';
+  bool _multiline;
+  Iterable<String> _quickExpressions;
 
   M.IsolateRef get isolate => _isolate;
   M.ObjectRef get context => _context;
@@ -36,7 +36,7 @@ class EvalBoxElement extends CustomElement implements Renderable {
       M.ObjectRepository objects, M.EvalRepository eval,
       {bool multiline: false,
       Iterable<String> quickExpressions: const [],
-      RenderingQueue? queue}) {
+      RenderingQueue queue}) {
     assert(isolate != null);
     assert(context != null);
     assert(objects != null);
@@ -139,7 +139,7 @@ class EvalBoxElement extends CustomElement implements Renderable {
     var area = new TextAreaElement()
       ..classes = ['textbox']
       ..placeholder = 'evaluate an expression'
-      ..value = _expression!
+      ..value = _expression
       ..onKeyUp.where((e) => e.key == '\n').listen((e) {
         e.preventDefault();
         _run();
@@ -155,7 +155,7 @@ class EvalBoxElement extends CustomElement implements Renderable {
     var textbox = new TextInputElement()
       ..classes = ['textbox']
       ..placeholder = 'evaluate an expression'
-      ..value = _expression!
+      ..value = _expression
       ..onKeyUp.where((e) => e.key == '\n').listen((e) {
         e.preventDefault();
         _run();
@@ -187,8 +187,8 @@ class EvalBoxElement extends CustomElement implements Renderable {
   }
 
   Future _run() async {
+    if (_expression == null || _expression.isEmpty) return;
     final expression = _expression;
-    if (expression == null || expression!.isEmpty) return;
     _expression = null;
     final result = new _ExpressionDescription.pending(expression);
     _results.add(result);
@@ -202,7 +202,7 @@ class EvalBoxElement extends CustomElement implements Renderable {
 
 class _ExpressionDescription {
   final String expression;
-  final M.ObjectRef? value;
+  final M.ObjectRef value;
   bool get isPending => value == null;
 
   _ExpressionDescription(this.expression, this.value);
diff --git a/runtime/observatory/lib/src/elements/field_ref.dart b/runtime/observatory/lib/src/elements/field_ref.dart
index 9962ee2cd5b..8074d44bd54 100644
--- a/runtime/observatory/lib/src/elements/field_ref.dart
+++ b/runtime/observatory/lib/src/elements/field_ref.dart
@@ -14,21 +14,21 @@ class FieldRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<FieldRefElement>('field-ref',
       dependencies: const [InstanceRefElement.tag]);
 
-  late RenderingScheduler<FieldRefElement> _r;
+  RenderingScheduler<FieldRefElement> _r;
 
   Stream<RenderedEvent<FieldRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.FieldRef _field;
-  late M.ObjectRepository _objects;
-  late bool _expandable;
+  M.IsolateRef _isolate;
+  M.FieldRef _field;
+  M.ObjectRepository _objects;
+  bool _expandable;
 
   M.IsolateRef get isolate => _isolate;
   M.FieldRef get field => _field;
 
   factory FieldRefElement(
       M.IsolateRef isolate, M.FieldRef field, M.ObjectRepository objects,
-      {RenderingQueue? queue, bool expandable: true}) {
+      {RenderingQueue queue, bool expandable: true}) {
     assert(isolate != null);
     assert(field != null);
     assert(objects != null);
@@ -58,21 +58,21 @@ class FieldRefElement extends CustomElement implements Renderable {
 
   void render() {
     var header = '';
-    if (_field.isStatic!) {
+    if (_field.isStatic) {
       if (_field.dartOwner is M.ClassRef) {
         header += 'static ';
       } else {
         header += 'top-level ';
       }
     }
-    if (_field.isFinal!) {
+    if (_field.isFinal) {
       header += 'final ';
-    } else if (_field.isConst!) {
+    } else if (_field.isConst) {
       header += 'const ';
-    } else if (_field.declaredType!.name == 'dynamic') {
+    } else if (_field.declaredType.name == 'dynamic') {
       header += 'var ';
     }
-    if (_field.declaredType!.name == 'dynamic') {
+    if (_field.declaredType.name == 'dynamic') {
       children = <Element>[
         new SpanElement()..text = header,
         new AnchorElement(href: Uris.inspect(_isolate, object: _field))
@@ -81,7 +81,7 @@ class FieldRefElement extends CustomElement implements Renderable {
     } else {
       children = <Element>[
         new SpanElement()..text = header,
-        new InstanceRefElement(_isolate, _field.declaredType!, _objects,
+        new InstanceRefElement(_isolate, _field.declaredType, _objects,
                 queue: _r.queue, expandable: _expandable)
             .element,
         new SpanElement()..text = ' ',
diff --git a/runtime/observatory/lib/src/elements/field_view.dart b/runtime/observatory/lib/src/elements/field_view.dart
index 50d53e0a39a..30b5c7c85eb 100644
--- a/runtime/observatory/lib/src/elements/field_view.dart
+++ b/runtime/observatory/lib/src/elements/field_view.dart
@@ -44,24 +44,24 @@ class FieldViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<FieldViewElement> _r;
+  RenderingScheduler<FieldViewElement> _r;
 
   Stream<RenderedEvent<FieldViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Field _field;
-  M.LibraryRef? _library;
-  late M.FieldRepository _fields;
-  late M.ClassRepository _classes;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ScriptRepository _scripts;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Field _field;
+  M.LibraryRef _library;
+  M.FieldRepository _fields;
+  M.ClassRepository _classes;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ScriptRepository _scripts;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -82,7 +82,7 @@ class FieldViewElement extends CustomElement implements Renderable {
       M.RetainingPathRepository retainingPaths,
       M.ScriptRepository scripts,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -112,7 +112,7 @@ class FieldViewElement extends CustomElement implements Renderable {
     e._scripts = scripts;
     e._objects = objects;
     if (field.dartOwner is M.LibraryRef) {
-      e._library = field.dartOwner as M.LibraryRef;
+      e._library = field.dartOwner;
     }
     return e;
   }
@@ -135,22 +135,22 @@ class FieldViewElement extends CustomElement implements Renderable {
 
   void render() {
     var header = '';
-    if (_field.isStatic!) {
+    if (_field.isStatic) {
       if (_field.dartOwner is M.ClassRef) {
         header += 'static ';
       } else {
         header += 'top-level ';
       }
     }
-    if (_field.isFinal!) {
+    if (_field.isFinal) {
       header += 'final ';
-    } else if (_field.isConst!) {
+    } else if (_field.isConst) {
       header += 'const ';
     }
-    if (_field.declaredType!.name == 'dynamic') {
+    if (_field.declaredType.name == 'dynamic') {
       header += 'var';
     } else {
-      header += _field.declaredType!.name!;
+      header += _field.declaredType.name;
     }
     children = <Element>[
       navBar(_createMenu()),
@@ -172,10 +172,10 @@ class FieldViewElement extends CustomElement implements Renderable {
             ..children = _field.location == null
                 ? const []
                 : [
-                    new ScriptInsetElement(_isolate, _field.location!.script,
+                    new ScriptInsetElement(_isolate, _field.location.script,
                             _scripts, _objects, _events,
-                            startPos: field.location!.tokenPos,
-                            endPos: field.location!.tokenPos,
+                            startPos: field.location.tokenPos,
+                            endPos: field.location.tokenPos,
                             queue: _r.queue)
                         .element
                   ],
@@ -191,15 +191,15 @@ class FieldViewElement extends CustomElement implements Renderable {
       new NavIsolateMenuElement(_isolate, _events, queue: _r.queue).element
     ];
     if (_library != null) {
-      menu.add(new NavLibraryMenuElement(_isolate, _library!, queue: _r.queue)
+      menu.add(new NavLibraryMenuElement(_isolate, _library, queue: _r.queue)
           .element);
     } else if (_field.dartOwner is M.ClassRef) {
-      menu.add(new NavClassMenuElement(_isolate, _field.dartOwner as M.ClassRef,
-              queue: _r.queue)
-          .element);
+      menu.add(
+          new NavClassMenuElement(_isolate, _field.dartOwner, queue: _r.queue)
+              .element);
     }
     menu.addAll(<Element>[
-      navMenu(_field.name!),
+      navMenu(_field.name),
       (new NavRefreshElement(queue: _r.queue)
             ..onRefresh.listen((e) {
               e.element.disabled = true;
@@ -237,13 +237,13 @@ class FieldViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberName']
             ..children = <Element>[
-              new SourceLinkElement(_isolate, field.location!, _scripts,
+              new SourceLinkElement(_isolate, field.location, _scripts,
                       queue: _r.queue)
                   .element
             ]
         ]
     ];
-    if (!_field.isStatic!) {
+    if (!_field.isStatic) {
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..title = 'The types observed for this field at runtime. '
@@ -286,23 +286,22 @@ class FieldViewElement extends CustomElement implements Renderable {
         break;
       case M.GuardClassKind.single:
         guard.add(
-            new ClassRefElement(_isolate, _field.guardClass!, queue: _r.queue)
+            new ClassRefElement(_isolate, _field.guardClass, queue: _r.queue)
                 .element);
         break;
     }
     guard.add(new SpanElement()
       ..text =
-          _field.guardNullable! ? '— null observed' : '— null not observed');
+          _field.guardNullable ? '— null observed' : '— null not observed');
     return guard;
   }
 
   Future _refresh() async {
-    _field = await _fields.get(_isolate, _field.id!);
+    _field = await _fields.get(_isolate, _field.id);
     if (_field.dartOwner is M.LibraryRef) {
-      _library = _field.dartOwner as M.LibraryRef;
+      _library = _field.dartOwner;
     } else if (_field.dartOwner is M.ClassRef) {
-      var cls = _field.dartOwner as M.ClassRef;
-      _library = (await _classes.get(_isolate, cls.id!)).library!;
+      _library = (await _classes.get(_isolate, _field.dartOwner.id)).library;
     }
     _r.dirty();
   }
diff --git a/runtime/observatory/lib/src/elements/flag_list.dart b/runtime/observatory/lib/src/elements/flag_list.dart
index c8e960e423f..0d0e090b7d4 100644
--- a/runtime/observatory/lib/src/elements/flag_list.dart
+++ b/runtime/observatory/lib/src/elements/flag_list.dart
@@ -28,21 +28,21 @@ class FlagListElement extends CustomElement implements Renderable {
     ViewFooterElement.tag,
   ]);
 
-  late RenderingScheduler<FlagListElement> _r;
+  RenderingScheduler<FlagListElement> _r;
 
   Stream<RenderedEvent<FlagListElement>> get onRendered => _r.onRendered;
 
-  late M.VMRef _vm;
-  late M.EventRepository _events;
-  late M.FlagsRepository _repository;
-  late M.NotificationRepository _notifications;
-  Iterable<M.Flag>? _flags;
+  M.VMRef _vm;
+  M.EventRepository _events;
+  M.FlagsRepository _repository;
+  M.NotificationRepository _notifications;
+  Iterable<M.Flag> _flags;
 
   M.VMRef get vm => _vm;
 
   factory FlagListElement(M.VMRef vm, M.EventRepository events,
       M.FlagsRepository repository, M.NotificationRepository notifications,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(events != null);
     assert(repository != null);
@@ -77,8 +77,8 @@ class FlagListElement extends CustomElement implements Renderable {
     if (_flags == null) {
       content.add(new HeadingElement.h1()..text = 'Loading Flags...');
     } else {
-      final modified = _flags!.where(_isModified);
-      final unmodified = _flags!.where(_isUnmodified);
+      final modified = _flags.where(_isModified);
+      final unmodified = _flags.where(_isUnmodified);
 
       if (modified.isNotEmpty) {
         content.add(new HeadingElement.h1()..text = 'Modified Flags');
@@ -100,7 +100,7 @@ class FlagListElement extends CustomElement implements Renderable {
     children = <Element>[
       navBar(<Element>[
         new NavTopMenuElement(queue: _r.queue).element,
-        new NavVMMenuElement(_vm as M.VM, _events, queue: _r.queue).element,
+        new NavVMMenuElement(_vm, _events, queue: _r.queue).element,
         navMenu('flags', link: Uris.flags()),
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
@@ -146,7 +146,7 @@ class FlagListElement extends CustomElement implements Renderable {
           new SpanElement()..text = '=',
           new SpanElement()
             ..classes = ['value']
-            ..text = flag.valueAsString
+            ..text = flag.valueAsString ?? 'NULL'
         ],
       new BRElement(),
     ];
diff --git a/runtime/observatory/lib/src/elements/function_ref.dart b/runtime/observatory/lib/src/elements/function_ref.dart
index 46b4aafee85..702eb39d2bd 100644
--- a/runtime/observatory/lib/src/elements/function_ref.dart
+++ b/runtime/observatory/lib/src/elements/function_ref.dart
@@ -22,20 +22,20 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class FunctionRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<FunctionRefElement>('function-ref');
 
-  late RenderingScheduler<FunctionRefElement> _r;
+  RenderingScheduler<FunctionRefElement> _r;
 
   Stream<RenderedEvent<FunctionRefElement>> get onRendered => _r.onRendered;
 
-  M.IsolateRef? _isolate;
-  late M.FunctionRef _function;
-  late bool _qualified;
+  M.IsolateRef _isolate;
+  M.FunctionRef _function;
+  bool _qualified;
 
-  M.IsolateRef? get isolate => _isolate;
+  M.IsolateRef get isolate => _isolate;
   M.FunctionRef get function => _function;
   bool get qualified => _qualified;
 
-  factory FunctionRefElement(M.IsolateRef? isolate, M.FunctionRef function,
-      {bool qualified: true, RenderingQueue? queue}) {
+  factory FunctionRefElement(M.IsolateRef isolate, M.FunctionRef function,
+      {bool qualified: true, RenderingQueue queue}) {
     assert(function != null);
     assert(qualified != null);
     FunctionRefElement e = new FunctionRefElement.created();
@@ -67,11 +67,11 @@ class FunctionRefElement extends CustomElement implements Renderable {
       new AnchorElement(
           href: (M.isSyntheticFunction(_function.kind) || (_isolate == null))
               ? null
-              : Uris.inspect(_isolate!, object: _function))
+              : Uris.inspect(_isolate, object: _function))
         ..text = _function.name
     ];
     if (qualified) {
-      M.ObjectRef? owner = _function.dartOwner;
+      M.ObjectRef owner = _function.dartOwner;
       while (owner is M.FunctionRef) {
         M.FunctionRef function = (owner as M.FunctionRef);
         content.addAll([
@@ -79,7 +79,7 @@ class FunctionRefElement extends CustomElement implements Renderable {
           new AnchorElement(
               href: (M.isSyntheticFunction(function.kind) || (_isolate == null))
                   ? null
-                  : Uris.inspect(_isolate!, object: function))
+                  : Uris.inspect(_isolate, object: function))
             ..text = function.name
         ]);
         owner = function.dartOwner;
@@ -87,8 +87,7 @@ class FunctionRefElement extends CustomElement implements Renderable {
       if (owner is M.ClassRef) {
         content.addAll([
           new SpanElement()..text = '.',
-          new ClassRefElement(_isolate!, owner as M.ClassRef, queue: _r.queue)
-              .element
+          new ClassRefElement(_isolate, owner, queue: _r.queue).element
         ]);
       }
     }
diff --git a/runtime/observatory/lib/src/elements/function_view.dart b/runtime/observatory/lib/src/elements/function_view.dart
index c39006924c9..64c44a3125d 100644
--- a/runtime/observatory/lib/src/elements/function_view.dart
+++ b/runtime/observatory/lib/src/elements/function_view.dart
@@ -50,24 +50,24 @@ class FunctionViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<FunctionViewElement> _r;
+  RenderingScheduler<FunctionViewElement> _r;
 
   Stream<RenderedEvent<FunctionViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.ServiceFunction _function;
-  M.LibraryRef? _library;
-  late M.FunctionRepository _functions;
-  late M.ClassRepository _classes;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ScriptRepository _scripts;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.ServiceFunction _function;
+  M.LibraryRef _library;
+  M.FunctionRepository _functions;
+  M.ClassRepository _classes;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ScriptRepository _scripts;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -88,7 +88,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
       M.RetainingPathRepository retainingPaths,
       M.ScriptRepository scripts,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -118,7 +118,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
     e._scripts = scripts;
     e._objects = objects;
     if (function.dartOwner is M.LibraryRef) {
-      e._library = function.dartOwner as M.LibraryRef;
+      e._library = function.dartOwner;
     }
     return e;
   }
@@ -160,7 +160,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
             ..children = _function.location == null
                 ? const []
                 : [
-                    new SourceInsetElement(_isolate, _function.location!,
+                    new SourceInsetElement(_isolate, _function.location,
                             _scripts, _objects, _events,
                             queue: _r.queue)
                         .element
@@ -177,16 +177,15 @@ class FunctionViewElement extends CustomElement implements Renderable {
       new NavIsolateMenuElement(_isolate, _events, queue: _r.queue).element
     ];
     if (_library != null) {
-      menu.add(new NavLibraryMenuElement(_isolate, _library!, queue: _r.queue)
+      menu.add(new NavLibraryMenuElement(_isolate, _library, queue: _r.queue)
           .element);
     } else if (_function.dartOwner is M.ClassRef) {
-      menu.add(new NavClassMenuElement(
-              _isolate, _function.dartOwner as M.ClassRef,
+      menu.add(new NavClassMenuElement(_isolate, _function.dartOwner,
               queue: _r.queue)
           .element);
     }
     menu.addAll(<Element>[
-      navMenu(_function.name!),
+      navMenu(_function.name),
       (new NavRefreshElement(queue: _r.queue)
             ..onRefresh.listen((e) {
               e.element.disabled = true;
@@ -210,8 +209,8 @@ class FunctionViewElement extends CustomElement implements Renderable {
             ..classes = ['memberName']
             ..children = <Element>[
               new SpanElement()
-                ..text = '${_function.isStatic! ? "static " : ""}'
-                    '${_function.isConst! ? "const " : ""}'
+                ..text = '${_function.isStatic ? "static " : ""}'
+                    '${_function.isConst ? "const " : ""}'
                     '${_functionKindToString(_function.kind)}'
             ]
         ],
@@ -241,7 +240,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberName']
             ..children = <Element>[
-              new FieldRefElement(_isolate, _function.field!, _objects,
+              new FieldRefElement(_isolate, _function.field, _objects,
                       queue: _r.queue)
                   .element
             ]
@@ -256,7 +255,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
         new DivElement()
           ..classes = ['memberName']
           ..children = <Element>[
-            new SourceLinkElement(_isolate, _function.location!, _scripts,
+            new SourceLinkElement(_isolate, _function.location, _scripts,
                     queue: _r.queue)
                 .element
           ]
@@ -271,7 +270,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberName']
             ..children = <Element>[
-              new CodeRefElement(_isolate, _function.code!, queue: _r.queue)
+              new CodeRefElement(_isolate, _function.code, queue: _r.queue)
                   .element
             ]
         ]);
@@ -286,7 +285,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberName']
             ..children = <Element>[
-              new CodeRefElement(_isolate, _function.unoptimizedCode!,
+              new CodeRefElement(_isolate, _function.unoptimizedCode,
                       queue: _r.queue)
                   .element,
               new SpanElement()
@@ -307,7 +306,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberName']
             ..children = <Element>[
-              new CodeRefElement(_isolate, _function.bytecode!, queue: _r.queue)
+              new CodeRefElement(_isolate, _function.bytecode, queue: _r.queue)
                   .element,
             ]
         ]);
@@ -326,7 +325,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberName']
             ..children = <Element>[
-              new InstanceRefElement(_isolate, _function.icDataArray!, _objects,
+              new InstanceRefElement(_isolate, _function.icDataArray, _objects,
                       queue: _r.queue)
                   .element
             ]
@@ -352,7 +351,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
             ..text = 'optimizable',
           new DivElement()
             ..classes = ['memberName']
-            ..text = _function.isOptimizable! ? 'yes' : 'no'
+            ..text = _function.isOptimizable ? 'yes' : 'no'
         ],
       new DivElement()
         ..classes = ['memberItem']
@@ -362,7 +361,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
             ..text = 'inlinable',
           new DivElement()
             ..classes = ['memberName']
-            ..text = _function.isInlinable! ? 'yes' : 'no'
+            ..text = _function.isInlinable ? 'yes' : 'no'
         ],
       new DivElement()
         ..classes = ['memberItem']
@@ -372,7 +371,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
             ..text = 'intrinsic',
           new DivElement()
             ..classes = ['memberName']
-            ..text = _function.hasIntrinsic! ? 'yes' : 'no'
+            ..text = _function.hasIntrinsic ? 'yes' : 'no'
         ],
       new DivElement()
         ..classes = ['memberItem']
@@ -382,7 +381,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
             ..text = 'recognized',
           new DivElement()
             ..classes = ['memberName']
-            ..text = _function.isRecognized! ? 'yes' : 'no'
+            ..text = _function.isRecognized ? 'yes' : 'no'
         ],
       new DivElement()
         ..classes = ['memberItem']
@@ -392,7 +391,7 @@ class FunctionViewElement extends CustomElement implements Renderable {
             ..text = 'native',
           new DivElement()
             ..classes = ['memberName']
-            ..text = _function.isNative! ? 'yes' : 'no'
+            ..text = _function.isNative ? 'yes' : 'no'
         ],
       new DivElement()
         ..classes = ['memberItem']
@@ -409,17 +408,16 @@ class FunctionViewElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _function = await _functions.get(_isolate, _function.id!);
+    _function = await _functions.get(_isolate, _function.id);
     if (_function.dartOwner is M.LibraryRef) {
-      _library = _function.dartOwner as M.LibraryRef;
+      _library = _function.dartOwner;
     } else if (_function.dartOwner is M.ClassRef) {
-      var cls = _function.dartOwner as M.ClassRef;
-      _library = (await _classes.get(_isolate, cls.id!)).library!;
+      _library = (await _classes.get(_isolate, _function.dartOwner.id)).library;
     }
     _r.dirty();
   }
 
-  static String _functionKindToString(M.FunctionKind? kind) {
+  static String _functionKindToString(M.FunctionKind kind) {
     switch (kind) {
       case M.FunctionKind.regular:
         return 'regular';
diff --git a/runtime/observatory/lib/src/elements/general_error.dart b/runtime/observatory/lib/src/elements/general_error.dart
index 05d0dbdbece..bc07f6ac808 100644
--- a/runtime/observatory/lib/src/elements/general_error.dart
+++ b/runtime/observatory/lib/src/elements/general_error.dart
@@ -17,19 +17,19 @@ class GeneralErrorElement extends CustomElement implements Renderable {
   static const tag = const Tag<GeneralErrorElement>('general-error',
       dependencies: const [NavTopMenuElement.tag, NavNotifyElement.tag]);
 
-  late RenderingScheduler<GeneralErrorElement> _r;
+  RenderingScheduler<GeneralErrorElement> _r;
 
   Stream<RenderedEvent<GeneralErrorElement>> get onRendered => _r.onRendered;
 
-  late M.NotificationRepository _notifications;
-  late String _message;
+  M.NotificationRepository _notifications;
+  String _message;
 
   String get message => _message;
 
   set message(String value) => _message = _r.checkAndReact(_message, value);
 
   factory GeneralErrorElement(M.NotificationRepository notifications,
-      {String message: '', RenderingQueue? queue}) {
+      {String message: '', RenderingQueue queue}) {
     assert(notifications != null);
     assert(message != null);
     GeneralErrorElement e = new GeneralErrorElement.created();
diff --git a/runtime/observatory/lib/src/elements/heap_map.dart b/runtime/observatory/lib/src/elements/heap_map.dart
index 21350e10925..f658591e3af 100644
--- a/runtime/observatory/lib/src/elements/heap_map.dart
+++ b/runtime/observatory/lib/src/elements/heap_map.dart
@@ -29,21 +29,21 @@ class HeapMapElement extends CustomElement implements Renderable {
     NavNotifyElement.tag,
   ]);
 
-  late RenderingScheduler<HeapMapElement> _r;
+  RenderingScheduler<HeapMapElement> _r;
 
   Stream<RenderedEvent<HeapMapElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
   M.NotificationRepository get notifications => _notifications;
 
   factory HeapMapElement(M.VM vm, M.IsolateRef isolate,
       M.EventRepository events, M.NotificationRepository notifications,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -73,9 +73,9 @@ class HeapMapElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  CanvasElement? _canvas;
-  dynamic _fragmentationData;
-  int? _pageHeight;
+  CanvasElement _canvas;
+  var _fragmentationData;
+  double _pageHeight;
   final _classIdToColor = {};
   final _colorToClassId = {};
   final _classIdToName = {};
@@ -87,8 +87,8 @@ class HeapMapElement extends CustomElement implements Renderable {
   // TODO(koda): Improve interface for huge heaps.
   static const _MAX_CANVAS_HEIGHT = 6000;
 
-  String _status = 'Loading';
-  S.ServiceMap? _fragmentation;
+  String _status;
+  S.ServiceMap _fragmentation;
 
   void render() {
     if (_canvas == null) {
@@ -99,7 +99,7 @@ class HeapMapElement extends CustomElement implements Renderable {
     }
 
     // Set hover text to describe the object under the cursor.
-    _canvas!.title = _status;
+    _canvas.title = _status;
 
     children = <Element>[
       navBar(<Element>[
@@ -129,7 +129,7 @@ class HeapMapElement extends CustomElement implements Renderable {
         ],
       new DivElement()
         ..classes = ['flex-row']
-        ..children = <Element>[_canvas!]
+        ..children = <Element>[_canvas]
     ];
   }
 
@@ -158,9 +158,9 @@ class HeapMapElement extends CustomElement implements Renderable {
         print('Ignoring non-class in class list');
         continue;
       }
-      var classId = int.parse(member.id!.split('/').last);
+      var classId = int.parse(member.id.split('/').last);
       var color = _classIdToRGBA(classId);
-      _addClass(classId, member.name!, color);
+      _addClass(classId, member.name, color);
     }
     _addClass(freeClassId, 'Free', _freeColor);
     _addClass(0, '', _pageSeparationColor);
@@ -177,15 +177,15 @@ class HeapMapElement extends CustomElement implements Renderable {
     return _classIdToName[_colorToClassId[_packColor(color)]];
   }
 
-  ObjectInfo? _objectAt(Point<num> point) {
+  ObjectInfo _objectAt(Point<num> point) {
     if (_fragmentation == null || _canvas == null) {
       return null;
     }
-    var pagePixels = _pageHeight! * _fragmentationData.width;
+    var pagePixels = _pageHeight * _fragmentationData.width;
     var index = new PixelReference(_fragmentationData, point).index;
     var pageIndex = index ~/ pagePixels;
     num pageOffset = index % pagePixels;
-    var pages = _fragmentation!['pages'];
+    var pages = _fragmentation['pages'];
     if (pageIndex < 0 || pageIndex >= pages.length) {
       return null;
     }
@@ -204,8 +204,8 @@ class HeapMapElement extends CustomElement implements Renderable {
     }
     return new ObjectInfo(
         int.parse(page['objectStart']) +
-            pageOffset * _fragmentation!['unitSizeBytes'],
-        size * _fragmentation!['unitSizeBytes']);
+            pageOffset * _fragmentation['unitSizeBytes'],
+        size * _fragmentation['unitSizeBytes']);
   }
 
   void _handleMouseMove(MouseEvent event) {
@@ -226,28 +226,28 @@ class HeapMapElement extends CustomElement implements Renderable {
       return;
     }
     _updateClassList(
-        _fragmentation!['classList'], _fragmentation!['freeClassId']);
-    var pages = _fragmentation!['pages'];
-    var width = max(_canvas!.parent!.client.width, 1) as int;
+        _fragmentation['classList'], _fragmentation['freeClassId']);
+    var pages = _fragmentation['pages'];
+    var width = max(_canvas.parent.client.width, 1);
     _pageHeight = _PAGE_SEPARATION_HEIGHT +
-        (_fragmentation!['pageSizeBytes'] as int) ~/
-            (_fragmentation!['unitSizeBytes'] as int) ~/
+        _fragmentation['pageSizeBytes'] ~/
+            _fragmentation['unitSizeBytes'] ~/
             width;
-    var height = min(_pageHeight! * pages.length, _MAX_CANVAS_HEIGHT) as int;
-    _fragmentationData = _canvas!.context2D.createImageData(width, height);
-    _canvas!.width = _fragmentationData.width;
-    _canvas!.height = _fragmentationData.height;
+    var height = min(_pageHeight * pages.length, _MAX_CANVAS_HEIGHT);
+    _fragmentationData = _canvas.context2D.createImageData(width, height);
+    _canvas.width = _fragmentationData.width;
+    _canvas.height = _fragmentationData.height;
     _renderPages(0);
   }
 
   // Renders and draws asynchronously, one page at a time to avoid
   // blocking the UI.
   void _renderPages(int startPage) {
-    var pages = _fragmentation!['pages'];
+    var pages = _fragmentation['pages'];
     _status = 'Loaded $startPage of ${pages.length} pages';
     _r.dirty();
-    var startY = (startPage * _pageHeight!).round();
-    var endY = startY + _pageHeight!.round();
+    var startY = (startPage * _pageHeight).round();
+    var endY = startY + _pageHeight.round();
     if (startPage >= pages.length || endY > _fragmentationData.height) {
       return;
     }
@@ -266,7 +266,7 @@ class HeapMapElement extends CustomElement implements Renderable {
       pixel.color = _pageSeparationColor;
       pixel = pixel.next();
     }
-    _canvas!.context2D.putImageData(
+    _canvas.context2D.putImageData(
         _fragmentationData, 0, 0, 0, startY, _fragmentationData.width, endY);
     // Continue with the next page, asynchronously.
     new Future(() {
@@ -274,14 +274,14 @@ class HeapMapElement extends CustomElement implements Renderable {
     });
   }
 
-  Future _refresh({String? gc}) {
+  Future _refresh({String gc}) {
     final isolate = _isolate as S.Isolate;
     var params = {};
     if (gc != null) {
       params['gc'] = gc;
     }
     return isolate.invokeRpc('_getHeapMap', params).then((serviceObject) {
-      S.ServiceMap response = serviceObject as S.ServiceMap;
+      S.ServiceMap response = serviceObject;
       assert(response['type'] == 'HeapMap');
       _fragmentation = response;
       _updateFragmentationData();
diff --git a/runtime/observatory/lib/src/elements/heap_snapshot.dart b/runtime/observatory/lib/src/elements/heap_snapshot.dart
index 19ce5e2eafd..f6505be85d9 100644
--- a/runtime/observatory/lib/src/elements/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/elements/heap_snapshot.dart
@@ -190,9 +190,9 @@ class MergedDominatorDiffTreeMap extends DiffTreeMap<MergedDominatorDiff> {
 
   String getType(MergedDominatorDiff node) => node.name;
   String getName(MergedDominatorDiff node) => "instances of ${node.name}";
-  MergedDominatorDiff? getParent(MergedDominatorDiff node) => node.parent;
+  MergedDominatorDiff getParent(MergedDominatorDiff node) => node.parent;
   Iterable<MergedDominatorDiff> getChildren(MergedDominatorDiff node) =>
-      node.children!;
+      node.children;
   void onSelect(MergedDominatorDiff node) {
     element.mergedDiffSelection = node;
     element._r.dirty();
@@ -220,7 +220,7 @@ class ClassesShallowTreeMap extends NormalTreeMap<SnapshotClass> {
       ? "${snapshot.classes.length} classes"
       : "${node.instanceCount} instances of ${node.name}";
 
-  SnapshotClass? getParent(SnapshotClass node) => null;
+  SnapshotClass getParent(SnapshotClass node) => null;
   Iterable<SnapshotClass> getChildren(SnapshotClass node) =>
       node == null ? snapshot.classes : <SnapshotClass>[];
   void onSelect(SnapshotClass node) {}
@@ -277,7 +277,7 @@ class ClassesShallowDiffTreeMap extends DiffTreeMap<SnapshotClassDiff> {
       node == null ? "Classes" : node.name;
   String getName(SnapshotClassDiff node) =>
       node == null ? "${classes.length} classes" : "instances of ${node.name}";
-  SnapshotClassDiff? getParent(SnapshotClassDiff node) => null;
+  SnapshotClassDiff getParent(SnapshotClassDiff node) => null;
   Iterable<SnapshotClassDiff> getChildren(SnapshotClassDiff node) =>
       node == null ? classes : <SnapshotClassDiff>[];
   void onSelect(SnapshotClassDiff node) {}
@@ -302,7 +302,7 @@ class ClassesOwnershipTreeMap extends NormalTreeMap<SnapshotClass> {
   String getName(SnapshotClass node) => node == null
       ? "${snapshot.classes.length} Classes"
       : "${node.instanceCount} instances of ${node.name}";
-  SnapshotClass? getParent(SnapshotClass node) => null;
+  SnapshotClass getParent(SnapshotClass node) => null;
   Iterable<SnapshotClass> getChildren(SnapshotClass node) =>
       node == null ? snapshot.classes : <SnapshotClass>[];
   void onSelect(SnapshotClass node) {}
@@ -359,7 +359,7 @@ class ClassesOwnershipDiffTreeMap extends DiffTreeMap<SnapshotClassDiff> {
       node == null ? "Classes" : node.name;
   String getName(SnapshotClassDiff node) =>
       node == null ? "${classes.length} classes" : "instances of ${node.name}";
-  SnapshotClassDiff? getParent(SnapshotClassDiff node) => null;
+  SnapshotClassDiff getParent(SnapshotClassDiff node) => null;
   Iterable<SnapshotClassDiff> getChildren(SnapshotClassDiff node) =>
       node == null ? classes : <SnapshotClassDiff>[];
   void onSelect(SnapshotClassDiff node) {}
@@ -378,10 +378,10 @@ class ProcessTreeMap extends NormalTreeMap<String> {
 
   ProcessTreeMap(this.element, this.snapshot);
 
-  int getSize(String node) => snapshot.processPartitions[node] ?? 0;
+  int getSize(String node) => snapshot.processPartitions[node];
   String getType(String node) => node;
   String getName(String node) => node;
-  String? getParent(String node) => node == root ? null : root;
+  String getParent(String node) => node == root ? null : root;
   Iterable<String> getChildren(String node) {
     if (node == root) {
       var children = snapshot.processPartitions.keys.toList();
@@ -407,22 +407,22 @@ class ProcessDiffTreeMap extends DiffTreeMap<PartitionDiff> {
   int getCommon(PartitionDiff node) => node.sizeCommon;
   String getType(PartitionDiff node) => node.name;
   String getName(PartitionDiff node) => node.name;
-  PartitionDiff? getParent(PartitionDiff node) => node.parent;
-  Iterable<PartitionDiff> getChildren(PartitionDiff node) => node.children!;
+  PartitionDiff getParent(PartitionDiff node) => node.parent;
+  Iterable<PartitionDiff> getChildren(PartitionDiff node) => node.children;
   void onSelect(PartitionDiff node) {}
   void onDetails(PartitionDiff node) {}
 }
 
 class PartitionDiff {
-  String name = "";
+  String name;
   int sizeA = 0;
   int sizeB = 0;
   int sizeGain = -1;
   int sizeLoss = -1;
   int sizeCommon = -1;
 
-  PartitionDiff? parent;
-  List<PartitionDiff>? children;
+  PartitionDiff parent;
+  List<PartitionDiff> children;
 
   static PartitionDiff from(SnapshotGraph graphA, SnapshotGraph graphB) {
     var partitions = new Set<String>();
@@ -460,7 +460,7 @@ class PartitionDiff {
       rootDiff.sizeGain += partitionDiff.sizeGain;
       rootDiff.sizeCommon += partitionDiff.sizeCommon;
       rootDiff.sizeLoss += partitionDiff.sizeLoss;
-      rootDiff.children!.add(partitionDiff);
+      rootDiff.children.add(partitionDiff);
     }
 
     var shallowA = rootDiff.sizeA - childrenA;
@@ -480,16 +480,16 @@ class PartitionDiff {
 }
 
 class SnapshotClassDiff {
-  SnapshotClass? _a;
-  SnapshotClass? _b;
+  SnapshotClass _a;
+  SnapshotClass _b;
 
-  int get shallowSizeA => _a == null ? 0 : _a!.shallowSize;
-  int get ownedSizeA => _a == null ? 0 : _a!.ownedSize;
-  int get instanceCountA => _a == null ? 0 : _a!.instanceCount;
+  int get shallowSizeA => _a == null ? 0 : _a.shallowSize;
+  int get ownedSizeA => _a == null ? 0 : _a.ownedSize;
+  int get instanceCountA => _a == null ? 0 : _a.instanceCount;
 
-  int get shallowSizeB => _b == null ? 0 : _b!.shallowSize;
-  int get ownedSizeB => _b == null ? 0 : _b!.ownedSize;
-  int get instanceCountB => _b == null ? 0 : _b!.instanceCount;
+  int get shallowSizeB => _b == null ? 0 : _b.shallowSize;
+  int get ownedSizeB => _b == null ? 0 : _b.ownedSize;
+  int get instanceCountB => _b == null ? 0 : _b.instanceCount;
 
   int get shallowSizeDiff => shallowSizeB - shallowSizeA;
   int get ownedSizeDiff => ownedSizeB - ownedSizeA;
@@ -507,12 +507,12 @@ class SnapshotClassDiff {
       shallowSizeB > shallowSizeA ? shallowSizeA : shallowSizeB;
   int get ownedSizeCommon => ownedSizeB > ownedSizeA ? ownedSizeA : ownedSizeB;
 
-  String get name => _a == null ? _b!.name : _a!.name;
+  String get name => _a == null ? _b.name : _a.name;
 
   List<SnapshotObject> get objectsA =>
-      _a == null ? const <SnapshotObject>[] : _a!.instances.toList();
+      _a == null ? <SnapshotObject>[] : _a.instances.toList();
   List<SnapshotObject> get objectsB =>
-      _b == null ? const <SnapshotObject>[] : _b!.instances.toList();
+      _b == null ? <SnapshotObject>[] : _b.instances.toList();
 
   static List<SnapshotClassDiff> from(
       SnapshotGraph graphA, SnapshotGraph graphB) {
@@ -544,32 +544,32 @@ class SnapshotClassDiff {
 }
 
 class MergedDominatorDiff {
-  SnapshotMergedDominator? _a;
-  SnapshotMergedDominator? _b;
-  MergedDominatorDiff? parent;
-  List<MergedDominatorDiff>? children;
+  SnapshotMergedDominator _a;
+  SnapshotMergedDominator _b;
+  MergedDominatorDiff parent;
+  List<MergedDominatorDiff> children;
   int retainedGain = -1;
   int retainedLoss = -1;
   int retainedCommon = -1;
 
-  int get shallowSizeA => _a == null ? 0 : _a!.shallowSize;
-  int get retainedSizeA => _a == null ? 0 : _a!.retainedSize;
-  int get instanceCountA => _a == null ? 0 : _a!.instanceCount;
+  int get shallowSizeA => _a == null ? 0 : _a.shallowSize;
+  int get retainedSizeA => _a == null ? 0 : _a.retainedSize;
+  int get instanceCountA => _a == null ? 0 : _a.instanceCount;
 
-  int get shallowSizeB => _b == null ? 0 : _b!.shallowSize;
-  int get retainedSizeB => _b == null ? 0 : _b!.retainedSize;
-  int get instanceCountB => _b == null ? 0 : _b!.instanceCount;
+  int get shallowSizeB => _b == null ? 0 : _b.shallowSize;
+  int get retainedSizeB => _b == null ? 0 : _b.retainedSize;
+  int get instanceCountB => _b == null ? 0 : _b.instanceCount;
 
   int get shallowSizeDiff => shallowSizeB - shallowSizeA;
   int get retainedSizeDiff => retainedSizeB - retainedSizeA;
   int get instanceCountDiff => instanceCountB - instanceCountA;
 
-  String get name => _a == null ? _b!.klass.name : _a!.klass.name;
+  String get name => _a == null ? _b.klass.name : _a.klass.name;
 
   List<SnapshotObject> get objectsA =>
-      _a == null ? const <SnapshotObject>[] : _a!.objects.toList();
+      _a == null ? <SnapshotObject>[] : _a.objects.toList();
   List<SnapshotObject> get objectsB =>
-      _b == null ? const <SnapshotObject>[] : _b!.objects.toList();
+      _b == null ? <SnapshotObject>[] : _b.objects.toList();
 
   static MergedDominatorDiff from(
       SnapshotMergedDominator a, SnapshotMergedDominator b) {
@@ -602,11 +602,11 @@ class MergedDominatorDiff {
     // Matching children by MergedObjectVertex.klass.qualifiedName.
     final childrenB = <String, SnapshotMergedDominator>{};
     if (_b != null)
-      for (var childB in _b!.children) {
+      for (var childB in _b.children) {
         childrenB[childB.klass.qualifiedName] = childB;
       }
     if (_a != null)
-      for (var childA in _a!.children) {
+      for (var childA in _a.children) {
         var childDiff = new MergedDominatorDiff();
         childDiff.parent = this;
         childDiff._a = childA;
@@ -616,18 +616,18 @@ class MergedDominatorDiff {
           childrenB.remove(qualifiedName);
           childDiff._b = childB;
         }
-        children!.add(childDiff);
+        children.add(childDiff);
         worklist.add(childDiff);
       }
     for (var childB in childrenB.values) {
       var childDiff = new MergedDominatorDiff();
       childDiff.parent = this;
       childDiff._b = childB;
-      children!.add(childDiff);
+      children.add(childDiff);
       worklist.add(childDiff);
     }
 
-    if (children!.length == 0) {
+    if (children.length == 0) {
       // Compress.
       children = const <MergedDominatorDiff>[];
     }
@@ -637,7 +637,7 @@ class MergedDominatorDiff {
     int g = 0;
     int l = 0;
     int c = 0;
-    for (var child in children!) {
+    for (var child in children) {
       g += child.retainedGain;
       l += child.retainedLoss;
       c += child.retainedCommon;
@@ -669,30 +669,30 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
     VirtualTreeElement.tag,
   ]);
 
-  late RenderingScheduler<HeapSnapshotElement> _r;
+  RenderingScheduler<HeapSnapshotElement> _r;
 
   Stream<RenderedEvent<HeapSnapshotElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.HeapSnapshotRepository _snapshots;
-  late M.ObjectRepository _objects;
-  SnapshotReader? _reader;
-  String? _status;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.HeapSnapshotRepository _snapshots;
+  M.ObjectRepository _objects;
+  SnapshotReader _reader;
+  String _status;
   List<SnapshotGraph> _loadedSnapshots = <SnapshotGraph>[];
-  SnapshotGraph? _snapshotA;
-  SnapshotGraph? _snapshotB;
+  SnapshotGraph _snapshotA;
+  SnapshotGraph _snapshotB;
   HeapSnapshotTreeMode _mode = HeapSnapshotTreeMode.mergedDominatorTreeMap;
   M.IsolateRef get isolate => _isolate;
   M.NotificationRepository get notifications => _notifications;
   M.HeapSnapshotRepository get profiles => _snapshots;
   M.VMRef get vm => _vm;
 
-  List<SnapshotObject>? selection;
-  SnapshotMergedDominator? mergedSelection;
-  MergedDominatorDiff? mergedDiffSelection;
+  List<SnapshotObject> selection;
+  SnapshotMergedDominator mergedSelection;
+  MergedDominatorDiff mergedDiffSelection;
 
   factory HeapSnapshotElement(
       M.VM vm,
@@ -701,7 +701,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
       M.NotificationRepository notifications,
       M.HeapSnapshotRepository snapshots,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -766,7 +766,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
     if (_reader != null) {
       // Loading
       content.addAll(_createStatusMessage('Loading snapshot...',
-          description: _status!, progress: 1));
+          description: _status, progress: 1));
     } else if (_snapshotA != null) {
       // Loaded
       content.addAll(_createReport());
@@ -780,7 +780,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
   }
 
   _save() {
-    var blob = new Blob(_snapshotA!.chunks, 'application/octet-stream');
+    var blob = new Blob(_snapshotA.chunks, 'application/octet-stream');
     var blobUrl = Url.createObjectUrl(blob);
     var link = new AnchorElement();
     link.href = blobUrl;
@@ -794,13 +794,13 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
     input.type = 'file';
     input.multiple = false;
     input.onChange.listen((event) {
-      var file = input.files![0];
+      var file = input.files[0];
       var reader = new FileReader();
       reader.onLoad.listen((event) async {
-        var encoded = <Uint8List>[reader.result as Uint8List];
+        var encoded = <Uint8List>[reader.result];
         var snapshotReader = new SnapshotReader();
         _snapshotLoading(snapshotReader);
-        snapshotReader.add(reader.result as Uint8List);
+        snapshotReader.add(reader.result);
         snapshotReader.close();
       });
       reader.readAsArrayBuffer(file);
@@ -854,7 +854,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
     ];
   }
 
-  VirtualTreeElement? _tree;
+  VirtualTreeElement _tree;
 
   void _createTreeMap<T>(List<HtmlElement> report, TreeMap<T> treemap, T root) {
     final content = new DivElement();
@@ -927,13 +927,13 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
     switch (_mode) {
       case HeapSnapshotTreeMode.dominatorTree:
         if (selection == null) {
-          selection = List.from(_snapshotA!.root.objects);
+          selection = List.from(_snapshotA.root.objects);
         }
         _tree = new VirtualTreeElement(
             _createDominator, _updateDominator, _getChildrenDominator,
-            items: selection!, queue: _r.queue);
-        if (selection!.length == 1) {
-          _tree!.expand(selection!.first);
+            items: selection, queue: _r.queue);
+        if (selection.length == 1) {
+          _tree.expand(selection.first);
         }
         final text = 'In a heap dominator tree, an object X is a parent of '
             'object Y if every path from the root to Y goes through '
@@ -945,49 +945,49 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['content-centered-big', 'explanation']
             ..text = text,
-          _tree!.element
+          _tree.element
         ]);
         break;
       case HeapSnapshotTreeMode.dominatorTreeMap:
         if (selection == null) {
-          selection = List.from(_snapshotA!.root.objects);
+          selection = List.from(_snapshotA.root.objects);
         }
-        _createTreeMap(report, new DominatorTreeMap(this), selection!.first);
+        _createTreeMap(report, new DominatorTreeMap(this), selection.first);
         break;
       case HeapSnapshotTreeMode.mergedDominatorTree:
         _tree = new VirtualTreeElement(_createMergedDominator,
             _updateMergedDominator, _getChildrenMergedDominator,
-            items: _getChildrenMergedDominator(_snapshotA!.mergedRoot),
+            items: _getChildrenMergedDominator(_snapshotA.mergedRoot),
             queue: _r.queue);
-        _tree!.expand(_snapshotA!.mergedRoot);
+        _tree.expand(_snapshotA.mergedRoot);
         final text = 'A heap dominator tree, where siblings with the same class'
             ' have been merged into a single node.';
         report.addAll([
           new DivElement()
             ..classes = ['content-centered-big', 'explanation']
             ..text = text,
-          _tree!.element
+          _tree.element
         ]);
         break;
       case HeapSnapshotTreeMode.mergedDominatorTreeDiff:
         var root = MergedDominatorDiff.from(
-            _snapshotA!.mergedRoot, _snapshotB!.mergedRoot);
+            _snapshotA.mergedRoot, _snapshotB.mergedRoot);
         _tree = new VirtualTreeElement(_createMergedDominatorDiff,
             _updateMergedDominatorDiff, _getChildrenMergedDominatorDiff,
             items: _getChildrenMergedDominatorDiff(root), queue: _r.queue);
-        _tree!.expand(root);
+        _tree.expand(root);
         final text = 'A heap dominator tree, where siblings with the same class'
             ' have been merged into a single node.';
         report.addAll([
           new DivElement()
             ..classes = ['content-centered-big', 'explanation']
             ..text = text,
-          _tree!.element
+          _tree.element
         ]);
         break;
       case HeapSnapshotTreeMode.mergedDominatorTreeMap:
         if (mergedSelection == null) {
-          mergedSelection = _snapshotA!.mergedRoot;
+          mergedSelection = _snapshotA.mergedRoot;
         }
         _createTreeMap(
             report, new MergedDominatorTreeMap(this), mergedSelection);
@@ -995,19 +995,18 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
       case HeapSnapshotTreeMode.mergedDominatorTreeMapDiff:
         if (mergedDiffSelection == null) {
           mergedDiffSelection = MergedDominatorDiff.from(
-              _snapshotA!.mergedRoot, _snapshotB!.mergedRoot);
+              _snapshotA.mergedRoot, _snapshotB.mergedRoot);
         }
         _createTreeMap(
             report, new MergedDominatorDiffTreeMap(this), mergedDiffSelection);
         break;
       case HeapSnapshotTreeMode.ownershipTable:
-        final items =
-            _snapshotA!.classes.where((c) => c.ownedSize > 0).toList();
+        final items = _snapshotA.classes.where((c) => c.ownedSize > 0).toList();
         items.sort((a, b) => b.ownedSize - a.ownedSize);
         _tree = new VirtualTreeElement(
             _createOwnership, _updateOwnership, _getChildrenOwnership,
             items: items, queue: _r.queue);
-        _tree!.expand(_snapshotA!.root);
+        _tree.expand(_snapshotA.root);
         final text = 'An object X is said to "own" object Y if X is the only '
             'object that references Y, or X owns the only object that '
             'references Y. In particular, objects "own" the space of any '
@@ -1016,18 +1015,18 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['content-centered-big', 'explanation']
             ..text = text,
-          _tree!.element
+          _tree.element
         ]);
         break;
       case HeapSnapshotTreeMode.ownershipTableDiff:
-        final items = SnapshotClassDiff.from(_snapshotA!, _snapshotB!);
+        final items = SnapshotClassDiff.from(_snapshotA, _snapshotB);
         items.sort((a, b) => b.ownedSizeB - a.ownedSizeB);
         items.sort((a, b) => b.ownedSizeA - a.ownedSizeA);
         items.sort((a, b) => b.ownedSizeDiff - a.ownedSizeDiff);
         _tree = new VirtualTreeElement(_createOwnershipDiff,
             _updateOwnershipDiff, _getChildrenOwnershipDiff,
             items: items, queue: _r.queue);
-        _tree!.expand(_snapshotA!.root);
+        _tree.expand(_snapshotA.root);
         final text = 'An object X is said to "own" object Y if X is the only '
             'object that references Y, or X owns the only object that '
             'references Y. In particular, objects "own" the space of any '
@@ -1036,88 +1035,88 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['content-centered-big', 'explanation']
             ..text = text,
-          _tree!.element
+          _tree.element
         ]);
         break;
       case HeapSnapshotTreeMode.ownershipTreeMap:
         _createTreeMap(
-            report, new ClassesOwnershipTreeMap(this, _snapshotA!), null);
+            report, new ClassesOwnershipTreeMap(this, _snapshotA), null);
         break;
       case HeapSnapshotTreeMode.ownershipTreeMapDiff:
-        final items = SnapshotClassDiff.from(_snapshotA!, _snapshotB!);
+        final items = SnapshotClassDiff.from(_snapshotA, _snapshotB);
         _createTreeMap(
             report, new ClassesOwnershipDiffTreeMap(this, items), null);
         break;
       case HeapSnapshotTreeMode.successors:
         if (selection == null) {
-          selection = List.from(_snapshotA!.root.objects);
+          selection = List.from(_snapshotA.root.objects);
         }
         _tree = new VirtualTreeElement(
             _createSuccessor, _updateSuccessor, _getChildrenSuccessor,
-            items: selection!, queue: _r.queue);
-        if (selection!.length == 1) {
-          _tree!.expand(selection!.first);
+            items: selection, queue: _r.queue);
+        if (selection.length == 1) {
+          _tree.expand(selection.first);
         }
         final text = '';
         report.addAll([
           new DivElement()
             ..classes = ['content-centered-big', 'explanation']
             ..text = text,
-          _tree!.element
+          _tree.element
         ]);
         break;
       case HeapSnapshotTreeMode.predecessors:
         if (selection == null) {
-          selection = List.from(_snapshotA!.root.objects);
+          selection = List.from(_snapshotA.root.objects);
         }
         _tree = new VirtualTreeElement(
             _createPredecessor, _updatePredecessor, _getChildrenPredecessor,
-            items: selection!, queue: _r.queue);
-        if (selection!.length == 1) {
-          _tree!.expand(selection!.first);
+            items: selection, queue: _r.queue);
+        if (selection.length == 1) {
+          _tree.expand(selection.first);
         }
         final text = '';
         report.addAll([
           new DivElement()
             ..classes = ['content-centered-big', 'explanation']
             ..text = text,
-          _tree!.element
+          _tree.element
         ]);
         break;
       case HeapSnapshotTreeMode.classesTable:
-        final items = _snapshotA!.classes.toList();
+        final items = _snapshotA.classes.toList();
         items.sort((a, b) => b.shallowSize - a.shallowSize);
         _tree = new VirtualTreeElement(
             _createClass, _updateClass, _getChildrenClass,
             items: items, queue: _r.queue);
-        report.add(_tree!.element);
+        report.add(_tree.element);
         break;
       case HeapSnapshotTreeMode.classesTableDiff:
-        final items = SnapshotClassDiff.from(_snapshotA!, _snapshotB!);
+        final items = SnapshotClassDiff.from(_snapshotA, _snapshotB);
         items.sort((a, b) => b.shallowSizeB - a.shallowSizeB);
         items.sort((a, b) => b.shallowSizeA - a.shallowSizeA);
         items.sort((a, b) => b.shallowSizeDiff - a.shallowSizeDiff);
         _tree = new VirtualTreeElement(
             _createClassDiff, _updateClassDiff, _getChildrenClassDiff,
             items: items, queue: _r.queue);
-        report.add(_tree!.element);
+        report.add(_tree.element);
         break;
       case HeapSnapshotTreeMode.classesTreeMap:
         _createTreeMap(
-            report, new ClassesShallowTreeMap(this, _snapshotA!), null);
+            report, new ClassesShallowTreeMap(this, _snapshotA), null);
         break;
 
       case HeapSnapshotTreeMode.classesTreeMapDiff:
-        final items = SnapshotClassDiff.from(_snapshotA!, _snapshotB!);
+        final items = SnapshotClassDiff.from(_snapshotA, _snapshotB);
         _createTreeMap(
             report, new ClassesShallowDiffTreeMap(this, items), null);
         break;
       case HeapSnapshotTreeMode.process:
         _createTreeMap(
-            report, new ProcessTreeMap(this, _snapshotA!), ProcessTreeMap.root);
+            report, new ProcessTreeMap(this, _snapshotA), ProcessTreeMap.root);
         break;
       case HeapSnapshotTreeMode.processDiff:
-        final root = PartitionDiff.from(_snapshotA!, _snapshotB!);
+        final root = PartitionDiff.from(_snapshotA, _snapshotB);
         _createTreeMap(report, new ProcessDiffTreeMap(this), root);
         break;
       default:
@@ -1365,7 +1364,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
 
   static Iterable _getChildrenMergedDominatorDiff(nodeDynamic) {
     MergedDominatorDiff node = nodeDynamic;
-    final list = node.children!
+    final list = node.children
         .where((child) =>
             child.retainedSizeA >= kMinRetainedSize ||
             child.retainedSizeB >= kMinRetainedSize)
@@ -1393,11 +1392,11 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
   void _updateDominator(HtmlElement element, nodeDynamic, int depth) {
     SnapshotObject node = nodeDynamic;
     element.children[0].text = Utils.formatPercentNormalized(
-        node.retainedSize * 1.0 / _snapshotA!.size);
+        node.retainedSize * 1.0 / _snapshotA.size);
     element.children[1].text = Utils.formatSize(node.retainedSize);
     _updateLines(element.children[2].children, depth);
     if (_getChildrenDominator(node).isNotEmpty) {
-      element.children[3].text = _tree!.isExpanded(node) ? '▼' : '►';
+      element.children[3].text = _tree.isExpanded(node) ? '▼' : '►';
     } else {
       element.children[3].text = '';
     }
@@ -1423,7 +1422,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
     SnapshotObject node = nodeDynamic;
     _updateLines(element.children[0].children, depth);
     if (_getChildrenSuccessor(node).isNotEmpty) {
-      element.children[1].text = _tree!.isExpanded(node) ? '▼' : '►';
+      element.children[1].text = _tree.isExpanded(node) ? '▼' : '►';
     } else {
       element.children[1].text = '';
     }
@@ -1451,7 +1450,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
     SnapshotObject node = nodeDynamic;
     _updateLines(element.children[0].children, depth);
     if (_getChildrenSuccessor(node).isNotEmpty) {
-      element.children[1].text = _tree!.isExpanded(node) ? '▼' : '►';
+      element.children[1].text = _tree.isExpanded(node) ? '▼' : '►';
     } else {
       element.children[1].text = '';
     }
@@ -1478,11 +1477,11 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
   void _updateMergedDominator(HtmlElement element, nodeDynamic, int depth) {
     SnapshotMergedDominator node = nodeDynamic;
     element.children[0].text = Utils.formatPercentNormalized(
-        node.retainedSize * 1.0 / _snapshotA!.size);
+        node.retainedSize * 1.0 / _snapshotA.size);
     element.children[1].text = Utils.formatSize(node.retainedSize);
     _updateLines(element.children[2].children, depth);
     if (_getChildrenMergedDominator(node).isNotEmpty) {
-      element.children[3].text = _tree!.isExpanded(node) ? '▼' : '►';
+      element.children[3].text = _tree.isExpanded(node) ? '▼' : '►';
     } else {
       element.children[3].text = '';
     }
@@ -1493,10 +1492,10 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
   void _updateMergedDominatorDiff(HtmlElement element, nodeDynamic, int depth) {
     MergedDominatorDiff node = nodeDynamic;
     element.children[0].text = Utils.formatPercentNormalized(
-        node.retainedSizeA * 1.0 / _snapshotA!.size);
+        node.retainedSizeA * 1.0 / _snapshotA.size);
     element.children[1].text = Utils.formatSize(node.retainedSizeA);
     element.children[2].text = Utils.formatPercentNormalized(
-        node.retainedSizeB * 1.0 / _snapshotB!.size);
+        node.retainedSizeB * 1.0 / _snapshotB.size);
     element.children[3].text = Utils.formatSize(node.retainedSizeB);
     element.children[4].text = (node.retainedSizeDiff > 0 ? '+' : '') +
         Utils.formatSize(node.retainedSizeDiff);
@@ -1504,7 +1503,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
         node.retainedSizeDiff > 0 ? "red" : "green";
     _updateLines(element.children[5].children, depth);
     if (_getChildrenMergedDominatorDiff(node).isNotEmpty) {
-      element.children[6].text = _tree!.isExpanded(node) ? '▼' : '►';
+      element.children[6].text = _tree.isExpanded(node) ? '▼' : '►';
     } else {
       element.children[6].text = '';
     }
@@ -1516,7 +1515,7 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
   void _updateOwnership(HtmlElement element, nodeDynamic, int depth) {
     SnapshotClass node = nodeDynamic;
     element.children[0].text =
-        Utils.formatPercentNormalized(node.ownedSize * 1.0 / _snapshotA!.size);
+        Utils.formatPercentNormalized(node.ownedSize * 1.0 / _snapshotA.size);
     element.children[1].text = Utils.formatSize(node.ownedSize);
     element.children[2].text = node.name;
   }
@@ -1524,10 +1523,10 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
   void _updateOwnershipDiff(HtmlElement element, nodeDynamic, int depth) {
     SnapshotClassDiff node = nodeDynamic;
     element.children[0].text =
-        Utils.formatPercentNormalized(node.ownedSizeA * 1.0 / _snapshotA!.size);
+        Utils.formatPercentNormalized(node.ownedSizeA * 1.0 / _snapshotA.size);
     element.children[1].text = Utils.formatSize(node.ownedSizeA);
     element.children[2].text =
-        Utils.formatPercentNormalized(node.ownedSizeB * 1.0 / _snapshotB!.size);
+        Utils.formatPercentNormalized(node.ownedSizeB * 1.0 / _snapshotB.size);
     element.children[3].text = Utils.formatSize(node.ownedSizeB);
     element.children[4].text = (node.ownedSizeDiff > 0 ? "+" : "") +
         Utils.formatSize(node.ownedSizeDiff);
@@ -1537,8 +1536,8 @@ class HeapSnapshotElement extends CustomElement implements Renderable {
 
   void _updateClass(HtmlElement element, nodeDynamic, int depth) {
     SnapshotClass node = nodeDynamic;
-    element.children[0].text = Utils.formatPercentNormalized(
-        node.shallowSize * 1.0 / _snapshotA!.size);
+    element.children[0].text =
+        Utils.formatPercentNormalized(node.shallowSize * 1.0 / _snapshotA.size);
     element.children[1].text = Utils.formatSize(node.shallowSize);
     element.children[2].text = node.instanceCount.toString();
     element.children[3].text = node.name;
diff --git a/runtime/observatory/lib/src/elements/helpers/any_ref.dart b/runtime/observatory/lib/src/elements/helpers/any_ref.dart
index 3324eb6aaad..a6ef2e9f2e9 100644
--- a/runtime/observatory/lib/src/elements/helpers/any_ref.dart
+++ b/runtime/observatory/lib/src/elements/helpers/any_ref.dart
@@ -28,7 +28,7 @@ import 'package:observatory/src/elements/unknown_ref.dart';
 import 'package:observatory/src/elements/unlinkedcall_ref.dart';
 
 Element anyRef(M.IsolateRef isolate, ref, M.ObjectRepository objects,
-    {RenderingQueue? queue, bool expandable: true}) {
+    {RenderingQueue queue, bool expandable: true}) {
   if (ref == null) {
     return new SpanElement()..text = "???";
   }
diff --git a/runtime/observatory/lib/src/elements/helpers/nav_bar.dart b/runtime/observatory/lib/src/elements/helpers/nav_bar.dart
index fac7f84ef85..050041b34dc 100644
--- a/runtime/observatory/lib/src/elements/helpers/nav_bar.dart
+++ b/runtime/observatory/lib/src/elements/helpers/nav_bar.dart
@@ -4,7 +4,7 @@
 
 import 'dart:html';
 
-Element navBar(List<Element> content) {
+Element navBar(Iterable<Element> content) {
   assert(content != null);
   return document.createElement('nav')
     ..classes = ['nav-bar']
diff --git a/runtime/observatory/lib/src/elements/helpers/nav_menu.dart b/runtime/observatory/lib/src/elements/helpers/nav_menu.dart
index 5a791902168..02cf8efbc30 100644
--- a/runtime/observatory/lib/src/elements/helpers/nav_menu.dart
+++ b/runtime/observatory/lib/src/elements/helpers/nav_menu.dart
@@ -4,7 +4,7 @@
 
 import 'dart:html';
 
-navMenu(String label, {String? link, List<Element> content: const []}) {
+navMenu(String label, {String link, Iterable<Element> content: const []}) {
   assert(label != null);
   assert(content != null);
   return new LIElement()
diff --git a/runtime/observatory/lib/src/elements/helpers/rendering_scheduler.dart b/runtime/observatory/lib/src/elements/helpers/rendering_scheduler.dart
index e1796429068..5d9706ee1bb 100644
--- a/runtime/observatory/lib/src/elements/helpers/rendering_scheduler.dart
+++ b/runtime/observatory/lib/src/elements/helpers/rendering_scheduler.dart
@@ -54,7 +54,7 @@ class RenderingScheduler<T extends Renderable> implements RenderingTask {
 
   /// Creates a new scheduler for an element.
   /// If no queue is provided it will create a new default configured queue.
-  factory RenderingScheduler(T element, {RenderingQueue? queue}) {
+  factory RenderingScheduler(T element, {RenderingQueue queue}) {
     assert(element != null);
     if (queue == null) {
       queue = new RenderingQueue();
diff --git a/runtime/observatory/lib/src/elements/helpers/tag.dart b/runtime/observatory/lib/src/elements/helpers/tag.dart
index c3a8d3894c7..65c269abdf9 100644
--- a/runtime/observatory/lib/src/elements/helpers/tag.dart
+++ b/runtime/observatory/lib/src/elements/helpers/tag.dart
@@ -9,8 +9,7 @@ HtmlElement element(CustomElement e) => e.element;
 
 class CustomElement {
   static Expando reverseElements = new Expando();
-  static CustomElement reverse(HtmlElement element) =>
-      reverseElements[element] as CustomElement;
+  static CustomElement reverse(HtmlElement element) => reverseElements[element];
 
   static List<CustomElement> toBeAttached = <CustomElement>[];
   static void drainAttached() {
@@ -47,8 +46,7 @@ class CustomElement {
   }
 
   final HtmlElement element;
-  CustomElement.created(Tag tag)
-      : element = document.createElement("shadow") as HtmlElement {
+  CustomElement.created(Tag tag) : element = document.createElement("shadow") {
     reverseElements[element] = this;
     element.classes = [tag.name];
 
@@ -61,7 +59,7 @@ class CustomElement {
   void attached() {}
   void detached() {}
 
-  Element? get parent => element.parent;
+  Element get parent => element.parent;
 
   List<Element> get children => element.children;
   set children(List<Element> c) => element.children = c;
@@ -72,7 +70,7 @@ class CustomElement {
   String get title => element.title;
   set title(String t) => element.title = t;
 
-  String? get text => element.text;
+  String get text => element.text;
   set text(String t) => element.text = t;
 
   CssStyleDeclaration get style => element.style;
diff --git a/runtime/observatory/lib/src/elements/helpers/uris.dart b/runtime/observatory/lib/src/elements/helpers/uris.dart
index 020cefe9724..8a88dec9a0c 100644
--- a/runtime/observatory/lib/src/elements/helpers/uris.dart
+++ b/runtime/observatory/lib/src/elements/helpers/uris.dart
@@ -7,7 +7,7 @@ import 'package:observatory/models.dart' as M;
 /// Utility class for URIs generation.
 abstract class Uris {
   static String _isolatePage(String path, M.IsolateRef isolate,
-      {M.ObjectRef? object}) {
+      {M.ObjectRef object}) {
     final parameters = {'isolateId': isolate.id};
     if (object != null) parameters['objectId'] = object.id;
     return '#' + new Uri(path: path, queryParameters: parameters).toString();
@@ -28,7 +28,7 @@ abstract class Uris {
       _isolatePage('/heap-map', isolate);
   static String heapSnapshot(M.IsolateRef isolate) =>
       _isolatePage('/heap-snapshot', isolate);
-  static String inspect(M.IsolateRef isolate, {M.ObjectRef? object, int? pos}) {
+  static String inspect(M.IsolateRef isolate, {M.ObjectRef object, int pos}) {
     if (pos == null) {
       return _isolatePage('/inspect', isolate, object: object);
     }
diff --git a/runtime/observatory/lib/src/elements/icdata_ref.dart b/runtime/observatory/lib/src/elements/icdata_ref.dart
index f98f080371d..bcf1c0450df 100644
--- a/runtime/observatory/lib/src/elements/icdata_ref.dart
+++ b/runtime/observatory/lib/src/elements/icdata_ref.dart
@@ -12,18 +12,18 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class ICDataRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<ICDataRefElement>('icdata-ref');
 
-  late RenderingScheduler<ICDataRefElement> _r;
+  RenderingScheduler<ICDataRefElement> _r;
 
   Stream<RenderedEvent<ICDataRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ICDataRef _icdata;
+  M.IsolateRef _isolate;
+  M.ICDataRef _icdata;
 
   M.IsolateRef get isolate => _isolate;
   M.ICDataRef get icdata => _icdata;
 
   factory ICDataRefElement(M.IsolateRef isolate, M.ICDataRef icdata,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(icdata != null);
     ICDataRefElement e = new ICDataRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/icdata_view.dart b/runtime/observatory/lib/src/elements/icdata_view.dart
index 324acdb81b5..f089e5bc659 100644
--- a/runtime/observatory/lib/src/elements/icdata_view.dart
+++ b/runtime/observatory/lib/src/elements/icdata_view.dart
@@ -32,21 +32,21 @@ class ICDataViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<ICDataViewElement> _r;
+  RenderingScheduler<ICDataViewElement> _r;
 
   Stream<RenderedEvent<ICDataViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.ICData _icdata;
-  late M.ICDataRepository _icdatas;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.ICData _icdata;
+  M.ICDataRepository _icdatas;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -65,7 +65,7 @@ class ICDataViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -118,7 +118,7 @@ class ICDataViewElement extends CustomElement implements Renderable {
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
-                _icdata = await _icdatas.get(_isolate, _icdata.id!);
+                _icdata = await _icdatas.get(_isolate, _icdata.id);
                 _r.dirty();
               }))
             .element,
diff --git a/runtime/observatory/lib/src/elements/inbound_references.dart b/runtime/observatory/lib/src/elements/inbound_references.dart
index 6f41542d14f..a1e24f69090 100644
--- a/runtime/observatory/lib/src/elements/inbound_references.dart
+++ b/runtime/observatory/lib/src/elements/inbound_references.dart
@@ -15,16 +15,16 @@ class InboundReferencesElement extends CustomElement implements Renderable {
   static const tag = const Tag<InboundReferencesElement>('inbound-references',
       dependencies: const [CurlyBlockElement.tag, InstanceRefElement.tag]);
 
-  late RenderingScheduler<InboundReferencesElement> _r;
+  RenderingScheduler<InboundReferencesElement> _r;
 
   Stream<RenderedEvent<InboundReferencesElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ObjectRef _object;
-  late M.InboundReferencesRepository _references;
-  late M.ObjectRepository _objects;
-  M.InboundReferences? _inbounds;
+  M.IsolateRef _isolate;
+  M.ObjectRef _object;
+  M.InboundReferencesRepository _references;
+  M.ObjectRepository _objects;
+  M.InboundReferences _inbounds;
   bool _expanded = false;
 
   M.IsolateRef get isolate => _isolate;
@@ -32,7 +32,7 @@ class InboundReferencesElement extends CustomElement implements Renderable {
 
   factory InboundReferencesElement(M.IsolateRef isolate, M.ObjectRef object,
       M.InboundReferencesRepository references, M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(object != null);
     assert(references != null);
@@ -78,7 +78,7 @@ class InboundReferencesElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _inbounds = await _references.get(_isolate, _object.id!);
+    _inbounds = await _references.get(_isolate, _object.id);
     _r.dirty();
   }
 
@@ -86,7 +86,7 @@ class InboundReferencesElement extends CustomElement implements Renderable {
     if (_inbounds == null) {
       return const [];
     }
-    return _inbounds!.elements.map<Element>(_createItem).toList();
+    return _inbounds.elements.map<Element>(_createItem).toList();
   }
 
   Element _createItem(M.InboundReference reference) {
diff --git a/runtime/observatory/lib/src/elements/instance_ref.dart b/runtime/observatory/lib/src/elements/instance_ref.dart
index a5cab1e35a9..96db281c81f 100644
--- a/runtime/observatory/lib/src/elements/instance_ref.dart
+++ b/runtime/observatory/lib/src/elements/instance_ref.dart
@@ -17,23 +17,23 @@ class InstanceRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<InstanceRefElement>('instance-ref',
       dependencies: const [CurlyBlockElement.tag]);
 
-  late RenderingScheduler<InstanceRefElement> _r;
+  RenderingScheduler<InstanceRefElement> _r;
 
   Stream<RenderedEvent<InstanceRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.InstanceRef _instance;
-  late M.ObjectRepository _objects;
-  M.Instance? _loadedInstance;
-  late bool _expandable;
-  late bool _expanded = false;
+  M.IsolateRef _isolate;
+  M.InstanceRef _instance;
+  M.ObjectRepository _objects;
+  M.Instance _loadedInstance;
+  bool _expandable;
+  bool _expanded = false;
 
   M.IsolateRef get isolate => _isolate;
   M.InstanceRef get instance => _instance;
 
   factory InstanceRefElement(
       M.IsolateRef isolate, M.InstanceRef instance, M.ObjectRepository objects,
-      {RenderingQueue? queue, bool expandable: true}) {
+      {RenderingQueue queue, bool expandable: true}) {
     assert(isolate != null);
     assert(instance != null);
     assert(objects != null);
@@ -89,20 +89,19 @@ class InstanceRefElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _loadedInstance = await _objects.get(_isolate, _instance.id!) as M.Instance;
+    _loadedInstance = await _objects.get(_isolate, _instance.id);
     _r.dirty();
   }
 
   List<Element> _createShowMoreButton() {
-    if (_loadedInstance!.count == null) {
+    if (_loadedInstance.count == null) {
       return [];
     }
-    final count = _loadedInstance!.count;
+    final count = _loadedInstance.count;
     final button = new ButtonElement()..text = 'show next ${count}';
     button.onClick.listen((_) async {
       button.disabled = true;
-      _loadedInstance =
-          await _objects.get(_isolate, _instance.id!) as M.Instance;
+      _loadedInstance = await _objects.get(_isolate, _instance.id);
       _r.dirty();
     });
     return [button];
@@ -125,7 +124,7 @@ class InstanceRefElement extends CustomElement implements Renderable {
         return [
           new AnchorElement(href: Uris.inspect(_isolate, object: _instance))
             ..text = Utils.formatStringAsLiteral(
-                _instance.valueAsString!, _instance.valueAsStringIsTruncated!)
+                _instance.valueAsString, _instance.valueAsStringIsTruncated)
         ];
       case M.InstanceKind.type:
       case M.InstanceKind.typeRef:
@@ -141,7 +140,7 @@ class InstanceRefElement extends CustomElement implements Renderable {
               new SpanElement()
                 ..classes = ['emphasize']
                 ..text = 'Closure',
-              new SpanElement()..text = ' (${_instance.closureFunction!.name})'
+              new SpanElement()..text = ' (${_instance.closureFunction.name})'
             ]
         ];
       case M.InstanceKind.regExp:
@@ -150,8 +149,8 @@ class InstanceRefElement extends CustomElement implements Renderable {
             ..children = <Element>[
               new SpanElement()
                 ..classes = ['emphasize']
-                ..text = _instance.clazz!.name,
-              new SpanElement()..text = ' (${_instance.pattern!.valueAsString})'
+                ..text = _instance.clazz.name,
+              new SpanElement()..text = ' (${_instance.pattern.valueAsString})'
             ]
         ];
       case M.InstanceKind.stackTrace:
@@ -160,14 +159,14 @@ class InstanceRefElement extends CustomElement implements Renderable {
             ..children = <Element>[
               new SpanElement()
                 ..classes = ['emphasize']
-                ..text = _instance.clazz!.name,
+                ..text = _instance.clazz.name,
             ]
         ];
       case M.InstanceKind.plainInstance:
         return [
           new AnchorElement(href: Uris.inspect(_isolate, object: _instance))
             ..classes = ['emphasize']
-            ..text = _instance.clazz!.name
+            ..text = _instance.clazz.name
         ];
       case M.InstanceKind.list:
       case M.InstanceKind.map:
@@ -190,7 +189,7 @@ class InstanceRefElement extends CustomElement implements Renderable {
             ..children = <Element>[
               new SpanElement()
                 ..classes = ['emphasize']
-                ..text = _instance.clazz!.name,
+                ..text = _instance.clazz.name,
               new SpanElement()..text = ' (${_instance.length})'
             ]
         ];
@@ -198,13 +197,13 @@ class InstanceRefElement extends CustomElement implements Renderable {
         return [
           new AnchorElement(href: Uris.inspect(_isolate, object: _instance))
             ..classes = ['emphasize']
-            ..text = _instance.clazz!.name
+            ..text = _instance.clazz.name
         ];
       case M.InstanceKind.weakProperty:
         return [
           new AnchorElement(href: Uris.inspect(_isolate, object: _instance))
             ..classes = ['emphasize']
-            ..text = _instance.clazz!.name
+            ..text = _instance.clazz.name
         ];
     }
     throw new Exception('Unknown InstanceKind: ${_instance.kind}');
@@ -234,7 +233,7 @@ class InstanceRefElement extends CustomElement implements Renderable {
       case M.InstanceKind.int32x4List:
       case M.InstanceKind.float32x4List:
       case M.InstanceKind.float64x2List:
-        return _instance.length! > 0;
+        return _instance.length > 0;
       default:
         return false;
     }
@@ -248,30 +247,29 @@ class InstanceRefElement extends CustomElement implements Renderable {
       case M.InstanceKind.closure:
         {
           var members = <Element>[];
-          if (_loadedInstance!.closureFunction != null) {
+          if (_loadedInstance.closureFunction != null) {
             members.add(new DivElement()
               ..children = <Element>[
                 new SpanElement()..text = 'function = ',
-                anyRef(_isolate, _loadedInstance!.closureFunction, _objects,
+                anyRef(_isolate, _loadedInstance.closureFunction, _objects,
                     queue: _r.queue)
               ]);
           }
-          if (_loadedInstance!.closureContext != null) {
+          if (_loadedInstance.closureContext != null) {
             members.add(new DivElement()
               ..children = <Element>[
                 new SpanElement()..text = 'context = ',
-                anyRef(_isolate, _loadedInstance!.closureContext, _objects,
+                anyRef(_isolate, _loadedInstance.closureContext, _objects,
                     queue: _r.queue)
               ]);
           }
           return members;
         }
       case M.InstanceKind.plainInstance:
-        return _loadedInstance!.fields!
+        return _loadedInstance.fields
             .map<Element>((f) => new DivElement()
               ..children = <Element>[
-                new FieldRefElement(_isolate, f.decl!, _objects,
-                        queue: _r.queue)
+                new FieldRefElement(_isolate, f.decl, _objects, queue: _r.queue)
                     .element,
                 new SpanElement()..text = ' = ',
                 anyRef(_isolate, f.value, _objects, queue: _r.queue)
@@ -279,7 +277,7 @@ class InstanceRefElement extends CustomElement implements Renderable {
             .toList();
       case M.InstanceKind.list:
         var index = 0;
-        return _loadedInstance!.elements!
+        return _loadedInstance.elements
             .map<Element>((element) => new DivElement()
               ..children = <Element>[
                 new SpanElement()..text = '[ ${index++} ] : ',
@@ -288,7 +286,7 @@ class InstanceRefElement extends CustomElement implements Renderable {
             .toList()
               ..addAll(_createShowMoreButton());
       case M.InstanceKind.map:
-        return _loadedInstance!.associations!
+        return _loadedInstance.associations
             .map<Element>((association) => new DivElement()
               ..children = <Element>[
                 new SpanElement()..text = '[ ',
@@ -313,14 +311,14 @@ class InstanceRefElement extends CustomElement implements Renderable {
       case M.InstanceKind.float32x4List:
       case M.InstanceKind.float64x2List:
         var index = 0;
-        return _loadedInstance!.typedElements!
+        return _loadedInstance.typedElements
             .map<Element>((e) => new DivElement()..text = '[ ${index++} ] : $e')
             .toList()
               ..addAll(_createShowMoreButton());
       case M.InstanceKind.mirrorReference:
         return [
           new SpanElement()..text = '<referent> : ',
-          anyRef(_isolate, _loadedInstance!.referent, _objects, queue: _r.queue)
+          anyRef(_isolate, _loadedInstance.referent, _objects, queue: _r.queue)
         ];
       case M.InstanceKind.stackTrace:
         return [
@@ -331,12 +329,12 @@ class InstanceRefElement extends CustomElement implements Renderable {
       case M.InstanceKind.weakProperty:
         return [
           new SpanElement()..text = '<key> : ',
-          new InstanceRefElement(_isolate, _loadedInstance!.key!, _objects,
+          new InstanceRefElement(_isolate, _loadedInstance.key, _objects,
                   queue: _r.queue)
               .element,
           new BRElement(),
           new SpanElement()..text = '<value> : ',
-          new InstanceRefElement(_isolate, _loadedInstance!.value!, _objects,
+          new InstanceRefElement(_isolate, _loadedInstance.value, _objects,
                   queue: _r.queue)
               .element,
         ];
diff --git a/runtime/observatory/lib/src/elements/instance_view.dart b/runtime/observatory/lib/src/elements/instance_view.dart
index cb99e01896d..596585ede50 100644
--- a/runtime/observatory/lib/src/elements/instance_view.dart
+++ b/runtime/observatory/lib/src/elements/instance_view.dart
@@ -54,29 +54,29 @@ class InstanceViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<InstanceViewElement> _r;
+  RenderingScheduler<InstanceViewElement> _r;
 
   Stream<RenderedEvent<InstanceViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Instance _instance;
-  M.LibraryRef? _library;
-  late M.ObjectRepository _objects;
-  late M.ClassRepository _classes;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ScriptRepository _scripts;
-  late M.EvalRepository _eval;
-  M.TypeArguments? _typeArguments;
-  late M.TypeArgumentsRepository _arguments;
-  late M.BreakpointRepository _breakpoints;
-  late M.FunctionRepository _functions;
-  M.SourceLocation? _location;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Instance _instance;
+  M.LibraryRef _library;
+  M.ObjectRepository _objects;
+  M.ClassRepository _classes;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ScriptRepository _scripts;
+  M.EvalRepository _eval;
+  M.TypeArguments _typeArguments;
+  M.TypeArgumentsRepository _arguments;
+  M.BreakpointRepository _breakpoints;
+  M.FunctionRepository _functions;
+  M.SourceLocation _location;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -100,7 +100,7 @@ class InstanceViewElement extends CustomElement implements Renderable {
       M.TypeArgumentsRepository arguments,
       M.BreakpointRepository breakpoints,
       M.FunctionRepository functions,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -159,7 +159,7 @@ class InstanceViewElement extends CustomElement implements Renderable {
       new HeadingElement.h2()
         ..text = M.isAbstractType(_instance.kind)
             ? 'type ${_instance.name}'
-            : 'instance of ${_instance.clazz!.name}',
+            : 'instance of ${_instance.clazz.name}',
       new HRElement(),
       new ObjectCommonElement(_isolate, _instance, _retainedSizes,
               _reachableSizes, _references, _retainingPaths, _objects,
@@ -178,8 +178,7 @@ class InstanceViewElement extends CustomElement implements Renderable {
     if (_location != null) {
       content.addAll([
         new HRElement(),
-        new SourceInsetElement(
-                _isolate, _location!, _scripts, _objects, _events,
+        new SourceInsetElement(_isolate, _location, _scripts, _objects, _events,
                 queue: _r.queue)
             .element
       ]);
@@ -201,11 +200,11 @@ class InstanceViewElement extends CustomElement implements Renderable {
       new NavIsolateMenuElement(_isolate, _events, queue: _r.queue).element
     ];
     if (_library != null) {
-      menu.add(new NavLibraryMenuElement(_isolate, _library!, queue: _r.queue)
+      menu.add(new NavLibraryMenuElement(_isolate, _library, queue: _r.queue)
           .element);
     }
     menu.addAll(<Element>[
-      new NavClassMenuElement(_isolate, _instance.clazz!, queue: _r.queue)
+      new NavClassMenuElement(_isolate, _instance.clazz, queue: _r.queue)
           .element,
       navMenu('instance'),
       (new NavRefreshElement(queue: _r.queue)
@@ -246,8 +245,8 @@ class InstanceViewElement extends CustomElement implements Renderable {
       if (_instance.kind == M.InstanceKind.string) {
         members.add(member(
             'value as literal',
-            Utils.formatStringAsLiteral(_instance.valueAsString!,
-                _instance.valueAsStringIsTruncated!)));
+            Utils.formatStringAsLiteral(
+                _instance.valueAsString, _instance.valueAsStringIsTruncated)));
       } else {
         members.add(member('value', _instance.valueAsString));
       }
@@ -255,7 +254,7 @@ class InstanceViewElement extends CustomElement implements Renderable {
     if (_instance.typeClass != null) {
       members.add(member('type class', _instance.typeClass));
     }
-    if (_typeArguments != null && _typeArguments!.types!.isNotEmpty) {
+    if (_typeArguments != null && _typeArguments.types.isNotEmpty) {
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
@@ -265,7 +264,7 @@ class InstanceViewElement extends CustomElement implements Renderable {
           new DivElement()
             ..classes = ['memberValue']
             ..children = ([new SpanElement()..text = '< ']
-              ..addAll(_typeArguments!.types!.expand((type) => [
+              ..addAll(_typeArguments.types.expand((type) => [
                     new InstanceRefElement(_isolate, type, _objects,
                             queue: _r.queue)
                         .element,
@@ -294,7 +293,7 @@ class InstanceViewElement extends CustomElement implements Renderable {
       members.add(member('closure context', _instance.closureContext));
     }
     if (_instance.kind == M.InstanceKind.closure) {
-      late ButtonElement btn;
+      ButtonElement btn;
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
@@ -316,24 +315,24 @@ class InstanceViewElement extends CustomElement implements Renderable {
         ]);
     }
 
-    if (_instance.nativeFields != null && _instance.nativeFields!.isNotEmpty) {
+    if (_instance.nativeFields != null && _instance.nativeFields.isNotEmpty) {
       int i = 0;
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
           new DivElement()
             ..classes = ['memberName']
-            ..text = 'native fields (${_instance.nativeFields!.length})',
+            ..text = 'native fields (${_instance.nativeFields.length})',
           new DivElement()
             ..classes = ['memberName']
             ..children = <Element>[
               (new CurlyBlockElement(
-                      expanded: _instance.nativeFields!.length <= 100,
+                      expanded: _instance.nativeFields.length <= 100,
                       queue: _r.queue)
                     ..content = <Element>[
                       new DivElement()
                         ..classes = ['memberList']
-                        ..children = _instance.nativeFields!
+                        ..children = _instance.nativeFields
                             .map<Element>(
                                 (f) => member('[ ${i++} ]', '[ ${f.value} ]'))
                             .toList()
@@ -343,8 +342,8 @@ class InstanceViewElement extends CustomElement implements Renderable {
         ]);
     }
 
-    if (_instance.fields != null && _instance.fields!.isNotEmpty) {
-      final fields = _instance.fields!.toList();
+    if (_instance.fields != null && _instance.fields.isNotEmpty) {
+      final fields = _instance.fields.toList();
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
@@ -368,8 +367,8 @@ class InstanceViewElement extends CustomElement implements Renderable {
         ]);
     }
 
-    if (_instance.elements != null && _instance.elements!.isNotEmpty) {
-      final elements = _instance.elements!.toList();
+    if (_instance.elements != null && _instance.elements.isNotEmpty) {
+      final elements = _instance.elements.toList();
       int i = 0;
       members.add(new DivElement()
         ..classes = ['memberItem']
@@ -395,12 +394,12 @@ class InstanceViewElement extends CustomElement implements Renderable {
         ]);
       if (_instance.length != elements.length) {
         members.add(member(
-            '...', '${_instance.length! - elements.length} omitted elements'));
+            '...', '${_instance.length - elements.length} omitted elements'));
       }
     }
 
-    if (_instance.associations != null && _instance.associations!.isNotEmpty) {
-      final associations = _instance.associations!.toList();
+    if (_instance.associations != null && _instance.associations.isNotEmpty) {
+      final associations = _instance.associations.toList();
       members.add(new DivElement()
         ..classes = ['memberItem']
         ..children = <Element>[
@@ -441,13 +440,12 @@ class InstanceViewElement extends CustomElement implements Renderable {
         ]);
       if (_instance.length != associations.length) {
         members.add(member('...',
-            '${_instance.length! - associations.length} omitted elements'));
+            '${_instance.length - associations.length} omitted elements'));
       }
     }
 
-    if (_instance.typedElements != null &&
-        _instance.typedElements!.isNotEmpty) {
-      final typedElements = _instance.typedElements!.toList();
+    if (_instance.typedElements != null && _instance.typedElements.isNotEmpty) {
+      final typedElements = _instance.typedElements.toList();
       int i = 0;
       members.add(new DivElement()
         ..classes = ['memberItem']
@@ -472,15 +470,15 @@ class InstanceViewElement extends CustomElement implements Renderable {
         ]);
       if (_instance.length != typedElements.length) {
         members.add(member('...',
-            '${_instance.length! - typedElements.length} omitted elements'));
+            '${_instance.length - typedElements.length} omitted elements'));
       }
     }
 
     if (_instance.kind == M.InstanceKind.regExp) {
       members.add(member('pattern', _instance.pattern));
       members.add(
-          member('isCaseSensitive', _instance.isCaseSensitive! ? 'yes' : 'no'));
-      members.add(member('isMultiLine', _instance.isMultiLine! ? 'yes' : 'no'));
+          member('isCaseSensitive', _instance.isCaseSensitive ? 'yes' : 'no'));
+      members.add(member('isMultiLine', _instance.isMultiLine ? 'yes' : 'no'));
       if (_instance.oneByteFunction != null) {
         members.add(member('oneByteFunction', _instance.oneByteFunction));
       }
@@ -516,26 +514,25 @@ class InstanceViewElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _instance = await _objects.get(_isolate, _instance.id!) as M.Instance;
+    _instance = await _objects.get(_isolate, _instance.id);
     await _loadExtraData();
     _r.dirty();
   }
 
   Future _loadExtraData() async {
-    _library = (await _classes.get(_isolate, _instance.clazz!.id!)).library!;
+    _library = (await _classes.get(_isolate, _instance.clazz.id)).library;
     if (_instance.typeArguments != null) {
       _typeArguments =
-          await _arguments.get(_isolate, _instance.typeArguments!.id!);
+          await _arguments.get(_isolate, _instance.typeArguments.id);
     } else {
       _typeArguments = null;
     }
     if (_instance.closureFunction != null) {
-      _location =
-          (await _functions.get(_isolate, _instance.closureFunction!.id!))
-              .location;
+      _location = (await _functions.get(_isolate, _instance.closureFunction.id))
+          .location;
     } else if (_instance.typeClass != null) {
       _location =
-          (await _classes.get(_isolate, _instance.typeClass!.id!)).location;
+          (await _classes.get(_isolate, _instance.typeClass.id)).location;
     }
     _r.dirty();
   }
@@ -544,7 +541,7 @@ class InstanceViewElement extends CustomElement implements Renderable {
     if (_instance.activationBreakpoint == null) {
       await _breakpoints.addOnActivation(_isolate, _instance);
     } else {
-      await _breakpoints.remove(_isolate, _instance.activationBreakpoint!);
+      await _breakpoints.remove(_isolate, _instance.activationBreakpoint);
     }
     await _refresh();
   }
diff --git a/runtime/observatory/lib/src/elements/isolate/counter_chart.dart b/runtime/observatory/lib/src/elements/isolate/counter_chart.dart
index fb403a51a3e..3c68990b7e4 100644
--- a/runtime/observatory/lib/src/elements/isolate/counter_chart.dart
+++ b/runtime/observatory/lib/src/elements/isolate/counter_chart.dart
@@ -11,15 +11,15 @@ class IsolateCounterChartElement extends CustomElement implements Renderable {
   static const tag =
       const Tag<IsolateCounterChartElement>('isolate-counter-chart');
 
-  late RenderingScheduler<IsolateCounterChartElement> _r;
+  RenderingScheduler<IsolateCounterChartElement> _r;
 
   Stream<RenderedEvent<IsolateCounterChartElement>> get onRendered =>
       _r.onRendered;
 
-  late Map _counters;
-  late StreamSubscription _subscription;
+  Map _counters;
+  StreamSubscription _subscription;
 
-  factory IsolateCounterChartElement(Map counters, {RenderingQueue? queue}) {
+  factory IsolateCounterChartElement(Map counters, {RenderingQueue queue}) {
     assert(counters != null);
     IsolateCounterChartElement e = new IsolateCounterChartElement.created();
     e._r = new RenderingScheduler<IsolateCounterChartElement>(e, queue: queue);
diff --git a/runtime/observatory/lib/src/elements/isolate/location.dart b/runtime/observatory/lib/src/elements/isolate/location.dart
index 8cabd97e429..e10e3d2a53f 100644
--- a/runtime/observatory/lib/src/elements/isolate/location.dart
+++ b/runtime/observatory/lib/src/elements/isolate/location.dart
@@ -14,19 +14,19 @@ class IsolateLocationElement extends CustomElement implements Renderable {
   static const tag = const Tag<IsolateLocationElement>('isolate-location',
       dependencies: const [FunctionRefElement.tag, SourceLinkElement.tag]);
 
-  late RenderingScheduler<IsolateLocationElement> _r;
+  RenderingScheduler<IsolateLocationElement> _r;
 
   Stream<RenderedEvent<IsolateLocationElement>> get onRendered => _r.onRendered;
 
-  late M.Isolate _isolate;
-  late M.EventRepository _events;
-  late M.ScriptRepository _scripts;
-  late StreamSubscription _debugSubscription;
-  late StreamSubscription _isolateSubscription;
+  M.Isolate _isolate;
+  M.EventRepository _events;
+  M.ScriptRepository _scripts;
+  StreamSubscription _debugSubscription;
+  StreamSubscription _isolateSubscription;
 
   factory IsolateLocationElement(
       M.Isolate isolate, M.EventRepository events, M.ScriptRepository scripts,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(events != null);
     assert(scripts != null);
@@ -66,12 +66,12 @@ class IsolateLocationElement extends CustomElement implements Renderable {
         children = <Element>[
           new SpanElement()..text = 'at ',
           new FunctionRefElement(
-                  _isolate, M.topFrame(_isolate.pauseEvent)!.function!,
+                  _isolate, M.topFrame(_isolate.pauseEvent).function,
                   queue: _r.queue)
               .element,
           new SpanElement()..text = ' (',
-          new SourceLinkElement(_isolate,
-                  M.topFrame(_isolate.pauseEvent)!.location!, _scripts,
+          new SourceLinkElement(
+                  _isolate, M.topFrame(_isolate.pauseEvent).location, _scripts,
                   queue: _r.queue)
               .element,
           new SpanElement()..text = ') '
@@ -95,12 +95,12 @@ class IsolateLocationElement extends CustomElement implements Renderable {
             content.addAll([
               new SpanElement()..text = ' at ',
               new FunctionRefElement(
-                      _isolate, M.topFrame(_isolate.pauseEvent)!.function!,
+                      _isolate, M.topFrame(_isolate.pauseEvent).function,
                       queue: _r.queue)
                   .element,
               new SpanElement()..text = ' (',
               new SourceLinkElement(_isolate,
-                      M.topFrame(_isolate.pauseEvent)!.location!, _scripts,
+                      M.topFrame(_isolate.pauseEvent).location, _scripts,
                       queue: _r.queue)
                   .element,
               new SpanElement()..text = ') '
diff --git a/runtime/observatory/lib/src/elements/isolate/run_state.dart b/runtime/observatory/lib/src/elements/isolate/run_state.dart
index 03624a09c3b..77ff052f479 100644
--- a/runtime/observatory/lib/src/elements/isolate/run_state.dart
+++ b/runtime/observatory/lib/src/elements/isolate/run_state.dart
@@ -11,17 +11,17 @@ import 'package:observatory/src/elements/helpers/tag.dart';
 class IsolateRunStateElement extends CustomElement implements Renderable {
   static const tag = const Tag<IsolateRunStateElement>('isolate-run-state');
 
-  late RenderingScheduler<IsolateRunStateElement> _r;
+  RenderingScheduler<IsolateRunStateElement> _r;
 
   Stream<RenderedEvent<IsolateRunStateElement>> get onRendered => _r.onRendered;
 
-  late M.Isolate _isolate;
-  late M.EventRepository _events;
-  late StreamSubscription _debugSubscription;
-  late StreamSubscription _isolateSubscription;
+  M.Isolate _isolate;
+  M.EventRepository _events;
+  StreamSubscription _debugSubscription;
+  StreamSubscription _isolateSubscription;
 
   factory IsolateRunStateElement(M.Isolate isolate, M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(events != null);
     IsolateRunStateElement e = new IsolateRunStateElement.created();
@@ -64,7 +64,7 @@ class IsolateRunStateElement extends CustomElement implements Renderable {
       case M.IsolateStatus.paused:
         children = <Element>[
           new SpanElement()
-            ..title = '${_isolate.pauseEvent!.timestamp}'
+            ..title = '${_isolate.pauseEvent.timestamp}'
             ..text = 'paused '
         ];
         break;
diff --git a/runtime/observatory/lib/src/elements/isolate/shared_summary.dart b/runtime/observatory/lib/src/elements/isolate/shared_summary.dart
index 79cd2e6ec21..89be8264548 100644
--- a/runtime/observatory/lib/src/elements/isolate/shared_summary.dart
+++ b/runtime/observatory/lib/src/elements/isolate/shared_summary.dart
@@ -16,18 +16,18 @@ class IsolateSharedSummaryElement extends CustomElement implements Renderable {
       'isolate-shared-summary',
       dependencies: const [IsolateCounterChartElement.tag]);
 
-  late RenderingScheduler<IsolateSharedSummaryElement> _r;
+  RenderingScheduler<IsolateSharedSummaryElement> _r;
 
   Stream<RenderedEvent<IsolateSharedSummaryElement>> get onRendered =>
       _r.onRendered;
 
-  late M.Isolate _isolate;
-  late M.EventRepository _events;
-  late StreamSubscription _isolateSubscription;
+  M.Isolate _isolate;
+  M.EventRepository _events;
+  StreamSubscription _isolateSubscription;
 
   factory IsolateSharedSummaryElement(
       M.Isolate isolate, M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(events != null);
     IsolateSharedSummaryElement e = new IsolateSharedSummaryElement.created();
@@ -55,10 +55,10 @@ class IsolateSharedSummaryElement extends CustomElement implements Renderable {
   }
 
   void render() {
-    final newHeapUsed = Utils.formatSize(_isolate.newSpace!.used);
-    final newHeapCapacity = Utils.formatSize(_isolate.newSpace!.capacity);
-    final oldHeapUsed = Utils.formatSize(_isolate.oldSpace!.used);
-    final oldHeapCapacity = Utils.formatSize(_isolate.oldSpace!.capacity);
+    final newHeapUsed = Utils.formatSize(_isolate.newSpace.used);
+    final newHeapCapacity = Utils.formatSize(_isolate.newSpace.capacity);
+    final oldHeapUsed = Utils.formatSize(_isolate.oldSpace.used);
+    final oldHeapCapacity = Utils.formatSize(_isolate.oldSpace.capacity);
     final content = <Element>[
       new DivElement()
         ..classes = ['menu']
@@ -151,14 +151,13 @@ class IsolateSharedSummaryElement extends CustomElement implements Renderable {
               new AnchorElement(href: Uris.logging(_isolate))..text = 'logging'
             ]
         ],
-      new IsolateCounterChartElement(_isolate.counters!, queue: _r.queue)
-          .element
+      new IsolateCounterChartElement(_isolate.counters, queue: _r.queue).element
     ];
     if (_isolate.error != null) {
       children = <Element>[
         new PreElement()
           ..classes = ['errorBox']
-          ..text = _isolate.error!.message,
+          ..text = _isolate.error.message,
         new DivElement()
           ..classes = ['summary']
           ..children = content
diff --git a/runtime/observatory/lib/src/elements/isolate/summary.dart b/runtime/observatory/lib/src/elements/isolate/summary.dart
index 3bf553b5e27..1ecd032b90a 100644
--- a/runtime/observatory/lib/src/elements/isolate/summary.dart
+++ b/runtime/observatory/lib/src/elements/isolate/summary.dart
@@ -23,22 +23,22 @@ class IsolateSummaryElement extends CustomElement implements Renderable {
     IsolateSharedSummaryElement.tag
   ]);
 
-  late RenderingScheduler<IsolateSummaryElement> _r;
+  RenderingScheduler<IsolateSummaryElement> _r;
 
   Stream<RenderedEvent<IsolateSummaryElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.IsolateRepository _isolates;
-  late M.ScriptRepository _scripts;
-  M.Isolate? _loadedIsolate;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.IsolateRepository _isolates;
+  M.ScriptRepository _scripts;
+  M.Isolate _loadedIsolate;
 
   factory IsolateSummaryElement(
       M.IsolateRef isolate,
       M.IsolateRepository isolates,
       M.EventRepository events,
       M.ScriptRepository scripts,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(isolates != null);
     assert(events != null);
@@ -98,10 +98,9 @@ class IsolateSummaryElement extends CustomElement implements Renderable {
         new DivElement()
           ..classes = ['flex-row', 'isolate-state-container']
           ..children = <Element>[
-            new IsolateRunStateElement(_isolate as M.Isolate, _events,
-                    queue: _r.queue)
+            new IsolateRunStateElement(_isolate, _events, queue: _r.queue)
                 .element,
-            new IsolateLocationElement(_isolate as M.Isolate, _events, _scripts,
+            new IsolateLocationElement(_isolate, _events, _scripts,
                     queue: _r.queue)
                 .element,
             new SpanElement()..text = ' [',
@@ -113,14 +112,14 @@ class IsolateSummaryElement extends CustomElement implements Renderable {
 
   Element memoryRow() {
     final isolate = _isolate as M.Isolate;
-    final newHeapUsed = Utils.formatSize(isolate.newSpace!.used);
-    final newHeapCapacity = Utils.formatSize(isolate.newSpace!.capacity);
-    final oldHeapUsed = Utils.formatSize(isolate.oldSpace!.used);
-    final oldHeapCapacity = Utils.formatSize(isolate.oldSpace!.capacity);
+    final newHeapUsed = Utils.formatSize(isolate.newSpace.used);
+    final newHeapCapacity = Utils.formatSize(isolate.newSpace.capacity);
+    final oldHeapUsed = Utils.formatSize(isolate.oldSpace.used);
+    final oldHeapCapacity = Utils.formatSize(isolate.oldSpace.capacity);
     final heapUsed =
-        Utils.formatSize(isolate.newSpace!.used + isolate.oldSpace!.used);
-    final heapCapacity = Utils.formatSize(
-        isolate.newSpace!.capacity + isolate.oldSpace!.capacity);
+        Utils.formatSize(isolate.newSpace.used + isolate.oldSpace.used);
+    final heapCapacity =
+        Utils.formatSize(isolate.newSpace.capacity + isolate.oldSpace.capacity);
     return new DivElement()
       ..classes = ['flex-row-wrap-right']
       ..children = <Element>[
diff --git a/runtime/observatory/lib/src/elements/isolate_reconnect.dart b/runtime/observatory/lib/src/elements/isolate_reconnect.dart
index 2c4d2f29746..8031aa3c214 100644
--- a/runtime/observatory/lib/src/elements/isolate_reconnect.dart
+++ b/runtime/observatory/lib/src/elements/isolate_reconnect.dart
@@ -23,25 +23,25 @@ class IsolateReconnectElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<IsolateReconnectElement> _r;
+  RenderingScheduler<IsolateReconnectElement> _r;
 
   Stream<RenderedEvent<IsolateReconnectElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late String _missing;
-  late Uri _uri;
-  late M.EventRepository _events;
-  late StreamSubscription _subscription;
+  M.VM _vm;
+  String _missing;
+  Uri _uri;
+  M.EventRepository _events;
+  StreamSubscription _subscription;
 
   M.VM get vm => _vm;
   String get missing => _missing;
   Uri get uri => _uri;
 
-  late M.NotificationRepository _notifications;
+  M.NotificationRepository _notifications;
   factory IsolateReconnectElement(M.VM vm, M.EventRepository events,
       M.NotificationRepository notifications, String missing, Uri uri,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(events != null);
     assert(missing != null);
@@ -63,7 +63,7 @@ class IsolateReconnectElement extends CustomElement implements Renderable {
   void attached() {
     super.attached();
     _subscription = _events.onVMUpdate.listen((e) {
-      _vm = e.vm as M.VM;
+      _vm = e.vm;
       _r.dirty();
     });
     _r.enable();
diff --git a/runtime/observatory/lib/src/elements/isolate_ref.dart b/runtime/observatory/lib/src/elements/isolate_ref.dart
index 24c66455da2..32e32a4ccdd 100644
--- a/runtime/observatory/lib/src/elements/isolate_ref.dart
+++ b/runtime/observatory/lib/src/elements/isolate_ref.dart
@@ -14,18 +14,18 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class IsolateRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<IsolateRefElement>('isolate-ref');
 
-  late RenderingScheduler<IsolateRefElement> _r;
+  RenderingScheduler<IsolateRefElement> _r;
 
   Stream<RenderedEvent<IsolateRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late StreamSubscription _updatesSubscription;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  StreamSubscription _updatesSubscription;
 
   M.IsolateRef get isolate => _isolate;
 
   factory IsolateRefElement(M.IsolateRef isolate, M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(events != null);
     IsolateRefElement e = new IsolateRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/isolate_view.dart b/runtime/observatory/lib/src/elements/isolate_view.dart
index 3a512c6b910..9db502bcbef 100644
--- a/runtime/observatory/lib/src/elements/isolate_view.dart
+++ b/runtime/observatory/lib/src/elements/isolate_view.dart
@@ -51,23 +51,23 @@ class IsolateViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<IsolateViewElement> _r;
+  RenderingScheduler<IsolateViewElement> _r;
 
   Stream<RenderedEvent<IsolateViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.Isolate _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.IsolateRepository _isolates;
-  late M.ScriptRepository _scripts;
-  late M.FunctionRepository _functions;
-  late M.LibraryRepository _libraries;
-  late M.ObjectRepository _objects;
-  late M.EvalRepository _eval;
-  M.ServiceFunction? _function;
-  M.ScriptRef? _rootScript;
-  late StreamSubscription _subscription;
+  M.VM _vm;
+  M.Isolate _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.IsolateRepository _isolates;
+  M.ScriptRepository _scripts;
+  M.FunctionRepository _functions;
+  M.LibraryRepository _libraries;
+  M.ObjectRepository _objects;
+  M.EvalRepository _eval;
+  M.ServiceFunction _function;
+  M.ScriptRef _rootScript;
+  StreamSubscription _subscription;
 
   M.VMRef get vm => _vm;
   M.Isolate get isolate => _isolate;
@@ -84,7 +84,7 @@ class IsolateViewElement extends CustomElement implements Renderable {
       M.LibraryRepository libraries,
       M.ObjectRepository objects,
       M.EvalRepository eval,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -134,9 +134,9 @@ class IsolateViewElement extends CustomElement implements Renderable {
   }
 
   void render() {
-    final uptime = new DateTime.now().difference(_isolate.startTime!);
-    final libraries = _isolate.libraries!.toList();
-    final List<M.Thread> threads = _isolate.threads!.toList();
+    final uptime = new DateTime.now().difference(_isolate.startTime);
+    final libraries = _isolate.libraries.toList();
+    final List<M.Thread> threads = _isolate.threads;
     children = <Element>[
       navBar(<Element>[
         new NavTopMenuElement(queue: _r.queue).element,
@@ -185,11 +185,11 @@ class IsolateViewElement extends CustomElement implements Renderable {
             ..children = _function != null
                 ? [
                     new BRElement(),
-                    (new SourceInsetElement(_isolate, _function!.location!,
+                    (new SourceInsetElement(_isolate, _function.location,
                             _scripts, _objects, _events,
                             currentPos: M
-                                .topFrame(isolate.pauseEvent)!
-                                .location!
+                                .topFrame(isolate.pauseEvent)
+                                .location
                                 .tokenPos,
                             queue: _r.queue)
                           ..classes = ['header_inset'])
@@ -235,7 +235,7 @@ class IsolateViewElement extends CustomElement implements Renderable {
                       _isolate.rootLibrary == null
                           ? (new SpanElement()..text = 'loading...')
                           : new LibraryRefElement(
-                                  _isolate, _isolate.rootLibrary!,
+                                  _isolate, _isolate.rootLibrary,
                                   queue: _r.queue)
                               .element
                     ]
@@ -250,7 +250,7 @@ class IsolateViewElement extends CustomElement implements Renderable {
                         new DivElement()
                           ..classes = ['memberValue']
                           ..children = <Element>[
-                            new FunctionRefElement(_isolate, _isolate.entry!,
+                            new FunctionRefElement(_isolate, _isolate.entry,
                                     queue: _r.queue)
                                 .element
                           ]
@@ -361,7 +361,7 @@ class IsolateViewElement extends CustomElement implements Renderable {
                 ]
             ],
           new HRElement(),
-          new EvalBoxElement(_isolate, _isolate.rootLibrary!, _objects, _eval,
+          new EvalBoxElement(_isolate, _isolate.rootLibrary, _objects, _eval,
                   queue: _r.queue)
               .element,
           new DivElement()
@@ -369,7 +369,7 @@ class IsolateViewElement extends CustomElement implements Renderable {
                 ? [
                     new HRElement(),
                     new ScriptInsetElement(
-                            _isolate, _rootScript!, _scripts, _objects, _events,
+                            _isolate, _rootScript, _scripts, _objects, _events,
                             queue: _r.queue)
                         .element
                   ]
@@ -410,11 +410,11 @@ class IsolateViewElement extends CustomElement implements Renderable {
     _rootScript = null;
     final frame = M.topFrame(_isolate.pauseEvent);
     if (frame != null) {
-      _function = await _functions.get(_isolate, frame.function!.id!);
+      _function = await _functions.get(_isolate, frame.function.id);
     }
     if (_isolate.rootLibrary != null) {
       final rootLibrary =
-          await _libraries.get(_isolate, _isolate.rootLibrary!.id!);
+          await _libraries.get(_isolate, _isolate.rootLibrary.id);
       _rootScript = rootLibrary.rootScript;
     }
     _r.dirty();
diff --git a/runtime/observatory/lib/src/elements/json_view.dart b/runtime/observatory/lib/src/elements/json_view.dart
index e76774ec175..7f1474bacdb 100644
--- a/runtime/observatory/lib/src/elements/json_view.dart
+++ b/runtime/observatory/lib/src/elements/json_view.dart
@@ -22,18 +22,18 @@ class JSONViewElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<JSONViewElement> _r;
+  RenderingScheduler<JSONViewElement> _r;
 
   Stream<RenderedEvent<JSONViewElement>> get onRendered => _r.onRendered;
 
-  late M.NotificationRepository _notifications;
-  late Map _map;
+  M.NotificationRepository _notifications;
+  Map _map;
 
   M.NotificationRepository get notifications => _notifications;
   Map get map => _map;
 
   factory JSONViewElement(Map map, M.NotificationRepository notifications,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(notifications != null);
     assert(map != null);
     JSONViewElement e = new JSONViewElement.created();
diff --git a/runtime/observatory/lib/src/elements/library_ref.dart b/runtime/observatory/lib/src/elements/library_ref.dart
index 7a855b167ce..a6d3ba8f2ad 100644
--- a/runtime/observatory/lib/src/elements/library_ref.dart
+++ b/runtime/observatory/lib/src/elements/library_ref.dart
@@ -14,18 +14,18 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class LibraryRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<LibraryRefElement>('library-ref');
 
-  late RenderingScheduler<LibraryRefElement> _r;
+  RenderingScheduler<LibraryRefElement> _r;
 
   Stream<RenderedEvent<LibraryRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.LibraryRef _library;
+  M.IsolateRef _isolate;
+  M.LibraryRef _library;
 
   M.IsolateRef get isolate => _isolate;
   M.LibraryRef get library => _library;
 
   factory LibraryRefElement(M.IsolateRef isolate, M.LibraryRef library,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(library != null);
     LibraryRefElement e = new LibraryRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/library_view.dart b/runtime/observatory/lib/src/elements/library_view.dart
index 816bab41726..b9c3d5c0e0b 100644
--- a/runtime/observatory/lib/src/elements/library_view.dart
+++ b/runtime/observatory/lib/src/elements/library_view.dart
@@ -49,25 +49,25 @@ class LibraryViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<LibraryViewElement> _r;
+  RenderingScheduler<LibraryViewElement> _r;
 
   Stream<RenderedEvent<LibraryViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Library _library;
-  late M.LibraryRepository _libraries;
-  late M.FieldRepository _fields;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ScriptRepository _scripts;
-  late M.ObjectRepository _objects;
-  late M.EvalRepository _eval;
-  Iterable<M.Field>? _variables;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Library _library;
+  M.LibraryRepository _libraries;
+  M.FieldRepository _fields;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ScriptRepository _scripts;
+  M.ObjectRepository _objects;
+  M.EvalRepository _eval;
+  Iterable<M.Field> _variables;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -89,7 +89,7 @@ class LibraryViewElement extends CustomElement implements Renderable {
       M.ScriptRepository scripts,
       M.ObjectRepository objects,
       M.EvalRepository eval,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -217,19 +217,19 @@ class LibraryViewElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _library = await _libraries.get(_isolate, _library.id!);
+    _library = await _libraries.get(_isolate, _library.id);
     _variables = null;
     _r.dirty();
     _variables = await Future.wait(
-        _library.variables!.map((field) => _fields.get(_isolate, field.id!)));
+        _library.variables.map((field) => _fields.get(_isolate, field.id)));
     _r.dirty();
   }
 
   Element _createDependencies() {
-    if (_library.dependencies!.isEmpty) {
+    if (_library.dependencies.isEmpty) {
       return new SpanElement();
     }
-    final dependencies = _library.dependencies!.toList();
+    final dependencies = _library.dependencies.toList();
     return new DivElement()
       ..children = <Element>[
         new SpanElement()..text = 'dependencies (${dependencies.length}) ',
@@ -252,10 +252,10 @@ class LibraryViewElement extends CustomElement implements Renderable {
   }
 
   Element _createScripts() {
-    if (_library.scripts!.isEmpty) {
+    if (_library.scripts.isEmpty) {
       return new SpanElement();
     }
-    final scripts = _library.scripts!.toList();
+    final scripts = _library.scripts.toList();
     return new DivElement()
       ..children = <Element>[
         new SpanElement()..text = 'scripts (${scripts.length}) ',
@@ -272,10 +272,10 @@ class LibraryViewElement extends CustomElement implements Renderable {
   }
 
   Element _createClasses() {
-    if (_library.classes!.isEmpty) {
+    if (_library.classes.isEmpty) {
       return new SpanElement();
     }
-    final classes = _library.classes!.toList();
+    final classes = _library.classes.toList();
     return new DivElement()
       ..children = <Element>[
         new SpanElement()..text = 'classes (${classes.length}) ',
@@ -292,10 +292,10 @@ class LibraryViewElement extends CustomElement implements Renderable {
   }
 
   Element _createVariables() {
-    if (_library.variables!.isEmpty) {
+    if (_library.variables.isEmpty) {
       return new SpanElement();
     }
-    final variables = _library.variables!.toList();
+    final variables = _library.variables.toList();
     return new DivElement()
       ..children = <Element>[
         new SpanElement()..text = 'variables (${variables.length}) ',
@@ -305,7 +305,7 @@ class LibraryViewElement extends CustomElement implements Renderable {
                     ? (new SpanElement()..text = 'loading...')
                     : (new DivElement()
                       ..classes = ['indent', 'memberList']
-                      ..children = _variables!
+                      ..children = _variables
                           .map<Element>((f) => new DivElement()
                             ..classes = ['memberItem']
                             ..children = <Element>[
@@ -331,10 +331,10 @@ class LibraryViewElement extends CustomElement implements Renderable {
   }
 
   Element _createFunctions() {
-    if (_library.functions!.isEmpty) {
+    if (_library.functions.isEmpty) {
       return new SpanElement();
     }
-    final functions = _library.functions!.toList();
+    final functions = _library.functions.toList();
     return new DivElement()
       ..children = <Element>[
         new SpanElement()..text = 'functions (${functions.length}) ',
diff --git a/runtime/observatory/lib/src/elements/local_var_descriptors_ref.dart b/runtime/observatory/lib/src/elements/local_var_descriptors_ref.dart
index 9ba3af507de..e5afd6373a0 100644
--- a/runtime/observatory/lib/src/elements/local_var_descriptors_ref.dart
+++ b/runtime/observatory/lib/src/elements/local_var_descriptors_ref.dart
@@ -14,20 +14,20 @@ class LocalVarDescriptorsRefElement extends CustomElement
     implements Renderable {
   static const tag = const Tag<LocalVarDescriptorsRefElement>('var-ref');
 
-  late RenderingScheduler<LocalVarDescriptorsRefElement> _r;
+  RenderingScheduler<LocalVarDescriptorsRefElement> _r;
 
   Stream<RenderedEvent<LocalVarDescriptorsRefElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.LocalVarDescriptorsRef _localVar;
+  M.IsolateRef _isolate;
+  M.LocalVarDescriptorsRef _localVar;
 
   M.IsolateRef get isolate => _isolate;
   M.LocalVarDescriptorsRef get localVar => _localVar;
 
   factory LocalVarDescriptorsRefElement(
       M.IsolateRef isolate, M.LocalVarDescriptorsRef localVar,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(localVar != null);
     LocalVarDescriptorsRefElement e =
diff --git a/runtime/observatory/lib/src/elements/logging.dart b/runtime/observatory/lib/src/elements/logging.dart
index 3196a088c89..523701033ef 100644
--- a/runtime/observatory/lib/src/elements/logging.dart
+++ b/runtime/observatory/lib/src/elements/logging.dart
@@ -34,15 +34,15 @@ class LoggingPageElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<LoggingPageElement> _r;
+  RenderingScheduler<LoggingPageElement> _r;
 
   Stream<RenderedEvent<LoggingPageElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late Level _level = Level.ALL;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  Level _level = Level.ALL;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -50,7 +50,7 @@ class LoggingPageElement extends CustomElement implements Renderable {
 
   factory LoggingPageElement(M.VM vm, M.IsolateRef isolate,
       M.EventRepository events, M.NotificationRepository notifications,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -79,11 +79,11 @@ class LoggingPageElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  LoggingListElement? _logs;
+  LoggingListElement _logs;
 
   void render() {
     _logs = _logs ?? new LoggingListElement(_isolate, _events);
-    _logs!.level = _level;
+    _logs.level = _level;
     children = <Element>[
       navBar(<Element>[
         new NavTopMenuElement(queue: _r.queue).element,
@@ -106,7 +106,7 @@ class LoggingPageElement extends CustomElement implements Renderable {
           new SpanElement()..text = 'Show messages with severity ',
           _createLevelSelector(),
           new HRElement(),
-          _logs!.element
+          _logs.element
         ]
     ];
   }
diff --git a/runtime/observatory/lib/src/elements/logging_list.dart b/runtime/observatory/lib/src/elements/logging_list.dart
index 804986dd2fc..4f935eedaff 100644
--- a/runtime/observatory/lib/src/elements/logging_list.dart
+++ b/runtime/observatory/lib/src/elements/logging_list.dart
@@ -13,13 +13,13 @@ import 'package:observatory/utils.dart';
 class LoggingListElement extends CustomElement implements Renderable {
   static const tag = const Tag<LoggingListElement>('logging-list');
 
-  late RenderingScheduler<LoggingListElement> _r;
+  RenderingScheduler<LoggingListElement> _r;
 
   Stream<RenderedEvent<LoggingListElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late StreamSubscription _subscription;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  StreamSubscription _subscription;
   Level _level = Level.ALL;
   final _logs = <Map>[];
 
@@ -29,7 +29,7 @@ class LoggingListElement extends CustomElement implements Renderable {
   set level(Level value) => _level = _r.checkAndReact(_level, value);
 
   factory LoggingListElement(M.IsolateRef isolate, M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(events != null);
     LoggingListElement e = new LoggingListElement.created();
diff --git a/runtime/observatory/lib/src/elements/megamorphiccache_ref.dart b/runtime/observatory/lib/src/elements/megamorphiccache_ref.dart
index 5380f53a523..d93696e4e6a 100644
--- a/runtime/observatory/lib/src/elements/megamorphiccache_ref.dart
+++ b/runtime/observatory/lib/src/elements/megamorphiccache_ref.dart
@@ -14,20 +14,20 @@ class MegamorphicCacheRefElement extends CustomElement implements Renderable {
   static const tag =
       const Tag<MegamorphicCacheRefElement>('megamorphic-cache-ref');
 
-  late RenderingScheduler<MegamorphicCacheRefElement> _r;
+  RenderingScheduler<MegamorphicCacheRefElement> _r;
 
   Stream<RenderedEvent<MegamorphicCacheRefElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.MegamorphicCacheRef _cache;
+  M.IsolateRef _isolate;
+  M.MegamorphicCacheRef _cache;
 
   M.IsolateRef get isolate => _isolate;
   M.MegamorphicCacheRef get cache => _cache;
 
   factory MegamorphicCacheRefElement(
       M.IsolateRef isolate, M.MegamorphicCacheRef cache,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(cache != null);
     MegamorphicCacheRefElement e = new MegamorphicCacheRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/megamorphiccache_view.dart b/runtime/observatory/lib/src/elements/megamorphiccache_view.dart
index 8354749312e..698ea598a20 100644
--- a/runtime/observatory/lib/src/elements/megamorphiccache_view.dart
+++ b/runtime/observatory/lib/src/elements/megamorphiccache_view.dart
@@ -37,22 +37,22 @@ class MegamorphicCacheViewElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<MegamorphicCacheViewElement> _r;
+  RenderingScheduler<MegamorphicCacheViewElement> _r;
 
   Stream<RenderedEvent<MegamorphicCacheViewElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.MegamorphicCache _cache;
-  late M.MegamorphicCacheRepository _caches;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.MegamorphicCache _cache;
+  M.MegamorphicCacheRepository _caches;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -71,7 +71,7 @@ class MegamorphicCacheViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -124,7 +124,7 @@ class MegamorphicCacheViewElement extends CustomElement implements Renderable {
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
-                _cache = await _caches.get(_isolate, _cache.id!);
+                _cache = await _caches.get(_isolate, _cache.id);
                 _r.dirty();
               }))
             .element,
diff --git a/runtime/observatory/lib/src/elements/metric/details.dart b/runtime/observatory/lib/src/elements/metric/details.dart
index 98b3116d7ce..72d218ea3e7 100644
--- a/runtime/observatory/lib/src/elements/metric/details.dart
+++ b/runtime/observatory/lib/src/elements/metric/details.dart
@@ -11,20 +11,20 @@ import 'package:observatory/src/elements/helpers/tag.dart';
 class MetricDetailsElement extends CustomElement implements Renderable {
   static const tag = const Tag<MetricDetailsElement>('metric-details');
 
-  late RenderingScheduler<MetricDetailsElement> _r;
+  RenderingScheduler<MetricDetailsElement> _r;
 
   Stream<RenderedEvent<MetricDetailsElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.Metric _metric;
-  late M.MetricRepository _metrics;
+  M.IsolateRef _isolate;
+  M.Metric _metric;
+  M.MetricRepository _metrics;
 
   M.IsolateRef get isolate => _isolate;
   M.Metric get metric => _metric;
 
   factory MetricDetailsElement(
       M.IsolateRef isolate, M.Metric metric, M.MetricRepository metrics,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(metric != null);
     assert(metrics != null);
diff --git a/runtime/observatory/lib/src/elements/metric/graph.dart b/runtime/observatory/lib/src/elements/metric/graph.dart
index 5c350c2bb91..b143895c7b8 100644
--- a/runtime/observatory/lib/src/elements/metric/graph.dart
+++ b/runtime/observatory/lib/src/elements/metric/graph.dart
@@ -11,21 +11,21 @@ import 'package:observatory/src/elements/helpers/tag.dart';
 class MetricGraphElement extends CustomElement implements Renderable {
   static const tag = const Tag<MetricGraphElement>('metric-graph');
 
-  late RenderingScheduler<MetricGraphElement> _r;
+  RenderingScheduler<MetricGraphElement> _r;
 
   Stream<RenderedEvent<MetricGraphElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.Metric _metric;
-  late M.MetricRepository _metrics;
-  late Timer _timer;
+  M.IsolateRef _isolate;
+  M.Metric _metric;
+  M.MetricRepository _metrics;
+  Timer _timer;
 
   M.IsolateRef get isolate => _isolate;
   M.Metric get metric => _metric;
 
   factory MetricGraphElement(
       M.IsolateRef isolate, M.Metric metric, M.MetricRepository metrics,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(metric != null);
     assert(metrics != null);
@@ -58,8 +58,8 @@ class MetricGraphElement extends CustomElement implements Renderable {
     final min = _metrics.getMinValue(_isolate, _metric);
     final max = _metrics.getMaxValue(_isolate, _metric);
     final rows = _metrics
-        .getSamples(_isolate, _metric)!
-        .map((s) => [s.time!.millisecondsSinceEpoch, s.value])
+        .getSamples(_isolate, _metric)
+        .map((s) => [s.time.millisecondsSinceEpoch, s.value])
         .toList();
     final current = rows.last.last;
 
diff --git a/runtime/observatory/lib/src/elements/metrics.dart b/runtime/observatory/lib/src/elements/metrics.dart
index 3f41cb03e0a..50e2da0b32a 100644
--- a/runtime/observatory/lib/src/elements/metrics.dart
+++ b/runtime/observatory/lib/src/elements/metrics.dart
@@ -31,17 +31,17 @@ class MetricsPageElement extends CustomElement implements Renderable {
     NavNotifyElement.tag,
   ]);
 
-  late RenderingScheduler<MetricsPageElement> _r;
+  RenderingScheduler<MetricsPageElement> _r;
 
   Stream<RenderedEvent<MetricsPageElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.MetricRepository _metrics;
-  List<M.Metric>? _available;
-  M.Metric? _selected;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.MetricRepository _metrics;
+  List<M.Metric> _available;
+  M.Metric _selected;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -53,7 +53,7 @@ class MetricsPageElement extends CustomElement implements Renderable {
       M.EventRepository events,
       M.NotificationRepository notifications,
       M.MetricRepository metrics,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -125,7 +125,7 @@ class MetricsPageElement extends CustomElement implements Renderable {
             ..children = _selected == null
                 ? const []
                 : [
-                    new MetricDetailsElement(_isolate, _selected!, _metrics,
+                    new MetricDetailsElement(_isolate, _selected, _metrics,
                             queue: _r.queue)
                         .element
                   ],
@@ -135,7 +135,7 @@ class MetricsPageElement extends CustomElement implements Renderable {
             ..children = _selected == null
                 ? const []
                 : [
-                    new MetricGraphElement(_isolate, _selected!, _metrics,
+                    new MetricGraphElement(_isolate, _selected, _metrics,
                             queue: _r.queue)
                         .element
                   ]
@@ -145,8 +145,8 @@ class MetricsPageElement extends CustomElement implements Renderable {
 
   Future _refresh() async {
     _available = (await _metrics.list(_isolate)).toList();
-    if (!_available!.contains(_selected)) {
-      _selected = _available!.first;
+    if (!_available.contains(_selected)) {
+      _selected = _available.first;
     }
     _r.dirty();
   }
@@ -155,14 +155,14 @@ class MetricsPageElement extends CustomElement implements Renderable {
     var s;
     return [
       s = new SelectElement()
-        ..value = _selected!.name!
-        ..children = _available!.map((metric) {
+        ..value = _selected.name
+        ..children = _available.map((metric) {
           return new OptionElement(
-              value: metric.name!, selected: _selected == metric)
-            ..text = metric.name!;
+              value: metric.name, selected: _selected == metric)
+            ..text = metric.name;
         }).toList(growable: false)
         ..onChange.listen((_) {
-          _selected = _available![s.selectedIndex];
+          _selected = _available[s.selectedIndex];
           _r.dirty();
         })
     ];
diff --git a/runtime/observatory/lib/src/elements/native_memory_profiler.dart b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
index 74e5d0900ab..dfbe45932e6 100644
--- a/runtime/observatory/lib/src/elements/native_memory_profiler.dart
+++ b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
@@ -33,21 +33,21 @@ class NativeMemoryProfileElement extends CustomElement implements Renderable {
         CpuProfileVirtualTreeElement.tag,
       ]);
 
-  late RenderingScheduler<NativeMemoryProfileElement> _r;
+  RenderingScheduler<NativeMemoryProfileElement> _r;
 
   Stream<RenderedEvent<NativeMemoryProfileElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.NativeMemorySampleProfileRepository _profiles;
-  Stream<M.SampleProfileLoadingProgressEvent>? _progressStream;
-  M.SampleProfileLoadingProgress? _progress;
-  late M.SampleProfileTag _tag = M.SampleProfileTag.none;
-  late ProfileTreeMode _mode = ProfileTreeMode.function;
-  late M.ProfileTreeDirection _direction = M.ProfileTreeDirection.exclusive;
-  late String _filter = '';
+  M.VM _vm;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.NativeMemorySampleProfileRepository _profiles;
+  Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
+  M.SampleProfileLoadingProgress _progress;
+  M.SampleProfileTag _tag = M.SampleProfileTag.none;
+  ProfileTreeMode _mode = ProfileTreeMode.function;
+  M.ProfileTreeDirection _direction = M.ProfileTreeDirection.exclusive;
+  String _filter = '';
 
   M.NotificationRepository get notifications => _notifications;
   M.NativeMemorySampleProfileRepository get profiles => _profiles;
@@ -57,7 +57,7 @@ class NativeMemoryProfileElement extends CustomElement implements Renderable {
       M.EventRepository events,
       M.NotificationRepository notifications,
       M.NativeMemorySampleProfileRepository profiles,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(events != null);
     assert(notifications != null);
@@ -102,16 +102,15 @@ class NativeMemoryProfileElement extends CustomElement implements Renderable {
       children = content;
       return;
     }
-    content.add((new SampleBufferControlElement(
-            _vm, _progress!, _progressStream!,
+    content.add((new SampleBufferControlElement(_vm, _progress, _progressStream,
             selectedTag: _tag, queue: _r.queue)
           ..onTagChange.listen((e) {
             _tag = e.element.selectedTag;
             _request();
           }))
         .element);
-    if (_progress!.status == M.SampleProfileLoadingStatus.loaded) {
-      late CpuProfileVirtualTreeElement tree;
+    if (_progress.status == M.SampleProfileLoadingStatus.loaded) {
+      CpuProfileVirtualTreeElement tree;
       content.addAll([
         new BRElement(),
         (new StackTraceTreeConfigElement(
@@ -137,7 +136,7 @@ class NativeMemoryProfileElement extends CustomElement implements Renderable {
               }))
             .element,
         new BRElement(),
-        (tree = new CpuProfileVirtualTreeElement(null, _progress!.profile,
+        (tree = new CpuProfileVirtualTreeElement(null, _progress.profile,
                 queue: _r.queue, type: M.SampleProfileType.memory))
             .element,
       ]);
@@ -147,17 +146,17 @@ class NativeMemoryProfileElement extends CustomElement implements Renderable {
 
   Future _request({bool forceFetch: false}) async {
     if (forceFetch) {
-      for (M.IsolateRef isolate in _vm.isolates) {
+      for (M.Isolate isolate in _vm.isolates) {
         await isolate.collectAllGarbage();
       }
     }
     _progress = null;
     _progressStream = _profiles.get(_vm, _tag, forceFetch: forceFetch);
     _r.dirty();
-    _progress = (await _progressStream!.first).progress;
+    _progress = (await _progressStream.first).progress;
     _r.dirty();
-    if (M.isSampleProcessRunning(_progress!.status)) {
-      _progress = (await _progressStream!.last).progress;
+    if (M.isSampleProcessRunning(_progress.status)) {
+      _progress = (await _progressStream.last).progress;
       _r.dirty();
     }
   }
diff --git a/runtime/observatory/lib/src/elements/nav/class_menu.dart b/runtime/observatory/lib/src/elements/nav/class_menu.dart
index 9603e313c0c..f0dd55df0cb 100644
--- a/runtime/observatory/lib/src/elements/nav/class_menu.dart
+++ b/runtime/observatory/lib/src/elements/nav/class_menu.dart
@@ -13,17 +13,17 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class NavClassMenuElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavClassMenuElement>('nav-class-menu');
 
-  late RenderingScheduler<NavClassMenuElement> _r;
+  RenderingScheduler<NavClassMenuElement> _r;
 
   Stream<RenderedEvent<NavClassMenuElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ClassRef _cls;
-  List<Element> _content = const [];
+  M.IsolateRef _isolate;
+  M.ClassRef _cls;
+  Iterable<Element> _content = const [];
 
   M.IsolateRef get isolate => _isolate;
   M.ClassRef get cls => _cls;
-  List<Element> get content => _content;
+  Iterable<Element> get content => _content;
 
   set content(Iterable<Element> value) {
     _content = value.toList();
@@ -31,7 +31,7 @@ class NavClassMenuElement extends CustomElement implements Renderable {
   }
 
   factory NavClassMenuElement(M.IsolateRef isolate, M.ClassRef cls,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(cls != null);
     NavClassMenuElement e = new NavClassMenuElement.created();
@@ -58,7 +58,7 @@ class NavClassMenuElement extends CustomElement implements Renderable {
 
   void render() {
     children = <Element>[
-      navMenu(cls.name!,
+      navMenu(cls.name,
           content: _content, link: Uris.inspect(isolate, object: cls))
     ];
   }
diff --git a/runtime/observatory/lib/src/elements/nav/isolate_menu.dart b/runtime/observatory/lib/src/elements/nav/isolate_menu.dart
index a059c4af63c..65cbca95e75 100644
--- a/runtime/observatory/lib/src/elements/nav/isolate_menu.dart
+++ b/runtime/observatory/lib/src/elements/nav/isolate_menu.dart
@@ -15,14 +15,14 @@ class NavIsolateMenuElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavIsolateMenuElement>('nav-isolate-menu',
       dependencies: const [NavMenuItemElement.tag]);
 
-  late RenderingScheduler<NavIsolateMenuElement> _r;
+  RenderingScheduler<NavIsolateMenuElement> _r;
 
   Stream<RenderedEvent<NavIsolateMenuElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  StreamSubscription? _updatesSubscription;
-  late Iterable<Element> _content = const [];
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  StreamSubscription _updatesSubscription;
+  Iterable<Element> _content = const [];
 
   M.IsolateRef get isolate => _isolate;
   Iterable<Element> get content => _content;
@@ -33,7 +33,7 @@ class NavIsolateMenuElement extends CustomElement implements Renderable {
   }
 
   factory NavIsolateMenuElement(M.IsolateRef isolate, M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(events != null);
     NavIsolateMenuElement e = new NavIsolateMenuElement.created();
@@ -63,7 +63,7 @@ class NavIsolateMenuElement extends CustomElement implements Renderable {
     children = <Element>[];
     _r.disable(notify: true);
     assert(_updatesSubscription != null);
-    _updatesSubscription!.cancel();
+    _updatesSubscription.cancel();
     _updatesSubscription = null;
   }
 
@@ -104,7 +104,7 @@ class NavIsolateMenuElement extends CustomElement implements Renderable {
           .element,
     ]..addAll(_content);
     children = <Element>[
-      navMenu(isolate.name!, content: content, link: Uris.inspect(isolate))
+      navMenu(isolate.name, content: content, link: Uris.inspect(isolate))
     ];
   }
 }
diff --git a/runtime/observatory/lib/src/elements/nav/library_menu.dart b/runtime/observatory/lib/src/elements/nav/library_menu.dart
index 70c4a249a84..e5705d4e58e 100644
--- a/runtime/observatory/lib/src/elements/nav/library_menu.dart
+++ b/runtime/observatory/lib/src/elements/nav/library_menu.dart
@@ -13,17 +13,17 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class NavLibraryMenuElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavLibraryMenuElement>('nav-library-menu');
 
-  late RenderingScheduler<NavLibraryMenuElement> _r;
+  RenderingScheduler<NavLibraryMenuElement> _r;
 
   Stream<RenderedEvent<NavLibraryMenuElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.LibraryRef _library;
-  List<Element> _content = const [];
+  M.IsolateRef _isolate;
+  M.LibraryRef _library;
+  Iterable<Element> _content = const [];
 
   M.IsolateRef get isolate => _isolate;
   M.LibraryRef get library => _library;
-  List<Element> get content => _content;
+  Iterable<Element> get content => _content;
 
   set content(Iterable<Element> value) {
     _content = value.toList();
@@ -31,7 +31,7 @@ class NavLibraryMenuElement extends CustomElement implements Renderable {
   }
 
   factory NavLibraryMenuElement(M.IsolateRef isolate, M.LibraryRef library,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(library != null);
     NavLibraryMenuElement e = new NavLibraryMenuElement.created();
@@ -58,7 +58,7 @@ class NavLibraryMenuElement extends CustomElement implements Renderable {
 
   void render() {
     children = <Element>[
-      navMenu(library.name!,
+      navMenu(library.name,
           content: _content,
           link: Uris.inspect(isolate, object: library).toString())
     ];
diff --git a/runtime/observatory/lib/src/elements/nav/menu_item.dart b/runtime/observatory/lib/src/elements/nav/menu_item.dart
index 865011a4fa4..5de1b9479c2 100644
--- a/runtime/observatory/lib/src/elements/nav/menu_item.dart
+++ b/runtime/observatory/lib/src/elements/nav/menu_item.dart
@@ -10,17 +10,17 @@ import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 class NavMenuItemElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavMenuItemElement>('nav-menu-item');
 
-  late RenderingScheduler<NavMenuItemElement> _r;
+  RenderingScheduler<NavMenuItemElement> _r;
 
   Stream<RenderedEvent<NavMenuItemElement>> get onRendered => _r.onRendered;
 
-  late String _label;
-  late String _link;
-  List<Element> _content = const <Element>[];
+  String _label;
+  String _link;
+  Iterable<Element> _content = const <Element>[];
 
   String get label => _label;
   String get link => _link;
-  List<Element> get content => _content;
+  Iterable<Element> get content => _content;
 
   set label(String value) => _label = _r.checkAndReact(_label, value);
   set link(String value) => _link = _r.checkAndReact(_link, value);
@@ -30,7 +30,7 @@ class NavMenuItemElement extends CustomElement implements Renderable {
   }
 
   factory NavMenuItemElement(String label,
-      {String link = '', RenderingQueue? queue}) {
+      {String link, RenderingQueue queue}) {
     assert(label != null);
     NavMenuItemElement e = new NavMenuItemElement.created();
     e._r = new RenderingScheduler<NavMenuItemElement>(e, queue: queue);
diff --git a/runtime/observatory/lib/src/elements/nav/notify.dart b/runtime/observatory/lib/src/elements/nav/notify.dart
index fde70f339ee..07ba4e00f49 100644
--- a/runtime/observatory/lib/src/elements/nav/notify.dart
+++ b/runtime/observatory/lib/src/elements/nav/notify.dart
@@ -17,14 +17,14 @@ class NavNotifyElement extends CustomElement implements Renderable {
         NavNotifyExceptionElement.tag
       ]);
 
-  late RenderingScheduler<NavNotifyElement> _r;
+  RenderingScheduler<NavNotifyElement> _r;
 
   Stream<RenderedEvent<NavNotifyElement>> get onRendered => _r.onRendered;
 
-  late M.NotificationRepository _repository;
-  StreamSubscription? _subscription;
+  M.NotificationRepository _repository;
+  StreamSubscription _subscription;
 
-  late bool _notifyOnPause;
+  bool _notifyOnPause;
 
   bool get notifyOnPause => _notifyOnPause;
 
@@ -32,7 +32,7 @@ class NavNotifyElement extends CustomElement implements Renderable {
       _notifyOnPause = _r.checkAndReact(_notifyOnPause, value);
 
   factory NavNotifyElement(M.NotificationRepository repository,
-      {bool notifyOnPause: true, RenderingQueue? queue}) {
+      {bool notifyOnPause: true, RenderingQueue queue}) {
     assert(repository != null);
     assert(notifyOnPause != null);
     NavNotifyElement e = new NavNotifyElement.created();
@@ -56,7 +56,7 @@ class NavNotifyElement extends CustomElement implements Renderable {
     super.detached();
     children = <Element>[];
     _r.disable(notify: true);
-    _subscription!.cancel();
+    _subscription.cancel();
   }
 
   void render() {
diff --git a/runtime/observatory/lib/src/elements/nav/notify_event.dart b/runtime/observatory/lib/src/elements/nav/notify_event.dart
index 677b3e3e4aa..5fa52d64a22 100644
--- a/runtime/observatory/lib/src/elements/nav/notify_event.dart
+++ b/runtime/observatory/lib/src/elements/nav/notify_event.dart
@@ -17,7 +17,7 @@ class EventDeleteEvent {
 class NavNotifyEventElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavNotifyEventElement>('nav-event');
 
-  late RenderingScheduler<NavNotifyEventElement> _r;
+  RenderingScheduler<NavNotifyEventElement> _r;
 
   Stream<RenderedEvent<NavNotifyEventElement>> get onRendered => _r.onRendered;
 
@@ -25,11 +25,11 @@ class NavNotifyEventElement extends CustomElement implements Renderable {
       new StreamController<EventDeleteEvent>.broadcast();
   Stream<EventDeleteEvent> get onDelete => _onDelete.stream;
 
-  late M.Event _event;
+  M.Event _event;
 
   M.Event get event => _event;
 
-  factory NavNotifyEventElement(M.Event event, {RenderingQueue? queue}) {
+  factory NavNotifyEventElement(M.Event event, {RenderingQueue queue}) {
     assert(event != null);
     NavNotifyEventElement e = new NavNotifyEventElement.created();
     e._r = new RenderingScheduler<NavNotifyEventElement>(e, queue: queue);
@@ -118,7 +118,7 @@ class NavNotifyEventElement extends CustomElement implements Renderable {
       M.PauseBreakpointEvent event) {
     String message = ' is paused';
     if (event.breakpoint != null) {
-      message += ' at breakpoint ${event.breakpoint!.number}';
+      message += ' at breakpoint ${event.breakpoint.number}';
     }
     return [
       new SpanElement()..text = 'Isolate ',
diff --git a/runtime/observatory/lib/src/elements/nav/notify_exception.dart b/runtime/observatory/lib/src/elements/nav/notify_exception.dart
index fd9f7fb0433..4100ef68c83 100644
--- a/runtime/observatory/lib/src/elements/nav/notify_exception.dart
+++ b/runtime/observatory/lib/src/elements/nav/notify_exception.dart
@@ -11,7 +11,7 @@ import 'package:observatory/models.dart' show ConnectionException;
 
 class ExceptionDeleteEvent {
   final dynamic exception;
-  final StackTrace? stacktrace;
+  final StackTrace stacktrace;
 
   ExceptionDeleteEvent(this.exception, {this.stacktrace});
 }
@@ -19,7 +19,7 @@ class ExceptionDeleteEvent {
 class NavNotifyExceptionElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavNotifyExceptionElement>('nav-exception');
 
-  late RenderingScheduler<NavNotifyExceptionElement> _r;
+  RenderingScheduler<NavNotifyExceptionElement> _r;
 
   Stream<RenderedEvent<NavNotifyExceptionElement>> get onRendered =>
       _r.onRendered;
@@ -28,14 +28,14 @@ class NavNotifyExceptionElement extends CustomElement implements Renderable {
       new StreamController<ExceptionDeleteEvent>.broadcast();
   Stream<ExceptionDeleteEvent> get onDelete => _onDelete.stream;
 
-  late dynamic _exception;
-  StackTrace? _stacktrace;
+  dynamic _exception;
+  StackTrace _stacktrace;
 
   dynamic get exception => _exception;
-  StackTrace? get stacktrace => _stacktrace;
+  StackTrace get stacktrace => _stacktrace;
 
   factory NavNotifyExceptionElement(dynamic exception,
-      {StackTrace? stacktrace: null, RenderingQueue? queue}) {
+      {StackTrace stacktrace: null, RenderingQueue queue}) {
     assert(exception != null);
     NavNotifyExceptionElement e = new NavNotifyExceptionElement.created();
     e._r = new RenderingScheduler<NavNotifyExceptionElement>(e, queue: queue);
diff --git a/runtime/observatory/lib/src/elements/nav/refresh.dart b/runtime/observatory/lib/src/elements/nav/refresh.dart
index 0a4e1e0e7ec..445d53ba87a 100644
--- a/runtime/observatory/lib/src/elements/nav/refresh.dart
+++ b/runtime/observatory/lib/src/elements/nav/refresh.dart
@@ -15,7 +15,7 @@ class RefreshEvent {
 class NavRefreshElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavRefreshElement>('nav-refresh');
 
-  late RenderingScheduler<NavRefreshElement> _r;
+  RenderingScheduler<NavRefreshElement> _r;
 
   Stream<RenderedEvent<NavRefreshElement>> get onRendered => _r.onRendered;
 
@@ -23,8 +23,8 @@ class NavRefreshElement extends CustomElement implements Renderable {
       new StreamController<RefreshEvent>.broadcast();
   Stream<RefreshEvent> get onRefresh => _onRefresh.stream;
 
-  late bool _disabled;
-  late String _label;
+  bool _disabled;
+  String _label;
 
   bool get disabled => _disabled;
   String get label => _label;
@@ -33,7 +33,7 @@ class NavRefreshElement extends CustomElement implements Renderable {
   set label(String value) => _label = _r.checkAndReact(_label, value);
 
   factory NavRefreshElement(
-      {String label: 'Refresh', bool disabled: false, RenderingQueue? queue}) {
+      {String label: 'Refresh', bool disabled: false, RenderingQueue queue}) {
     assert(label != null);
     assert(disabled != null);
     NavRefreshElement e = new NavRefreshElement.created();
diff --git a/runtime/observatory/lib/src/elements/nav/reload.dart b/runtime/observatory/lib/src/elements/nav/reload.dart
index f9bdb80d211..da6813b6d2d 100644
--- a/runtime/observatory/lib/src/elements/nav/reload.dart
+++ b/runtime/observatory/lib/src/elements/nav/reload.dart
@@ -18,7 +18,7 @@ class ReloadEvent {
 class NavReloadElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavReloadElement>('nav-reload');
 
-  late RenderingScheduler<NavReloadElement> _r;
+  RenderingScheduler<NavReloadElement> _r;
 
   Stream<RenderedEvent<NavReloadElement>> get onRendered => _r.onRendered;
 
@@ -26,15 +26,15 @@ class NavReloadElement extends CustomElement implements Renderable {
       new StreamController<ReloadEvent>.broadcast();
   Stream<ReloadEvent> get onReload => _onReload.stream;
 
-  late M.IsolateRef _isolate;
-  late M.IsolateRepository _isolates;
-  late M.EventRepository _events;
-  StreamSubscription? _sub;
+  M.IsolateRef _isolate;
+  M.IsolateRepository _isolates;
+  M.EventRepository _events;
+  StreamSubscription _sub;
   bool _disabled = false;
 
   factory NavReloadElement(M.IsolateRef isolate, M.IsolateRepository isolates,
       M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(isolates != null);
     assert(events != null);
@@ -59,7 +59,7 @@ class NavReloadElement extends CustomElement implements Renderable {
   void detached() {
     super.detached();
     children = <Element>[];
-    _sub!.cancel();
+    _sub.cancel();
     _sub = null;
     _r.disable(notify: true);
   }
@@ -84,15 +84,13 @@ class NavReloadElement extends CustomElement implements Renderable {
                 .listen((_) => _reload(_isolates.reloadSourcesServices.single))
         ]);
     } else {
-      final content = _isolates.reloadSourcesServices
-          .map((s) => new LIElement()
-            ..children = <Element>[
-              new ButtonElement()
-                ..text = s.alias
-                ..disabled = _disabled
-                ..onClick.listen((_) => _reload(s))
-            ])
-          .toList();
+      final content = _isolates.reloadSourcesServices.map((s) => new LIElement()
+        ..children = <Element>[
+          new ButtonElement()
+            ..text = s.alias
+            ..disabled = _disabled
+            ..onClick.listen((_) => _reload(s))
+        ]);
       children.add(navMenu('Reload Source', content: content));
     }
     this.children = children;
diff --git a/runtime/observatory/lib/src/elements/nav/top_menu.dart b/runtime/observatory/lib/src/elements/nav/top_menu.dart
index 1a9c0e21f2f..3415fdedc0a 100644
--- a/runtime/observatory/lib/src/elements/nav/top_menu.dart
+++ b/runtime/observatory/lib/src/elements/nav/top_menu.dart
@@ -14,7 +14,7 @@ class NavTopMenuElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavTopMenuElement>('nav-top-menu',
       dependencies: const [NavMenuItemElement.tag]);
 
-  late RenderingScheduler<NavTopMenuElement> _r;
+  RenderingScheduler<NavTopMenuElement> _r;
 
   Stream<RenderedEvent<NavTopMenuElement>> get onRendered => _r.onRendered;
 
@@ -27,7 +27,7 @@ class NavTopMenuElement extends CustomElement implements Renderable {
     _r.dirty();
   }
 
-  factory NavTopMenuElement({RenderingQueue? queue}) {
+  factory NavTopMenuElement({RenderingQueue queue}) {
     NavTopMenuElement e = new NavTopMenuElement.created();
     e._r = new RenderingScheduler<NavTopMenuElement>(e, queue: queue);
     return e;
diff --git a/runtime/observatory/lib/src/elements/nav/vm_menu.dart b/runtime/observatory/lib/src/elements/nav/vm_menu.dart
index 105c2c773d3..0216a01aded 100644
--- a/runtime/observatory/lib/src/elements/nav/vm_menu.dart
+++ b/runtime/observatory/lib/src/elements/nav/vm_menu.dart
@@ -15,13 +15,13 @@ class NavVMMenuElement extends CustomElement implements Renderable {
   static const tag = const Tag<NavVMMenuElement>('nav-vm-menu',
       dependencies: const [NavMenuItemElement.tag]);
 
-  late RenderingScheduler<NavVMMenuElement> _r;
+  RenderingScheduler<NavVMMenuElement> _r;
 
   Stream<RenderedEvent<NavVMMenuElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.EventRepository _events;
-  late StreamSubscription _updatesSubscription;
+  M.VM _vm;
+  M.EventRepository _events;
+  StreamSubscription _updatesSubscription;
   Iterable<Element> _content = const [];
 
   M.VM get vm => _vm;
@@ -33,7 +33,7 @@ class NavVMMenuElement extends CustomElement implements Renderable {
   }
 
   factory NavVMMenuElement(M.VM vm, M.EventRepository events,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(events != null);
     NavVMMenuElement e = new NavVMMenuElement.created();
@@ -49,7 +49,7 @@ class NavVMMenuElement extends CustomElement implements Renderable {
   void attached() {
     super.attached();
     _updatesSubscription = _events.onVMUpdate.listen((e) {
-      _vm = e.vm as M.VM;
+      _vm = e.vm;
       _r.dirty();
     });
     _r.enable();
@@ -65,13 +65,13 @@ class NavVMMenuElement extends CustomElement implements Renderable {
 
   void render() {
     final content = (_vm.isolates.map<Element>((isolate) {
-      return new NavMenuItemElement(isolate.name!,
+      return new NavMenuItemElement(isolate.name,
               queue: _r.queue, link: Uris.inspect(isolate))
           .element;
     }).toList()
       ..addAll(_content));
     children = <Element>[
-      navMenu(vm.displayName!, link: Uris.vm(), content: content)
+      navMenu(vm.displayName, link: Uris.vm(), content: content)
     ];
   }
 }
diff --git a/runtime/observatory/lib/src/elements/object_common.dart b/runtime/observatory/lib/src/elements/object_common.dart
index 5515c3bdc0e..c2fd2c38775 100644
--- a/runtime/observatory/lib/src/elements/object_common.dart
+++ b/runtime/observatory/lib/src/elements/object_common.dart
@@ -22,20 +22,20 @@ class ObjectCommonElement extends CustomElement implements Renderable {
     SentinelValueElement.tag
   ]);
 
-  late RenderingScheduler<ObjectCommonElement> _r;
+  RenderingScheduler<ObjectCommonElement> _r;
 
   Stream<RenderedEvent<ObjectCommonElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.Object _object;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
-  M.Guarded<M.Instance>? _retainedSize = null;
+  M.IsolateRef _isolate;
+  M.Object _object;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
+  M.Guarded<M.Instance> _retainedSize = null;
   bool _loadingRetainedBytes = false;
-  M.Guarded<M.Instance>? _reachableSize = null;
+  M.Guarded<M.Instance> _reachableSize = null;
   bool _loadingReachableBytes = false;
 
   M.IsolateRef get isolate => _isolate;
@@ -49,7 +49,7 @@ class ObjectCommonElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(object != null);
     assert(retainedSizes != null);
@@ -84,8 +84,8 @@ class ObjectCommonElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  RetainingPathElement? _path;
-  InboundReferencesElement? _inbounds;
+  RetainingPathElement _path;
+  InboundReferencesElement _inbounds;
 
   void render() {
     _path = _path ??
@@ -109,7 +109,7 @@ class ObjectCommonElement extends CustomElement implements Renderable {
                 ..children = <Element>[
                   _object.clazz == null
                       ? (new SpanElement()..text = '...')
-                      : new ClassRefElement(_isolate, _object.clazz!,
+                      : new ClassRefElement(_isolate, _object.clazz,
                               queue: _r.queue)
                           .element
                 ]
@@ -157,7 +157,7 @@ class ObjectCommonElement extends CustomElement implements Renderable {
                 ..text = 'Retaining path ',
               new DivElement()
                 ..classes = ['memberValue']
-                ..children = <Element>[_path!.element]
+                ..children = <Element>[_path.element]
             ],
           new DivElement()
             ..classes = ['memberItem']
@@ -168,7 +168,7 @@ class ObjectCommonElement extends CustomElement implements Renderable {
                 ..text = 'Inbound references ',
               new DivElement()
                 ..classes = ['memberValue']
-                ..children = <Element>[_inbounds!.element]
+                ..children = <Element>[_inbounds.element]
             ]
         ]
     ];
@@ -177,14 +177,14 @@ class ObjectCommonElement extends CustomElement implements Renderable {
   List<Element> _createReachableSizeValue() {
     final content = <Element>[];
     if (_reachableSize != null) {
-      if (_reachableSize!.isSentinel) {
-        content.add(new SentinelValueElement(_reachableSize!.asSentinel!,
-                queue: _r.queue)
-            .element);
+      if (_reachableSize.isSentinel) {
+        content.add(
+            new SentinelValueElement(_reachableSize.asSentinel, queue: _r.queue)
+                .element);
       } else {
         content.add(new SpanElement()
           ..text = Utils.formatSize(
-              int.parse(_reachableSize!.asValue!.valueAsString!)));
+              int.parse(_reachableSize.asValue.valueAsString)));
       }
     } else {
       content.add(new SpanElement()..text = '...');
@@ -196,7 +196,7 @@ class ObjectCommonElement extends CustomElement implements Renderable {
     button.onClick.listen((_) async {
       button.disabled = true;
       _loadingReachableBytes = true;
-      _reachableSize = await _reachableSizes.get(_isolate, _object.id!);
+      _reachableSize = await _reachableSizes.get(_isolate, _object.id);
       _r.dirty();
     });
     content.add(button);
@@ -206,14 +206,14 @@ class ObjectCommonElement extends CustomElement implements Renderable {
   List<Element> _createRetainedSizeValue() {
     final content = <Element>[];
     if (_retainedSize != null) {
-      if (_retainedSize!.isSentinel) {
-        content.add(new SentinelValueElement(_retainedSize!.asSentinel!,
-                queue: _r.queue)
-            .element);
+      if (_retainedSize.isSentinel) {
+        content.add(
+            new SentinelValueElement(_retainedSize.asSentinel, queue: _r.queue)
+                .element);
       } else {
         content.add(new SpanElement()
-          ..text = Utils.formatSize(
-              int.parse(_retainedSize!.asValue!.valueAsString!)));
+          ..text =
+              Utils.formatSize(int.parse(_retainedSize.asValue.valueAsString)));
       }
     } else {
       content.add(new SpanElement()..text = '...');
@@ -225,7 +225,7 @@ class ObjectCommonElement extends CustomElement implements Renderable {
     button.onClick.listen((_) async {
       button.disabled = true;
       _loadingRetainedBytes = true;
-      _retainedSize = await _retainedSizes.get(_isolate, _object.id!);
+      _retainedSize = await _retainedSizes.get(_isolate, _object.id);
       _r.dirty();
     });
     content.add(button);
diff --git a/runtime/observatory/lib/src/elements/object_view.dart b/runtime/observatory/lib/src/elements/object_view.dart
index b7bb5030bf9..2d7620f371b 100644
--- a/runtime/observatory/lib/src/elements/object_view.dart
+++ b/runtime/observatory/lib/src/elements/object_view.dart
@@ -33,24 +33,25 @@ class ObjectViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<ObjectViewElement> _r;
+  RenderingScheduler<ObjectViewElement> _r;
 
   Stream<RenderedEvent<ObjectViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Object _object;
-  late M.ObjectRepository _objects;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Object _object;
+  M.ObjectRepository _objects;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
   M.NotificationRepository get notifications => _notifications;
+  M.Context get object => _object;
 
   factory ObjectViewElement(
       M.VM vm,
@@ -63,7 +64,7 @@ class ObjectViewElement extends CustomElement implements Renderable {
       M.ReachableSizeRepository reachableSizes,
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -114,7 +115,7 @@ class ObjectViewElement extends CustomElement implements Renderable {
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
-                _object = await _objects.get(_isolate, _object.id!);
+                _object = await _objects.get(_isolate, _object.id);
                 _r.dirty();
               }))
             .element,
diff --git a/runtime/observatory/lib/src/elements/objectpool_ref.dart b/runtime/observatory/lib/src/elements/objectpool_ref.dart
index 19a2c05a89d..cd9902a0cbc 100644
--- a/runtime/observatory/lib/src/elements/objectpool_ref.dart
+++ b/runtime/observatory/lib/src/elements/objectpool_ref.dart
@@ -12,18 +12,18 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class ObjectPoolRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<ObjectPoolRefElement>('object-pool-ref');
 
-  late RenderingScheduler<ObjectPoolRefElement> _r;
+  RenderingScheduler<ObjectPoolRefElement> _r;
 
   Stream<RenderedEvent<ObjectPoolRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ObjectPoolRef _pool;
+  M.IsolateRef _isolate;
+  M.ObjectPoolRef _pool;
 
   M.IsolateRef get isolate => _isolate;
   M.ObjectPoolRef get pool => _pool;
 
   factory ObjectPoolRefElement(M.IsolateRef isolate, M.ObjectPoolRef pool,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(pool != null);
     ObjectPoolRefElement e = new ObjectPoolRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/objectpool_view.dart b/runtime/observatory/lib/src/elements/objectpool_view.dart
index 3d5a1b95678..0d4c2326668 100644
--- a/runtime/observatory/lib/src/elements/objectpool_view.dart
+++ b/runtime/observatory/lib/src/elements/objectpool_view.dart
@@ -36,21 +36,21 @@ class ObjectPoolViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<ObjectPoolViewElement> _r;
+  RenderingScheduler<ObjectPoolViewElement> _r;
 
   Stream<RenderedEvent<ObjectPoolViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.ObjectPool _pool;
-  late M.ObjectPoolRepository _pools;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.ObjectPool _pool;
+  M.ObjectPoolRepository _pools;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -69,7 +69,7 @@ class ObjectPoolViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -122,7 +122,7 @@ class ObjectPoolViewElement extends CustomElement implements Renderable {
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
-                _pool = await _pools.get(_isolate, _pool.id!);
+                _pool = await _pools.get(_isolate, _pool.id);
                 _r.dirty();
               }))
             .element,
@@ -138,10 +138,10 @@ class ObjectPoolViewElement extends CustomElement implements Renderable {
                   queue: _r.queue)
               .element,
           new HRElement(),
-          new HeadingElement.h3()..text = 'entries (${_pool.entries!.length})',
+          new HeadingElement.h3()..text = 'entries (${_pool.entries.length})',
           new DivElement()
             ..classes = ['memberList']
-            ..children = _pool.entries!
+            ..children = _pool.entries
                 .map<Element>((entry) => new DivElement()
                   ..classes = ['memberItem']
                   ..children = <Element>[
@@ -167,12 +167,12 @@ class ObjectPoolViewElement extends CustomElement implements Renderable {
       case M.ObjectPoolEntryKind.immediate:
         return [
           new SpanElement()
-            ..text = 'Immediate 0x${entry.asInteger!.toRadixString(16)}'
+            ..text = 'Immediate 0x${entry.asInteger.toRadixString(16)}'
         ];
       case M.ObjectPoolEntryKind.nativeEntry:
         return [
           new SpanElement()
-            ..text = 'NativeEntry 0x${entry.asInteger!.toRadixString(16)}'
+            ..text = 'NativeEntry 0x${entry.asInteger.toRadixString(16)}'
         ];
     }
     throw new Exception('Unknown ObjectPoolEntryKind (${entry.kind})');
diff --git a/runtime/observatory/lib/src/elements/objectstore_view.dart b/runtime/observatory/lib/src/elements/objectstore_view.dart
index 8879a43f35d..66b50dc558b 100644
--- a/runtime/observatory/lib/src/elements/objectstore_view.dart
+++ b/runtime/observatory/lib/src/elements/objectstore_view.dart
@@ -31,17 +31,17 @@ class ObjectStoreViewElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<ObjectStoreViewElement> _r;
+  RenderingScheduler<ObjectStoreViewElement> _r;
 
   Stream<RenderedEvent<ObjectStoreViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  M.ObjectStore? _store;
-  late M.ObjectStoreRepository _stores;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.ObjectStore _store;
+  M.ObjectStoreRepository _stores;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -54,7 +54,7 @@ class ObjectStoreViewElement extends CustomElement implements Renderable {
       M.NotificationRepository notifications,
       M.ObjectStoreRepository stores,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
diff --git a/runtime/observatory/lib/src/elements/observatory_application.dart b/runtime/observatory/lib/src/elements/observatory_application.dart
index 1ccad0fa0c3..49e166507fa 100644
--- a/runtime/observatory/lib/src/elements/observatory_application.dart
+++ b/runtime/observatory/lib/src/elements/observatory_application.dart
@@ -14,7 +14,7 @@ class ObservatoryApplicationElement extends CustomElement {
   static const tag =
       const Tag<ObservatoryApplicationElement>('observatory-application');
 
-  late ObservatoryApplication app;
+  ObservatoryApplication app;
 
   ObservatoryApplicationElement.created() : super.created(tag);
 
diff --git a/runtime/observatory/lib/src/elements/pc_descriptors_ref.dart b/runtime/observatory/lib/src/elements/pc_descriptors_ref.dart
index 2b2da04ae7d..fa43bbe13c2 100644
--- a/runtime/observatory/lib/src/elements/pc_descriptors_ref.dart
+++ b/runtime/observatory/lib/src/elements/pc_descriptors_ref.dart
@@ -12,20 +12,20 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class PcDescriptorsRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<PcDescriptorsRefElement>('pc-ref');
 
-  late RenderingScheduler<PcDescriptorsRefElement> _r;
+  RenderingScheduler<PcDescriptorsRefElement> _r;
 
   Stream<RenderedEvent<PcDescriptorsRefElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.PcDescriptorsRef _descriptors;
+  M.IsolateRef _isolate;
+  M.PcDescriptorsRef _descriptors;
 
   M.IsolateRef get isolate => _isolate;
   M.PcDescriptorsRef get descriptors => _descriptors;
 
   factory PcDescriptorsRefElement(
       M.IsolateRef isolate, M.PcDescriptorsRef descriptors,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(descriptors != null);
     PcDescriptorsRefElement e = new PcDescriptorsRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/persistent_handles.dart b/runtime/observatory/lib/src/elements/persistent_handles.dart
index f1bd5464921..9a0d15dffb8 100644
--- a/runtime/observatory/lib/src/elements/persistent_handles.dart
+++ b/runtime/observatory/lib/src/elements/persistent_handles.dart
@@ -38,18 +38,18 @@ class PersistentHandlesPageElement extends CustomElement implements Renderable {
         VirtualCollectionElement.tag
       ]);
 
-  late RenderingScheduler<PersistentHandlesPageElement> _r;
+  RenderingScheduler<PersistentHandlesPageElement> _r;
 
   Stream<RenderedEvent<PersistentHandlesPageElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.PersistentHandlesRepository _repository;
-  late M.ObjectRepository _objects;
-  M.PersistentHandles? _handles;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.PersistentHandlesRepository _repository;
+  M.ObjectRepository _objects;
+  M.PersistentHandles _handles;
   _SortingField _sortingField = _SortingField.externalSize;
   _SortingDirection _sortingDirection = _SortingDirection.descending;
 
@@ -64,7 +64,7 @@ class PersistentHandlesPageElement extends CustomElement implements Renderable {
       M.NotificationRepository notifications,
       M.PersistentHandlesRepository repository,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -119,13 +119,13 @@ class PersistentHandlesPageElement extends CustomElement implements Renderable {
           'Weak Persistent Handles',
           _handles == null
               ? null
-              : (_handles!.weakElements.toList()..sort(_createSorter())),
+              : (_handles.weakElements.toList()..sort(_createSorter())),
           _createWeakLine,
           _updateWeakLine,
           createHeader: _createWeakHeader));
   }
 
-  List<Element> _createHandlers(String name, List? items, create, update,
+  List<Element> _createHandlers(String name, List items, create, update,
       {createHeader}) {
     return [
       new DivElement()
diff --git a/runtime/observatory/lib/src/elements/ports.dart b/runtime/observatory/lib/src/elements/ports.dart
index 03c73236654..6561ffea6d5 100644
--- a/runtime/observatory/lib/src/elements/ports.dart
+++ b/runtime/observatory/lib/src/elements/ports.dart
@@ -29,17 +29,17 @@ class PortsElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<PortsElement> _r;
+  RenderingScheduler<PortsElement> _r;
 
   Stream<RenderedEvent<PortsElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.PortsRepository _ports;
-  late M.ObjectRepository _objects;
-  M.Ports? _isolatePorts;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.PortsRepository _ports;
+  M.ObjectRepository _objects;
+  M.Ports _isolatePorts;
 
   M.IsolateRef get isolate => _isolate;
   M.NotificationRepository get notifications => _notifications;
@@ -53,7 +53,7 @@ class PortsElement extends CustomElement implements Renderable {
       M.NotificationRepository notifications,
       M.PortsRepository ports,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -74,7 +74,7 @@ class PortsElement extends CustomElement implements Renderable {
   PortsElement.created() : super.created(tag);
 
   int get portCount {
-    return _isolatePorts == null ? 0 : _isolatePorts!.elements.length;
+    return _isolatePorts == null ? 0 : _isolatePorts.elements.length;
   }
 
   @override
@@ -120,7 +120,7 @@ class PortsElement extends CustomElement implements Renderable {
       return const [];
     }
     int i = 0;
-    return _isolatePorts!.elements
+    return _isolatePorts.elements
         .map<Element>((port) => new DivElement()
           ..classes = ['memberItem']
           ..children = <Element>[
diff --git a/runtime/observatory/lib/src/elements/retaining_path.dart b/runtime/observatory/lib/src/elements/retaining_path.dart
index 4fa835838b0..c27b9120934 100644
--- a/runtime/observatory/lib/src/elements/retaining_path.dart
+++ b/runtime/observatory/lib/src/elements/retaining_path.dart
@@ -15,15 +15,15 @@ class RetainingPathElement extends CustomElement implements Renderable {
   static const tag = const Tag<RetainingPathElement>('retaining-path',
       dependencies: const [CurlyBlockElement.tag, InstanceRefElement.tag]);
 
-  late RenderingScheduler<RetainingPathElement> _r;
+  RenderingScheduler<RetainingPathElement> _r;
 
   Stream<RenderedEvent<RetainingPathElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ObjectRef _object;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
-  M.RetainingPath? _path;
+  M.IsolateRef _isolate;
+  M.ObjectRef _object;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
+  M.RetainingPath _path;
   bool _expanded = false;
 
   M.IsolateRef get isolate => _isolate;
@@ -31,7 +31,7 @@ class RetainingPathElement extends CustomElement implements Renderable {
 
   factory RetainingPathElement(M.IsolateRef isolate, M.ObjectRef object,
       M.RetainingPathRepository retainingPaths, M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(object != null);
     assert(retainingPaths != null);
@@ -78,7 +78,7 @@ class RetainingPathElement extends CustomElement implements Renderable {
 
   Future _refresh() async {
     _path = null;
-    _path = await _retainingPaths.get(_isolate, _object.id!);
+    _path = await _retainingPaths.get(_isolate, _object.id);
     _r.dirty();
   }
 
@@ -89,11 +89,11 @@ class RetainingPathElement extends CustomElement implements Renderable {
 
     var elements = <Element>[];
     bool first = true;
-    for (var item in _path!.elements) {
+    for (var item in _path.elements) {
       elements.add(_createItem(item, first));
       first = false;
     }
-    elements.add(_createGCRootItem(_path!.gcRootType));
+    elements.add(_createGCRootItem(_path.gcRootType));
     return elements;
   }
 
diff --git a/runtime/observatory/lib/src/elements/sample_buffer_control.dart b/runtime/observatory/lib/src/elements/sample_buffer_control.dart
index 6386315d084..10ba12f4371 100644
--- a/runtime/observatory/lib/src/elements/sample_buffer_control.dart
+++ b/runtime/observatory/lib/src/elements/sample_buffer_control.dart
@@ -18,7 +18,7 @@ class SampleBufferControlElement extends CustomElement implements Renderable {
   static const tag =
       const Tag<SampleBufferControlElement>('sample-buffer-control');
 
-  late RenderingScheduler<SampleBufferControlElement> _r;
+  RenderingScheduler<SampleBufferControlElement> _r;
 
   Stream<RenderedEvent<SampleBufferControlElement>> get onRendered =>
       _r.onRendered;
@@ -28,13 +28,13 @@ class SampleBufferControlElement extends CustomElement implements Renderable {
   Stream<SampleBufferControlChangedElement> get onTagChange =>
       _onTagChange.stream;
 
-  late M.VM _vm;
-  late Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
-  late M.SampleProfileLoadingProgress _progress;
-  late M.SampleProfileTag _tag;
+  M.VM _vm;
+  Stream<M.SampleProfileLoadingProgressEvent> _progressStream;
+  M.SampleProfileLoadingProgress _progress;
+  M.SampleProfileTag _tag;
   bool _showTag = false;
   bool _profileVM = false;
-  late StreamSubscription _subscription;
+  StreamSubscription _subscription;
 
   M.SampleProfileLoadingProgress get progress => _progress;
   M.SampleProfileTag get selectedTag => _tag;
@@ -52,7 +52,7 @@ class SampleBufferControlElement extends CustomElement implements Renderable {
       Stream<M.SampleProfileLoadingProgressEvent> progressStream,
       {M.SampleProfileTag selectedTag: M.SampleProfileTag.none,
       bool showTag: true,
-      RenderingQueue? queue}) {
+      RenderingQueue queue}) {
     assert(progress != null);
     assert(progressStream != null);
     assert(selectedTag != null);
diff --git a/runtime/observatory/lib/src/elements/script_inset.dart b/runtime/observatory/lib/src/elements/script_inset.dart
index 4632cb7e593..032072b8ce1 100644
--- a/runtime/observatory/lib/src/elements/script_inset.dart
+++ b/runtime/observatory/lib/src/elements/script_inset.dart
@@ -19,22 +19,22 @@ import 'package:observatory/utils.dart';
 class ScriptInsetElement extends CustomElement implements Renderable {
   static const tag = const Tag<ScriptInsetElement>('script-inset');
 
-  late RenderingScheduler<ScriptInsetElement> _r;
+  RenderingScheduler<ScriptInsetElement> _r;
 
   Stream<RenderedEvent<ScriptInsetElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ScriptRef _script;
-  M.Script? _loadedScript;
-  late M.ScriptRepository _scripts;
-  late M.ObjectRepository _objects;
-  late M.EventRepository _events;
-  late StreamSubscription _subscription;
-  int? _startPos;
-  int? _endPos;
-  int? _currentPos;
-  late bool _inDebuggerContext;
-  Iterable? _variables;
+  M.IsolateRef _isolate;
+  M.ScriptRef _script;
+  M.Script _loadedScript;
+  M.ScriptRepository _scripts;
+  M.ObjectRepository _objects;
+  M.EventRepository _events;
+  StreamSubscription _subscription;
+  int _startPos;
+  int _endPos;
+  int _currentPos;
+  bool _inDebuggerContext;
+  Iterable _variables;
 
   M.IsolateRef get isolate => _isolate;
   M.ScriptRef get script => _script;
@@ -45,12 +45,12 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       M.ScriptRepository scripts,
       M.ObjectRepository objects,
       M.EventRepository events,
-      {int? startPos,
-      int? endPos,
-      int? currentPos,
+      {int startPos,
+      int endPos,
+      int currentPos,
       bool inDebuggerContext: false,
       Iterable variables: const [],
-      RenderingQueue? queue}) {
+      RenderingQueue queue}) {
     assert(isolate != null);
     assert(script != null);
     assert(scripts != null);
@@ -75,7 +75,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
   ScriptInsetElement.created() : super.created(tag);
 
-  bool get noSource => _startPos == -1 || _loadedScript!.source == null;
+  bool get noSource => _startPos == -1 || _loadedScript.source == null;
 
   @override
   void attached() {
@@ -85,11 +85,11 @@ class ScriptInsetElement extends CustomElement implements Renderable {
         .where((e) => e is M.BreakpointEvent)
         .map((e) => (e as M.BreakpointEvent).breakpoint)
         .listen((M.Breakpoint b) async {
-      final M.Location loc = b.location!;
-      int? line;
+      final M.Location loc = b.location;
+      int line;
       if (loc.script.id == script.id) {
         if (loc.tokenPos != null) {
-          line = _loadedScript!.tokenToLine(loc.tokenPos!);
+          line = _loadedScript.tokenToLine(loc.tokenPos);
         } else {
           line = (loc as dynamic).line;
         }
@@ -98,12 +98,12 @@ class ScriptInsetElement extends CustomElement implements Renderable {
           line = (loc as dynamic).line;
         } on NoSuchMethodError {
           if (loc.tokenPos != null) {
-            M.Script scriptUsed = await _scripts.get(_isolate, loc.script.id!);
-            line = scriptUsed.tokenToLine(loc.tokenPos!);
+            M.Script scriptUsed = await _scripts.get(_isolate, loc.script.id);
+            line = scriptUsed.tokenToLine(loc.tokenPos);
           }
         }
       }
-      if ((line == null) || ((line >= _startLine!) && (line <= _endLine!))) {
+      if ((line == null) || ((line >= _startLine) && (line <= _endLine))) {
         _r.dirty();
       }
     });
@@ -132,9 +132,9 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
         firstBuild = true;
       }
-      children = <Element>[container!];
-      container!.children.clear();
-      container!.children.add(table);
+      children = <Element>[container];
+      container.children.clear();
+      container.children.add(table);
       _makeCssClassUncopyable(table, "noCopy");
       if (firstBuild) {
         _scrollToCurrentPos();
@@ -143,19 +143,19 @@ class ScriptInsetElement extends CustomElement implements Renderable {
   }
 
   Future _refresh() async {
-    _loadedScript = await _scripts.get(_isolate, _script.id!);
+    _loadedScript = await _scripts.get(_isolate, _script.id);
     await _refreshSourceReport();
     await _computeAnnotations();
     _r.dirty();
   }
 
-  ButtonElement? _refreshButton;
-  ButtonElement? _toggleProfileButton;
+  ButtonElement _refreshButton;
+  ButtonElement _toggleProfileButton;
 
-  int? _currentLine;
-  int? _currentCol;
-  int? _startLine;
-  int? _endLine;
+  int _currentLine;
+  int _currentCol;
+  int _startLine;
+  int _endLine;
 
   Map/*<int, List<S.ServiceMap>>*/ _rangeMap = {};
   Set _callSites = new Set<S.CallSite>();
@@ -167,7 +167,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
   bool _includeProfile = false;
 
-  String makeLineClass(int? line) {
+  String makeLineClass(int line) {
     return 'script-inset-line-$line';
   }
 
@@ -217,7 +217,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     return element;
   }
 
-  Element? container;
+  Element container;
 
   // Build _rangeMap and _callSites from a source report.
   Future _refreshSourceReport() async {
@@ -231,16 +231,16 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       reports.add(S.Isolate.kProfileReport);
     }
     S.Isolate isolate = _isolate as S.Isolate;
-    dynamic sourceReport = await isolate.getSourceReport(
-        reports, script as S.Script, _startPos, _endPos);
+    dynamic sourceReport =
+        await isolate.getSourceReport(reports, script, _startPos, _endPos);
     _possibleBreakpointLines =
-        S.getPossibleBreakpointLines(sourceReport, script as S.Script);
+        S.getPossibleBreakpointLines(sourceReport, script);
     _rangeMap.clear();
     _callSites.clear();
     _profileMap.clear();
     for (var range in sourceReport['ranges']) {
-      int? startLine = _loadedScript!.tokenToLine(range['startPos']);
-      int? endLine = _loadedScript!.tokenToLine(range['endPos']);
+      int startLine = _loadedScript.tokenToLine(range['startPos']);
+      int endLine = _loadedScript.tokenToLine(range['endPos']);
       // TODO(turnidge): Track down the root cause of null startLine/endLine.
       if ((startLine != null) && (endLine != null)) {
         for (var line = startLine; line <= endLine; line++) {
@@ -265,10 +265,10 @@ class ScriptInsetElement extends CustomElement implements Renderable {
             // TODO(johnmccutchan): Add classifier data to UI.
             continue;
           }
-          int? line = _loadedScript!.tokenToLine(positions[i]);
-          ScriptLineProfile? lineProfile = _profileMap[line];
+          int line = _loadedScript.tokenToLine(positions[i]);
+          ScriptLineProfile lineProfile = _profileMap[line];
           if (lineProfile == null) {
-            lineProfile = new ScriptLineProfile(line!, sampleCount);
+            lineProfile = new ScriptLineProfile(line, sampleCount);
             _profileMap[line] = lineProfile;
           }
           lineProfile.process(exclusiveTicks[i], inclusiveTicks[i]);
@@ -278,8 +278,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
         var rangeCallSites = range['callSites'];
         if (rangeCallSites != null) {
           for (var callSiteMap in rangeCallSites) {
-            _callSites
-                .add(new S.CallSite.fromMap(callSiteMap, script as S.Script));
+            _callSites.add(new S.CallSite.fromMap(callSiteMap, script));
           }
         }
       }
@@ -290,22 +289,21 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     if (noSource) return;
 
     _startLine = (_startPos != null
-        ? _loadedScript!.tokenToLine(_startPos!)
-        : 1 + _loadedScript!.lineOffset!);
+        ? _loadedScript.tokenToLine(_startPos)
+        : 1 + _loadedScript.lineOffset);
     _currentLine =
-        (_currentPos != null ? _loadedScript!.tokenToLine(_currentPos!) : null);
-    _currentCol = (_currentPos != null
-        ? (_loadedScript!.tokenToCol(_currentPos!))
-        : null);
+        (_currentPos != null ? _loadedScript.tokenToLine(_currentPos) : null);
+    _currentCol =
+        (_currentPos != null ? (_loadedScript.tokenToCol(_currentPos)) : null);
     if (_currentCol != null) {
-      _currentCol = _currentCol! - 1; // make this 0-based.
+      _currentCol--; // make this 0-based.
     }
 
     S.Script script = _loadedScript as S.Script;
 
     _endLine = (_endPos != null
-        ? _loadedScript!.tokenToLine(_endPos!)
-        : script.lines.length + _loadedScript!.lineOffset!);
+        ? _loadedScript.tokenToLine(_endPos)
+        : script.lines.length + _loadedScript.lineOffset);
 
     if (_startLine == null || _endLine == null) {
       return;
@@ -317,7 +315,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     addBreakpointAnnotations();
 
     if (!_inDebuggerContext && script.library != null) {
-      await loadDeclarationsOfLibrary(script.library!);
+      await loadDeclarationsOfLibrary(script.library);
       addLibraryAnnotations();
       addDependencyAnnotations();
       addPartAnnotations();
@@ -335,22 +333,22 @@ class ScriptInsetElement extends CustomElement implements Renderable {
   void addCurrentExecutionAnnotation() {
     if (_currentLine != null) {
       var a = new CurrentExecutionAnnotation(_isolate, _objects, _r.queue);
-      a.line = _currentLine!;
-      a.columnStart = _currentCol!;
+      a.line = _currentLine;
+      a.columnStart = _currentCol;
       S.Script script = _loadedScript as S.Script;
-      var length = script.guessTokenLength(_currentLine!, _currentCol!);
+      var length = script.guessTokenLength(_currentLine, _currentCol);
       if (length == null) {
         length = 1;
       }
-      a.columnStop = _currentCol! + length;
+      a.columnStop = _currentCol + length;
       _annotations.add(a);
     }
   }
 
   void addBreakpointAnnotations() {
     S.Script script = _loadedScript as S.Script;
-    for (var line = _startLine!; line <= _endLine!; line++) {
-      var bpts = script.getLine(line)!.breakpoints;
+    for (var line = _startLine; line <= _endLine; line++) {
+      var bpts = script.getLine(line).breakpoints;
       if (bpts != null) {
         for (var bpt in bpts) {
           if (bpt.location != null) {
@@ -364,7 +362,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
   Future loadDeclarationsOfLibrary(S.Library lib) {
     return lib.load().then((serviceObject) {
-      S.Library lib = serviceObject as S.Library;
+      S.Library lib = serviceObject;
       var loads = <Future>[];
       for (var func in lib.functions) {
         loads.add(func.load());
@@ -381,7 +379,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
   Future loadDeclarationsOfClass(S.Class cls) {
     return cls.load().then((serviceObject) {
-      S.Class cls = serviceObject as S.Class;
+      S.Class cls = serviceObject;
       var loads = <Future>[];
       for (var func in cls.functions) {
         loads.add(func.load());
@@ -397,30 +395,30 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     S.Script script = _loadedScript as S.Script;
     for (S.ScriptLine line in script.lines) {
       // TODO(rmacnak): Use a real scanner.
-      var pattern = new RegExp("library ${script.library!.name!}");
+      var pattern = new RegExp("library ${script.library.name}");
       var match = pattern.firstMatch(line.text);
       if (match != null) {
         var anno = new LibraryAnnotation(
             _isolate,
             _objects,
             _r.queue,
-            _loadedScript!.library as S.Library,
-            Uris.inspect(isolate, object: _loadedScript!.library));
+            _loadedScript.library,
+            Uris.inspect(isolate, object: _loadedScript.library));
         anno.line = line.line;
         anno.columnStart = match.start + 8;
         anno.columnStop = match.end;
         _annotations.add(anno);
       }
       // TODO(rmacnak): Use a real scanner.
-      pattern = new RegExp("part of ${script.library!.name!}");
+      pattern = new RegExp("part of ${script.library.name}");
       match = pattern.firstMatch(line.text);
       if (match != null) {
         var anno = new LibraryAnnotation(
             _isolate,
             _objects,
             _r.queue,
-            _loadedScript!.library as S.Library,
-            Uris.inspect(isolate, object: _loadedScript!.library));
+            _loadedScript.library,
+            Uris.inspect(isolate, object: _loadedScript.library));
         anno.line = line.line;
         anno.columnStart = match.start + 8;
         anno.columnStop = match.end;
@@ -429,23 +427,22 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     }
   }
 
-  S.Library? resolveDependency(String relativeUri) {
+  M.Library resolveDependency(String relativeUri) {
     S.Script script = _loadedScript as S.Script;
     // This isn't really correct: we need to ask the embedder to do the
     // uri canonicalization for us, but Observatory isn't in a position
     // to invoke the library tag handler. Handle the most common cases.
-    var targetUri =
-        Uri.parse(_loadedScript!.library!.uri!).resolve(relativeUri);
-    for (M.Library l in script.isolate!.libraries) {
+    var targetUri = Uri.parse(_loadedScript.library.uri).resolve(relativeUri);
+    for (M.Library l in script.isolate.libraries) {
       if (targetUri.toString() == l.uri) {
-        return l as S.Library;
+        return l;
       }
     }
     if (targetUri.scheme == 'package') {
       var targetUriString = "packages/${targetUri.path}";
-      for (M.Library l in script.isolate!.libraries) {
+      for (M.Library l in script.isolate.libraries) {
         if (targetUriString == l.uri) {
-          return l as S.Library;
+          return l;
         }
       }
     }
@@ -467,10 +464,10 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       for (var pattern in patterns) {
         var match = pattern.firstMatch(line.text);
         if (match != null) {
-          M.Library? target = resolveDependency(match[1]!);
+          M.Library target = resolveDependency(match[1]);
           if (target != null) {
             var anno = new LibraryAnnotation(_isolate, _objects, _r.queue,
-                target as S.Library, Uris.inspect(isolate, object: target));
+                target, Uris.inspect(isolate, object: target));
             anno.line = line.line;
             anno.columnStart = match.start + 8;
             anno.columnStop = match.end - 1;
@@ -481,17 +478,17 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     }
   }
 
-  S.Script? resolvePart(String relativeUri) {
+  M.Script resolvePart(String relativeUri) {
     S.Script script = _loadedScript as S.Script;
-    var rootUri = Uri.parse(script.library!.uri!);
+    var rootUri = Uri.parse(script.library.uri);
     if (rootUri.scheme == 'dart') {
       // The relative paths from dart:* libraries to their parts are not valid.
-      rootUri = Uri.parse(script.library!.uri! + '/');
+      rootUri = Uri.parse(script.library.uri + '/');
     }
     var targetUri = rootUri.resolve(relativeUri);
-    for (M.Script s in script.library!.scripts) {
+    for (M.Script s in script.library.scripts) {
       if (targetUri.toString() == s.uri) {
-        return s as S.Script?;
+        return s;
       }
     }
     print("Could not resolve part: $relativeUri");
@@ -509,7 +506,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       for (var pattern in patterns) {
         var match = pattern.firstMatch(line.text);
         if (match != null) {
-          S.Script? part = resolvePart(match[1]!);
+          S.Script part = resolvePart(match[1]);
           if (part != null) {
             var anno = new PartAnnotation(_isolate, _objects, _r.queue, part,
                 Uris.inspect(isolate, object: part));
@@ -525,8 +522,8 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
   void addClassAnnotations() {
     S.Script script = _loadedScript as S.Script;
-    for (var cls in script.library!.classes) {
-      if ((cls.location != null) && (cls.location!.script == script)) {
+    for (var cls in script.library.classes) {
+      if ((cls.location != null) && (cls.location.script == script)) {
         var a = new ClassDeclarationAnnotation(_isolate, _objects, _r.queue,
             cls, Uris.inspect(isolate, object: cls));
         _annotations.add(a);
@@ -536,16 +533,16 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
   void addFieldAnnotations() {
     S.Script script = _loadedScript as S.Script;
-    for (var field in script.library!.variables) {
-      if ((field.location != null) && (field.location!.script == script)) {
+    for (var field in script.library.variables) {
+      if ((field.location != null) && (field.location.script == script)) {
         var a = new FieldDeclarationAnnotation(_isolate, _objects, _r.queue,
             field, Uris.inspect(isolate, object: field));
         _annotations.add(a);
       }
     }
-    for (var cls in script.library!.classes) {
+    for (var cls in script.library.classes) {
       for (var field in cls.fields) {
-        if ((field.location != null) && (field.location!.script == script)) {
+        if ((field.location != null) && (field.location.script == script)) {
           var a = new FieldDeclarationAnnotation(_isolate, _objects, _r.queue,
               field, Uris.inspect(isolate, object: field));
           _annotations.add(a);
@@ -556,9 +553,9 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
   void addFunctionAnnotations() {
     S.Script script = _loadedScript as S.Script;
-    for (var func in script.library!.functions) {
+    for (var func in script.library.functions) {
       if ((func.location != null) &&
-          (func.location!.script == script) &&
+          (func.location.script == script) &&
           (func.kind != M.FunctionKind.implicitGetter) &&
           (func.kind != M.FunctionKind.implicitSetter)) {
         // We annotate a field declaration with the field instead of the
@@ -568,11 +565,11 @@ class ScriptInsetElement extends CustomElement implements Renderable {
         _annotations.add(a);
       }
     }
-    for (var cls in script.library!.classes) {
+    for (var cls in script.library.classes) {
       S.Script script = _loadedScript as S.Script;
       for (var func in cls.functions) {
         if ((func.location != null) &&
-            (func.location!.script == script) &&
+            (func.location.script == script) &&
             (func.kind != M.FunctionKind.implicitGetter) &&
             (func.kind != M.FunctionKind.implicitSetter)) {
           // We annotate a field declaration with the field instead of the
@@ -597,7 +594,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     // We have local variable information.
     if (_variables != null) {
       // For each variable.
-      for (var variable in _variables!) {
+      for (var variable in _variables) {
         // Find variable usage locations.
         var locations = script.scanForLocalVariableLocations(
             variable['name'], variable['_tokenPos'], variable['_endTokenPos']);
@@ -620,7 +617,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       button.disabled = false;
     });
     button.title = 'Refresh coverage';
-    button.children = <Element>[_iconRefresh.clone(true) as Element];
+    button.children = <Element>[_iconRefresh.clone(true)];
     return button;
   }
 
@@ -636,7 +633,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       _refresh();
       button.disabled = false;
     });
-    button.children = <Element>[_iconWhatsHot.clone(true) as Element];
+    button.children = <Element>[_iconWhatsHot.clone(true)];
     return button;
   }
 
@@ -647,23 +644,23 @@ class ScriptInsetElement extends CustomElement implements Renderable {
 
     _refreshButton = _newRefreshButton();
     _toggleProfileButton = _newToggleProfileButton();
-    table.append(_refreshButton!);
-    table.append(_toggleProfileButton!);
+    table.append(_refreshButton);
+    table.append(_toggleProfileButton);
 
     if (_startLine == null || _endLine == null) {
       return table;
     }
 
     var endLine = (_endPos != null
-        ? _loadedScript!.tokenToLine(_endPos!)
-        : script.lines.length + _loadedScript!.lineOffset!);
+        ? _loadedScript.tokenToLine(_endPos)
+        : script.lines.length + _loadedScript.lineOffset);
     var lineNumPad = endLine.toString().length;
 
     _annotationsCursor = 0;
 
     int blankLineCount = 0;
-    for (int i = _startLine!; i <= _endLine!; i++) {
-      var line = script.getLine(i)!;
+    for (int i = _startLine; i <= _endLine; i++) {
+      var line = script.getLine(i);
       if (line.isBlank) {
         // Try to introduce ellipses if there are 4 or more contiguous
         // blank lines.
@@ -693,7 +690,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
   }
 
   // Assumes annotations are sorted.
-  Annotation? nextAnnotationOnLine(int line) {
+  Annotation nextAnnotationOnLine(int line) {
     if (_annotationsCursor >= _annotations.length) return null;
     var annotation = _annotations[_annotationsCursor];
 
@@ -711,7 +708,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     return annotation;
   }
 
-  Element lineElement(S.ScriptLine? line, int lineNumPad) {
+  Element lineElement(S.ScriptLine line, int lineNumPad) {
     var e = new DivElement();
     e.classes.add("sourceRow");
     e.append(lineBreakpointElement(line));
@@ -724,7 +721,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     return e;
   }
 
-  Element lineProfileElement(S.ScriptLine? line, bool self) {
+  Element lineProfileElement(S.ScriptLine line, bool self) {
     var e = span('');
     e.classes.add('noCopy');
     if (self) {
@@ -746,7 +743,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       return e;
     }
 
-    ScriptLineProfile? lineProfile = _profileMap[line.line];
+    ScriptLineProfile lineProfile = _profileMap[line.line];
     if (lineProfile == null) {
       e.classes.add('noProfile');
       e.text = nbsp;
@@ -770,7 +767,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     return e;
   }
 
-  Element lineBreakpointElement(S.ScriptLine? line) {
+  Element lineBreakpointElement(S.ScriptLine line) {
     var e = new DivElement();
     if (line == null || !_possibleBreakpointLines.contains(line.line)) {
       e.classes.add('noCopy');
@@ -788,8 +785,8 @@ class ScriptInsetElement extends CustomElement implements Renderable {
         e.classes.add("busyBreakpoint");
       } else if (line.breakpoints != null) {
         bool resolved = false;
-        for (var bpt in line.breakpoints!) {
-          if (bpt.resolved!) {
+        for (var bpt in line.breakpoints) {
+          if (bpt.resolved) {
             resolved = true;
             break;
           }
@@ -811,7 +808,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       busy = true;
       if (line.breakpoints == null) {
         // No breakpoint.  Add it.
-        line.script.isolate!
+        line.script.isolate
             .addBreakpoint(line.script, line.line)
             .catchError((e, st) {
           if (e is! S.ServerRpcException ||
@@ -826,8 +823,8 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       } else {
         // Existing breakpoint.  Remove it.
         List<Future> pending = [];
-        for (var bpt in line.breakpoints!) {
-          pending.add(line.script.isolate!.removeBreakpoint(bpt));
+        for (var bpt in line.breakpoints) {
+          pending.add(line.script.isolate.removeBreakpoint(bpt));
         }
         Future.wait(pending).then((_) {
           busy = false;
@@ -840,7 +837,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     return e;
   }
 
-  Element lineNumberElement(S.ScriptLine? line, int lineNumPad) {
+  Element lineNumberElement(S.ScriptLine line, int lineNumPad) {
     var lineNumber = line == null ? "..." : line.line;
     var e =
         span("$nbsp${lineNumber.toString().padLeft(lineNumPad, nbsp)}$nbsp");
@@ -861,7 +858,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     for (var range in ranges) {
       if (range['compiled']) {
         for (var callSite in range['callSites']) {
-          var callLine = line!.script.tokenToLine(callSite['tokenPos']);
+          var callLine = line.script.tokenToLine(callSite['tokenPos']);
           if (lineNumber == callLine) {
             // The call site is on the current line.
             hasCallInfo = true;
@@ -893,7 +890,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     return e;
   }
 
-  Element lineSourceElement(S.ScriptLine? line) {
+  Element lineSourceElement(S.ScriptLine line) {
     var e = new DivElement();
     e.classes.add("sourceItem");
 
@@ -921,11 +918,10 @@ class ScriptInsetElement extends CustomElement implements Renderable {
       }
 
       // TODO(rmacnak): Tolerate overlapping annotations.
-      var annotation = nextAnnotationOnLine(line.line);
-      while (annotation != null) {
+      var annotation;
+      while ((annotation = nextAnnotationOnLine(line.line)) != null) {
         consumeUntil(annotation.columnStart);
         annotation.applyStyleTo(consumeUntil(annotation.columnStop));
-        annotation = nextAnnotationOnLine(line.line);
       }
       consumeUntil(line.text.length);
     }
@@ -941,8 +937,7 @@ class ScriptInsetElement extends CustomElement implements Renderable {
   /// children have been added, and only supports one node at a time.
   static void _makeCssClassUncopyable(Element root, String className) {
     var noCopyNodes = root.getElementsByClassName(className);
-    for (Node n in noCopyNodes) {
-      var node = n as HtmlElement;
+    for (HtmlElement node in noCopyNodes) {
       node.style.setProperty('-moz-user-select', 'none');
       node.style.setProperty('-khtml-user-select', 'none');
       node.style.setProperty('-webkit-user-select', 'none');
@@ -952,13 +947,11 @@ class ScriptInsetElement extends CustomElement implements Renderable {
     root.onCopy.listen((event) {
       // Mark the nodes as hidden before the copy happens, then mark them as
       // visible on the next event loop turn.
-      for (Node n in noCopyNodes) {
-        var node = n as HtmlElement;
+      for (HtmlElement node in noCopyNodes) {
         node.style.visibility = 'hidden';
       }
       Timer.run(() {
-        for (Node n in noCopyNodes) {
-          var node = n as HtmlElement;
+        for (HtmlElement node in noCopyNodes) {
           node.style.visibility = 'visible';
         }
       });
@@ -1008,10 +1001,10 @@ void addLink(Element content, String target) {
 abstract class Annotation implements Comparable<Annotation> {
   M.IsolateRef _isolate;
   M.ObjectRepository _objects;
-  RenderingQueue? queue;
-  int? line;
-  int? columnStart;
-  int? columnStop;
+  RenderingQueue queue;
+  int line;
+  int columnStart;
+  int columnStop;
   int get priority;
 
   Annotation(this._isolate, this._objects, this.queue);
@@ -1023,9 +1016,9 @@ abstract class Annotation implements Comparable<Annotation> {
       if (columnStart == other.columnStart) {
         return priority.compareTo(other.priority);
       }
-      return columnStart!.compareTo(other.columnStart!);
+      return columnStart.compareTo(other.columnStart);
     }
-    return line!.compareTo(other.line!);
+    return line.compareTo(other.line);
   }
 
   Element table() {
@@ -1062,7 +1055,7 @@ class CurrentExecutionAnnotation extends Annotation {
   int priority = 0; // highest priority.
 
   CurrentExecutionAnnotation(
-      M.IsolateRef isolate, M.ObjectRepository objects, RenderingQueue? queue)
+      M.IsolateRef isolate, M.ObjectRepository objects, RenderingQueue queue)
       : super(isolate, objects, queue);
 
   void applyStyleTo(element) {
@@ -1079,34 +1072,37 @@ class BreakpointAnnotation extends Annotation {
   int priority = 1;
 
   BreakpointAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, this.bpt)
+      RenderingQueue queue, this.bpt)
       : super(isolate, objects, queue) {
-    S.Script script = bpt.location!.script as S.Script;
-    var location = bpt.location!;
+    S.Script script = bpt.location.script;
+    var location = bpt.location;
     if (location.tokenPos != null) {
-      var pos = location.tokenPos!;
+      var pos = location.tokenPos;
       line = script.tokenToLine(pos);
-      columnStart = script.tokenToCol(pos)! - 1; // tokenToCol is 1-origin.
+      columnStart = script.tokenToCol(pos) - 1; // tokenToCol is 1-origin.
     } else if (location is M.UnresolvedSourceLocation) {
-      line = location.line!;
-      columnStart = location.column ?? 0;
+      line = location.line;
+      columnStart = location.column;
+      if (columnStart == null) {
+        columnStart = 0;
+      }
     }
-    var length = script.guessTokenLength(line!, columnStart!);
+    var length = script.guessTokenLength(line, columnStart);
     if (length == null) {
       length = 1;
     }
-    columnStop = columnStart! + length;
+    columnStop = columnStart + length;
   }
 
   void applyStyleTo(element) {
     if (element == null) {
       return; // TODO(rmacnak): Handling overlapping annotations.
     }
-    S.Script script = bpt.location!.script as S.Script;
-    int? pos = bpt.location!.tokenPos;
-    int? line = script.tokenToLine(pos);
-    int? column = script.tokenToCol(pos);
-    if (bpt.resolved!) {
+    S.Script script = bpt.location.script;
+    var pos = bpt.location.tokenPos;
+    int line = script.tokenToLine(pos);
+    int column = script.tokenToCol(pos);
+    if (bpt.resolved) {
       element.classes.add("resolvedBreakAnnotation");
     } else {
       element.classes.add("unresolvedBreakAnnotation");
@@ -1121,7 +1117,7 @@ class LibraryAnnotation extends Annotation {
   int priority = 2;
 
   LibraryAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, this.target, this.url)
+      RenderingQueue queue, this.target, this.url)
       : super(isolate, objects, queue);
 
   void applyStyleTo(element) {
@@ -1139,7 +1135,7 @@ class PartAnnotation extends Annotation {
   int priority = 2;
 
   PartAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, this.part, this.url)
+      RenderingQueue queue, this.part, this.url)
       : super(isolate, objects, queue);
 
   void applyStyleTo(element) {
@@ -1156,7 +1152,7 @@ class LocalVariableAnnotation extends Annotation {
   int priority = 2;
 
   LocalVariableAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, S.LocalVarLocation location, this.value)
+      RenderingQueue queue, S.LocalVarLocation location, this.value)
       : super(isolate, objects, queue) {
     line = location.line;
     columnStart = location.column;
@@ -1177,17 +1173,17 @@ class CallSiteAnnotation extends Annotation {
   int priority = 2;
 
   CallSiteAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, this.callSite)
+      RenderingQueue queue, this.callSite)
       : super(isolate, objects, queue) {
     line = callSite.line;
     columnStart = callSite.column - 1; // Call site is 1-origin.
-    var tokenLength = callSite.script.guessTokenLength(line!, columnStart!);
+    var tokenLength = callSite.script.guessTokenLength(line, columnStart);
     if (tokenLength == null) {
       tokenLength = callSite.name.length; // Approximate.
       if (callSite.name.startsWith("get:") || callSite.name.startsWith("set:"))
         tokenLength -= 4;
     }
-    columnStop = columnStart! + tokenLength;
+    columnStop = columnStart + tokenLength;
   }
 
   void applyStyleTo(element) {
@@ -1230,7 +1226,7 @@ abstract class DeclarationAnnotation extends Annotation {
   int priority = 2;
 
   DeclarationAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, decl, this.url)
+      RenderingQueue queue, decl, this.url)
       : super(isolate, objects, queue) {
     assert(decl.loaded);
     S.SourceLocation location = decl.location;
@@ -1249,18 +1245,18 @@ abstract class DeclarationAnnotation extends Annotation {
       columnStart = 0;
       columnStop = 0;
     } else {
-      columnStart = columnStart! - 1; // 1-origin -> 0-origin.
+      columnStart--; // 1-origin -> 0-origin.
 
       // The method's token position is at the beginning of the method
       // declaration, which may be a return type annotation, metadata, static
       // modifier, etc. Try to scan forward to position this annotation on the
       // function's name instead.
-      var lineSource = script.getLine(line!)!.text;
-      var betterStart = lineSource.indexOf(decl.name, columnStart!);
+      var lineSource = script.getLine(line).text;
+      var betterStart = lineSource.indexOf(decl.name, columnStart);
       if (betterStart != -1) {
         columnStart = betterStart;
       }
-      columnStop = columnStart! + (decl.name.length as int);
+      columnStop = columnStart + decl.name.length;
     }
   }
 }
@@ -1269,7 +1265,7 @@ class ClassDeclarationAnnotation extends DeclarationAnnotation {
   S.Class klass;
 
   ClassDeclarationAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, S.Class cls, String url)
+      RenderingQueue queue, S.Class cls, String url)
       : klass = cls,
         super(isolate, objects, queue, cls, url);
 
@@ -1286,7 +1282,7 @@ class FieldDeclarationAnnotation extends DeclarationAnnotation {
   S.Field field;
 
   FieldDeclarationAnnotation(M.IsolateRef isolate, M.ObjectRepository objects,
-      RenderingQueue? queue, S.Field fld, String url)
+      RenderingQueue queue, S.Field fld, String url)
       : field = fld,
         super(isolate, objects, queue, fld, url);
 
@@ -1306,7 +1302,7 @@ class FunctionDeclarationAnnotation extends DeclarationAnnotation {
   FunctionDeclarationAnnotation(
       M.IsolateRef isolate,
       M.ObjectRepository objects,
-      RenderingQueue? queue,
+      RenderingQueue queue,
       S.ServiceFunction func,
       String url)
       : function = func,
@@ -1323,14 +1319,14 @@ class FunctionDeclarationAnnotation extends DeclarationAnnotation {
     if (function.isInlinable == false) {
       tooltip += "\nNot inlinable!";
     }
-    if (function.deoptimizations! > 0) {
+    if (function.deoptimizations > 0) {
       tooltip += "\nDeoptimized ${function.deoptimizations} times!";
     }
     element.title = tooltip;
 
     if (function.isOptimizable == false ||
         function.isInlinable == false ||
-        function.deoptimizations! > 0) {
+        function.deoptimizations > 0) {
       element.style.backgroundColor = "#EEA7A7"; // Low-saturation red.
     }
 
diff --git a/runtime/observatory/lib/src/elements/script_ref.dart b/runtime/observatory/lib/src/elements/script_ref.dart
index 3133aab2f2a..91f8e689d0a 100644
--- a/runtime/observatory/lib/src/elements/script_ref.dart
+++ b/runtime/observatory/lib/src/elements/script_ref.dart
@@ -14,18 +14,18 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class ScriptRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<ScriptRefElement>('script-ref');
 
-  late RenderingScheduler<ScriptRefElement> _r;
+  RenderingScheduler<ScriptRefElement> _r;
 
   Stream<RenderedEvent<ScriptRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ScriptRef _script;
+  M.IsolateRef _isolate;
+  M.ScriptRef _script;
 
   M.IsolateRef get isolate => _isolate;
   M.ScriptRef get script => _script;
 
   factory ScriptRefElement(M.IsolateRef isolate, M.ScriptRef script,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(script != null);
     ScriptRefElement e = new ScriptRefElement.created();
@@ -51,14 +51,14 @@ class ScriptRefElement extends CustomElement implements Renderable {
   }
 
   void render() {
-    var displayUri = script.uri!.split('/').last;
+    var displayUri = script.uri.split('/').last;
     if (displayUri.isEmpty) {
       displayUri = 'N/A';
     }
 
     children = <Element>[
       new AnchorElement(href: Uris.inspect(isolate, object: script))
-        ..title = script.uri!
+        ..title = script.uri
         ..text = displayUri
     ];
   }
diff --git a/runtime/observatory/lib/src/elements/script_view.dart b/runtime/observatory/lib/src/elements/script_view.dart
index bf6fd3aa0b3..7be611b3a11 100644
--- a/runtime/observatory/lib/src/elements/script_view.dart
+++ b/runtime/observatory/lib/src/elements/script_view.dart
@@ -39,22 +39,22 @@ class ScriptViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<ScriptViewElement> _r;
+  RenderingScheduler<ScriptViewElement> _r;
 
   Stream<RenderedEvent<ScriptViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.Script _script;
-  late M.ScriptRepository _scripts;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
-  int? _pos;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.Script _script;
+  M.ScriptRepository _scripts;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
+  int _pos;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -73,8 +73,8 @@ class ScriptViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {int? pos,
-      RenderingQueue? queue}) {
+      {int pos,
+      RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -124,13 +124,13 @@ class ScriptViewElement extends CustomElement implements Renderable {
         new NavTopMenuElement(queue: _r.queue).element,
         new NavVMMenuElement(_vm, _events, queue: _r.queue).element,
         new NavIsolateMenuElement(_isolate, _events, queue: _r.queue).element,
-        new NavLibraryMenuElement(_isolate, _script.library!, queue: _r.queue)
+        new NavLibraryMenuElement(_isolate, _script.library, queue: _r.queue)
             .element,
         navMenu('object'),
         (new NavRefreshElement(queue: _r.queue)
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
-                _script = await _scripts.get(_isolate, _script.id!);
+                _script = await _scripts.get(_isolate, _script.id);
                 _r.dirty();
               }))
             .element,
diff --git a/runtime/observatory/lib/src/elements/sentinel_value.dart b/runtime/observatory/lib/src/elements/sentinel_value.dart
index 0f74b06adb4..b646d9e0494 100644
--- a/runtime/observatory/lib/src/elements/sentinel_value.dart
+++ b/runtime/observatory/lib/src/elements/sentinel_value.dart
@@ -11,15 +11,15 @@ import 'package:observatory/src/elements/helpers/tag.dart';
 class SentinelValueElement extends CustomElement implements Renderable {
   static const tag = const Tag<SentinelValueElement>('sentinel-value');
 
-  late RenderingScheduler<SentinelValueElement> _r;
+  RenderingScheduler<SentinelValueElement> _r;
 
   Stream<RenderedEvent<SentinelValueElement>> get onRendered => _r.onRendered;
 
-  late M.Sentinel _sentinel;
+  M.Sentinel _sentinel;
 
   M.Sentinel get sentinel => _sentinel;
 
-  factory SentinelValueElement(M.Sentinel sentinel, {RenderingQueue? queue}) {
+  factory SentinelValueElement(M.Sentinel sentinel, {RenderingQueue queue}) {
     assert(sentinel != null);
     SentinelValueElement e = new SentinelValueElement.created();
     e._r = new RenderingScheduler<SentinelValueElement>(e, queue: queue);
diff --git a/runtime/observatory/lib/src/elements/sentinel_view.dart b/runtime/observatory/lib/src/elements/sentinel_view.dart
index d4c1912fd28..496c930fcb4 100644
--- a/runtime/observatory/lib/src/elements/sentinel_view.dart
+++ b/runtime/observatory/lib/src/elements/sentinel_view.dart
@@ -25,15 +25,15 @@ class SentinelViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<SentinelViewElement> _r;
+  RenderingScheduler<SentinelViewElement> _r;
 
   Stream<RenderedEvent<SentinelViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.Sentinel _sentinel;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.Sentinel _sentinel;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
 
   M.Sentinel get sentinel => _sentinel;
 
@@ -43,7 +43,7 @@ class SentinelViewElement extends CustomElement implements Renderable {
       M.Sentinel sentinel,
       M.EventRepository events,
       M.NotificationRepository notifications,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(sentinel != null);
diff --git a/runtime/observatory/lib/src/elements/singletargetcache_ref.dart b/runtime/observatory/lib/src/elements/singletargetcache_ref.dart
index a2ab6b36934..9b8dd33a68c 100644
--- a/runtime/observatory/lib/src/elements/singletargetcache_ref.dart
+++ b/runtime/observatory/lib/src/elements/singletargetcache_ref.dart
@@ -14,20 +14,20 @@ class SingleTargetCacheRefElement extends CustomElement implements Renderable {
   static const tag =
       const Tag<SingleTargetCacheRefElement>('singletargetcache-ref');
 
-  late RenderingScheduler<SingleTargetCacheRefElement> _r;
+  RenderingScheduler<SingleTargetCacheRefElement> _r;
 
   Stream<RenderedEvent<SingleTargetCacheRefElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.SingleTargetCacheRef _singleTargetCache;
+  M.IsolateRef _isolate;
+  M.SingleTargetCacheRef _singleTargetCache;
 
   M.IsolateRef get isolate => _isolate;
   M.SingleTargetCacheRef get singleTargetCache => _singleTargetCache;
 
   factory SingleTargetCacheRefElement(
       M.IsolateRef isolate, M.SingleTargetCacheRef singleTargetCache,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(singleTargetCache != null);
     SingleTargetCacheRefElement e = new SingleTargetCacheRefElement.created();
@@ -60,7 +60,7 @@ class SingleTargetCacheRefElement extends CustomElement implements Renderable {
           new SpanElement()
             ..classes = ['emphasize']
             ..text = 'SingleTargetCache',
-          new SpanElement()..text = ' (${_singleTargetCache.target!.name})'
+          new SpanElement()..text = ' (${_singleTargetCache.target.name})'
         ]
     ];
   }
diff --git a/runtime/observatory/lib/src/elements/singletargetcache_view.dart b/runtime/observatory/lib/src/elements/singletargetcache_view.dart
index 08a0fb35db4..05a306ae1df 100644
--- a/runtime/observatory/lib/src/elements/singletargetcache_view.dart
+++ b/runtime/observatory/lib/src/elements/singletargetcache_view.dart
@@ -33,22 +33,22 @@ class SingleTargetCacheViewElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<SingleTargetCacheViewElement> _r;
+  RenderingScheduler<SingleTargetCacheViewElement> _r;
 
   Stream<RenderedEvent<SingleTargetCacheViewElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.SingleTargetCache _singleTargetCache;
-  late M.SingleTargetCacheRepository _singleTargetCaches;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.SingleTargetCache _singleTargetCache;
+  M.SingleTargetCacheRepository _singleTargetCaches;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -67,7 +67,7 @@ class SingleTargetCacheViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -122,7 +122,7 @@ class SingleTargetCacheViewElement extends CustomElement implements Renderable {
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
                 _singleTargetCache = await _singleTargetCaches.get(
-                    _isolate, _singleTargetCache.id!);
+                    _isolate, _singleTargetCache.id);
                 _r.dirty();
               }))
             .element,
diff --git a/runtime/observatory/lib/src/elements/source_inset.dart b/runtime/observatory/lib/src/elements/source_inset.dart
index f14f17aa458..9e3b78d39bf 100644
--- a/runtime/observatory/lib/src/elements/source_inset.dart
+++ b/runtime/observatory/lib/src/elements/source_inset.dart
@@ -14,18 +14,18 @@ import 'package:observatory/src/elements/script_inset.dart';
 class SourceInsetElement extends CustomElement implements Renderable {
   static const tag = const Tag<SourceInsetElement>('source-inset');
 
-  late RenderingScheduler<SourceInsetElement> _r;
+  RenderingScheduler<SourceInsetElement> _r;
 
   Stream<RenderedEvent<SourceInsetElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.SourceLocation _location;
-  late M.ScriptRepository _scripts;
-  late M.ObjectRepository _objects;
-  late M.EventRepository _events;
-  int? _currentPos;
-  late bool _inDebuggerContext;
-  late Iterable _variables;
+  M.IsolateRef _isolate;
+  M.SourceLocation _location;
+  M.ScriptRepository _scripts;
+  M.ObjectRepository _objects;
+  M.EventRepository _events;
+  int _currentPos;
+  bool _inDebuggerContext;
+  Iterable _variables;
 
   M.IsolateRef get isolate => _isolate;
   M.SourceLocation get location => _location;
@@ -36,10 +36,10 @@ class SourceInsetElement extends CustomElement implements Renderable {
       M.ScriptRepository scripts,
       M.ObjectRepository objects,
       M.EventRepository events,
-      {int? currentPos,
+      {int currentPos,
       bool inDebuggerContext: false,
       Iterable variables: const [],
-      RenderingQueue? queue}) {
+      RenderingQueue queue}) {
     assert(isolate != null);
     assert(location != null);
     assert(scripts != null);
@@ -80,7 +80,7 @@ class SourceInsetElement extends CustomElement implements Renderable {
       new ScriptInsetElement(
               _isolate, _location.script, _scripts, _objects, _events,
               startPos: _location.tokenPos,
-              endPos: _location.endTokenPos!,
+              endPos: _location.endTokenPos,
               currentPos: _currentPos,
               inDebuggerContext: _inDebuggerContext,
               variables: _variables,
diff --git a/runtime/observatory/lib/src/elements/source_link.dart b/runtime/observatory/lib/src/elements/source_link.dart
index 784b15cfcb2..b05ac573d33 100644
--- a/runtime/observatory/lib/src/elements/source_link.dart
+++ b/runtime/observatory/lib/src/elements/source_link.dart
@@ -16,21 +16,21 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class SourceLinkElement extends CustomElement implements Renderable {
   static const tag = const Tag<SourceLinkElement>('source-link');
 
-  late RenderingScheduler<SourceLinkElement> _r;
+  RenderingScheduler<SourceLinkElement> _r;
 
   Stream<RenderedEvent<SourceLinkElement>> get onRendered => _r.onRendered;
 
-  late IsolateRef _isolate;
-  late SourceLocation _location;
-  Script? _script;
-  late ScriptRepository _repository;
+  IsolateRef _isolate;
+  SourceLocation _location;
+  Script _script;
+  ScriptRepository _repository;
 
   IsolateRef get isolate => _isolate;
   SourceLocation get location => _location;
 
   factory SourceLinkElement(
       IsolateRef isolate, SourceLocation location, ScriptRepository repository,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(location != null);
     SourceLinkElement e = new SourceLinkElement.created();
@@ -46,7 +46,7 @@ class SourceLinkElement extends CustomElement implements Renderable {
   @override
   void attached() {
     super.attached();
-    _repository.get(_isolate, _location.script.id!).then((script) {
+    _repository.get(_isolate, _location.script.id).then((script) {
       _script = script;
       _r.dirty();
     }, onError: (e) {
@@ -77,14 +77,14 @@ class SourceLinkElement extends CustomElement implements Renderable {
     if (_script == null) {
       children = <Element>[new SpanElement()..text = '<LOADING>'];
     } else {
-      String label = _script!.uri!.split('/').last;
-      int? token = _location.tokenPos;
-      int? line = _script!.tokenToLine(token);
-      int? column = _script!.tokenToCol(token);
+      String label = _script.uri.split('/').last;
+      int token = _location.tokenPos;
+      int line = _script.tokenToLine(token);
+      int column = _script.tokenToCol(token);
       children = <Element>[
         new AnchorElement(
-            href: Uris.inspect(isolate, object: _script!, pos: token))
-          ..title = _script!.uri!
+            href: Uris.inspect(isolate, object: _script, pos: token))
+          ..title = _script.uri
           ..text = '${label}:${line}:${column}'
       ];
     }
diff --git a/runtime/observatory/lib/src/elements/stack_trace_tree_config.dart b/runtime/observatory/lib/src/elements/stack_trace_tree_config.dart
index 04c34b61b5c..396b93822de 100644
--- a/runtime/observatory/lib/src/elements/stack_trace_tree_config.dart
+++ b/runtime/observatory/lib/src/elements/stack_trace_tree_config.dart
@@ -22,7 +22,7 @@ class StackTraceTreeConfigElement extends CustomElement implements Renderable {
   static const tag =
       const Tag<StackTraceTreeConfigElement>('stack-trace-tree-config');
 
-  late RenderingScheduler<StackTraceTreeConfigElement> _r;
+  RenderingScheduler<StackTraceTreeConfigElement> _r;
 
   Stream<RenderedEvent<StackTraceTreeConfigElement>> get onRendered =>
       _r.onRendered;
@@ -40,12 +40,12 @@ class StackTraceTreeConfigElement extends CustomElement implements Renderable {
   Stream<StackTraceTreeConfigChangedEvent> get onFilterChange =>
       _onFilterChange.stream;
 
-  late bool _showMode;
-  late bool _showDirection;
-  late bool _showFilter;
-  late ProfileTreeMode _mode;
-  late M.ProfileTreeDirection _direction;
-  late String _filter;
+  bool _showMode;
+  bool _showDirection;
+  bool _showFilter;
+  ProfileTreeMode _mode;
+  M.ProfileTreeDirection _direction;
+  String _filter;
 
   bool get showMode => _showMode;
   bool get showDirection => _showDirection;
@@ -71,7 +71,7 @@ class StackTraceTreeConfigElement extends CustomElement implements Renderable {
       String filter: '',
       ProfileTreeMode mode: ProfileTreeMode.function,
       M.ProfileTreeDirection direction: M.ProfileTreeDirection.exclusive,
-      RenderingQueue? queue}) {
+      RenderingQueue queue}) {
     assert(showMode != null);
     assert(showDirection != null);
     assert(showFilter != null);
diff --git a/runtime/observatory/lib/src/elements/strongly_reachable_instances.dart b/runtime/observatory/lib/src/elements/strongly_reachable_instances.dart
index ee35d948110..a772497dc60 100644
--- a/runtime/observatory/lib/src/elements/strongly_reachable_instances.dart
+++ b/runtime/observatory/lib/src/elements/strongly_reachable_instances.dart
@@ -17,16 +17,16 @@ class StronglyReachableInstancesElement extends CustomElement
       'strongly-reachable-instances',
       dependencies: const [CurlyBlockElement.tag, InstanceRefElement.tag]);
 
-  late RenderingScheduler<StronglyReachableInstancesElement> _r;
+  RenderingScheduler<StronglyReachableInstancesElement> _r;
 
   Stream<RenderedEvent<StronglyReachableInstancesElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.ClassRef _cls;
-  late M.StronglyReachableInstancesRepository _stronglyReachableInstances;
-  late M.ObjectRepository _objects;
-  M.InstanceSet? _result;
+  M.IsolateRef _isolate;
+  M.ClassRef _cls;
+  M.StronglyReachableInstancesRepository _stronglyReachableInstances;
+  M.ObjectRepository _objects;
+  M.InstanceSet _result;
   bool _expanded = false;
 
   M.IsolateRef get isolate => _isolate;
@@ -37,7 +37,7 @@ class StronglyReachableInstancesElement extends CustomElement
       M.ClassRef cls,
       M.StronglyReachableInstancesRepository stronglyReachable,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(cls != null);
     assert(stronglyReachable != null);
@@ -92,7 +92,7 @@ class StronglyReachableInstancesElement extends CustomElement
     if (_result == null) {
       return [new SpanElement()..text = 'Loading...'];
     }
-    final content = _result!.instances!
+    final content = _result.instances
         .map<Element>((sample) => new DivElement()
           ..children = <Element>[
             anyRef(_isolate, sample, _objects, queue: _r.queue)
@@ -101,13 +101,13 @@ class StronglyReachableInstancesElement extends CustomElement
     content.add(new DivElement()
       ..children = ([]
         ..addAll(_createShowMoreButton())
-        ..add(new SpanElement()..text = ' of total ${_result!.count}')));
+        ..add(new SpanElement()..text = ' of total ${_result.count}')));
     return content;
   }
 
   List<Element> _createShowMoreButton() {
-    final samples = _result!.instances!.toList();
-    if (samples.length == _result!.count) {
+    final samples = _result.instances.toList();
+    if (samples.length == _result.count) {
       return [];
     }
     final count = samples.length;
diff --git a/runtime/observatory/lib/src/elements/subtypetestcache_ref.dart b/runtime/observatory/lib/src/elements/subtypetestcache_ref.dart
index 6662bb16e9c..6d7085b0cd8 100644
--- a/runtime/observatory/lib/src/elements/subtypetestcache_ref.dart
+++ b/runtime/observatory/lib/src/elements/subtypetestcache_ref.dart
@@ -14,20 +14,20 @@ class SubtypeTestCacheRefElement extends CustomElement implements Renderable {
   static const tag =
       const Tag<SubtypeTestCacheRefElement>('subtypetestcache-ref');
 
-  late RenderingScheduler<SubtypeTestCacheRefElement> _r;
+  RenderingScheduler<SubtypeTestCacheRefElement> _r;
 
   Stream<RenderedEvent<SubtypeTestCacheRefElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.SubtypeTestCacheRef _subtypeTestCache;
+  M.IsolateRef _isolate;
+  M.SubtypeTestCacheRef _subtypeTestCache;
 
   M.IsolateRef get isolate => _isolate;
   M.SubtypeTestCacheRef get subtypeTestCache => _subtypeTestCache;
 
   factory SubtypeTestCacheRefElement(
       M.IsolateRef isolate, M.SubtypeTestCacheRef subtypeTestCache,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(subtypeTestCache != null);
     SubtypeTestCacheRefElement e = new SubtypeTestCacheRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/subtypetestcache_view.dart b/runtime/observatory/lib/src/elements/subtypetestcache_view.dart
index a1e6c24be68..b53cb434fe4 100644
--- a/runtime/observatory/lib/src/elements/subtypetestcache_view.dart
+++ b/runtime/observatory/lib/src/elements/subtypetestcache_view.dart
@@ -33,22 +33,22 @@ class SubtypeTestCacheViewElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<SubtypeTestCacheViewElement> _r;
+  RenderingScheduler<SubtypeTestCacheViewElement> _r;
 
   Stream<RenderedEvent<SubtypeTestCacheViewElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.SubtypeTestCache _subtypeTestCache;
-  late M.SubtypeTestCacheRepository _subtypeTestCaches;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.SubtypeTestCache _subtypeTestCache;
+  M.SubtypeTestCacheRepository _subtypeTestCaches;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -67,7 +67,7 @@ class SubtypeTestCacheViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -121,7 +121,7 @@ class SubtypeTestCacheViewElement extends CustomElement implements Renderable {
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
                 _subtypeTestCache = await _subtypeTestCaches.get(
-                    _isolate, _subtypeTestCache.id!);
+                    _isolate, _subtypeTestCache.id);
                 _r.dirty();
               }))
             .element,
diff --git a/runtime/observatory/lib/src/elements/timeline/dashboard.dart b/runtime/observatory/lib/src/elements/timeline/dashboard.dart
index 51253dfbd3e..8995be8823c 100644
--- a/runtime/observatory/lib/src/elements/timeline/dashboard.dart
+++ b/runtime/observatory/lib/src/elements/timeline/dashboard.dart
@@ -37,15 +37,15 @@ class TimelineDashboardElement extends CustomElement implements Renderable {
   static const tag = const Tag<TimelineDashboardElement>('timeline-dashboard',
       dependencies: const [NavNotifyElement.tag]);
 
-  late RenderingScheduler<TimelineDashboardElement> _r;
+  RenderingScheduler<TimelineDashboardElement> _r;
 
   Stream<RenderedEvent<TimelineDashboardElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.TimelineRepository _repository;
-  late M.NotificationRepository _notifications;
-  late M.TimelineFlags _flags;
+  M.VM _vm;
+  M.TimelineRepository _repository;
+  M.NotificationRepository _notifications;
+  M.TimelineFlags _flags;
   _TimelineView _view = _TimelineView.strict;
 
   M.VM get vm => _vm;
@@ -53,7 +53,7 @@ class TimelineDashboardElement extends CustomElement implements Renderable {
 
   factory TimelineDashboardElement(M.VM vm, M.TimelineRepository repository,
       M.NotificationRepository notifications,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(repository != null);
     assert(notifications != null);
@@ -84,8 +84,8 @@ class TimelineDashboardElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  IFrameElement? _frame;
-  DivElement? _content;
+  IFrameElement _frame;
+  DivElement _content;
 
   void render() {
     if (_frame == null) {
@@ -94,8 +94,8 @@ class TimelineDashboardElement extends CustomElement implements Renderable {
     if (_content == null) {
       _content = new DivElement()..classes = ['content-centered-big'];
     }
-    _frame!.src = _makeFrameUrl();
-    _content!.children = <Element>[
+    _frame.src = _makeFrameUrl();
+    _content.children = <Element>[
       new HeadingElement.h2()
         ..nodes = ([new Text("Timeline View")]
           ..addAll(_createButtons())
@@ -112,10 +112,10 @@ class TimelineDashboardElement extends CustomElement implements Renderable {
         navBar(<Element>[
           new NavNotifyElement(_notifications, queue: _r.queue).element
         ]),
-        _content!,
+        _content,
         new DivElement()
           ..classes = ['iframe']
-          ..children = <Element>[_frame!]
+          ..children = <Element>[_frame]
       ];
     }
   }
@@ -225,7 +225,7 @@ class TimelineDashboardElement extends CustomElement implements Renderable {
   Future _postMessage(String method,
       [Map<String, dynamic> params = const <String, dynamic>{}]) async {
     var message = {'method': method, 'params': params};
-    _frame!.contentWindow!
+    _frame.contentWindow
         .postMessage(json.encode(message), window.location.href);
     return null;
   }
diff --git a/runtime/observatory/lib/src/elements/timeline_page.dart b/runtime/observatory/lib/src/elements/timeline_page.dart
index 27f10c9d8cf..e5f3f84e17f 100644
--- a/runtime/observatory/lib/src/elements/timeline_page.dart
+++ b/runtime/observatory/lib/src/elements/timeline_page.dart
@@ -27,25 +27,25 @@ class TimelinePageElement extends CustomElement implements Renderable {
     NavNotifyElement.tag
   ]);
 
-  late RenderingScheduler<TimelinePageElement> _r;
+  RenderingScheduler<TimelinePageElement> _r;
 
   Stream<RenderedEvent<TimelinePageElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.TimelineRepository _repository;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  M.TimelineRecorder? _recorder;
-  late Set<M.TimelineStream> _availableStreams;
-  late Set<M.TimelineStream> _recordedStreams;
-  late Set<M.TimelineProfile> _profiles;
+  M.VM _vm;
+  M.TimelineRepository _repository;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.TimelineRecorder _recorder;
+  Set<M.TimelineStream> _availableStreams;
+  Set<M.TimelineStream> _recordedStreams;
+  Set<M.TimelineProfile> _profiles;
 
   M.VM get vm => _vm;
   M.NotificationRepository get notifications => _notifications;
 
   factory TimelinePageElement(M.VM vm, M.TimelineRepository repository,
       M.EventRepository events, M.NotificationRepository notifications,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(repository != null);
     assert(events != null);
@@ -75,25 +75,25 @@ class TimelinePageElement extends CustomElement implements Renderable {
     children = <Element>[];
   }
 
-  IFrameElement? _frame;
-  DivElement? _content;
+  IFrameElement _frame;
+  DivElement _content;
 
   bool get usingVMRecorder =>
-      _recorder!.name != "Fuchsia" &&
-      _recorder!.name != "Systrace" &&
-      _recorder!.name != "Macos";
+      _recorder.name != "Fuchsia" &&
+      _recorder.name != "Systrace" &&
+      _recorder.name != "Macos";
 
   void render() {
     if (_frame == null) {
       _frame = new IFrameElement()..src = 'timeline.html';
-      _frame!.onLoad.listen((event) {
+      _frame.onLoad.listen((event) {
         _refresh();
       });
     }
     if (_content == null) {
       _content = new DivElement()..classes = ['content-centered-big'];
     }
-    _content!.children = <Element>[
+    _content.children = <Element>[
       new HeadingElement.h1()..text = 'Timeline settings',
       _recorder == null
           ? (new DivElement()..text = 'Loading...')
@@ -108,7 +108,7 @@ class TimelinePageElement extends CustomElement implements Renderable {
                     ..text = 'Recorder:',
                   new DivElement()
                     ..classes = ['memberValue']
-                    ..text = _recorder!.name
+                    ..text = _recorder.name
                 ],
               new DivElement()
                 ..classes = ['memberItem']
@@ -170,20 +170,19 @@ class TimelinePageElement extends CustomElement implements Renderable {
             .element,
         new NavNotifyElement(_notifications, queue: _r.queue).element
       ]),
-      _content!,
+      _content,
       _createIFrameOrMessage(),
     ];
   }
 
   HtmlElement _createIFrameOrMessage() {
-    final recorder = _recorder;
-    if (recorder == null) {
+    if (_recorder == null) {
       return new DivElement()
         ..classes = ['content-centered-big']
         ..text = 'Loading...';
     }
 
-    if (recorder.name == "Fuchsia") {
+    if (_recorder.name == "Fuchsia") {
       return new DivElement()
         ..classes = ['content-centered-big']
         ..children = <Element>[
@@ -198,7 +197,7 @@ class TimelinePageElement extends CustomElement implements Renderable {
         ];
     }
 
-    if (recorder.name == "Systrace") {
+    if (_recorder.name == "Systrace") {
       return new DivElement()
         ..classes = ['content-centered-big']
         ..children = <Element>[
@@ -214,7 +213,7 @@ class TimelinePageElement extends CustomElement implements Renderable {
         ];
     }
 
-    if (recorder.name == "Macos") {
+    if (_recorder.name == "Macos") {
       return new DivElement()
         ..classes = ['content-centered-big']
         ..children = <Element>[
@@ -232,7 +231,7 @@ class TimelinePageElement extends CustomElement implements Renderable {
 
     return new DivElement()
       ..classes = ['iframe']
-      ..children = <Element>[_frame!];
+      ..children = <Element>[_frame];
   }
 
   List<Element> _createProfileSelect() {
@@ -272,12 +271,12 @@ class TimelinePageElement extends CustomElement implements Renderable {
   }
 
   Future _postMessage(String method,
-      [Map<dynamic, dynamic> params = const <dynamic, dynamic>{}]) async {
-    if (_frame!.contentWindow == null) {
+      [Map<String, dynamic> params = const <String, dynamic>{}]) async {
+    if (_frame.contentWindow == null) {
       return null;
     }
     var message = {'method': method, 'params': params};
-    _frame!.contentWindow!
+    _frame.contentWindow
         .postMessage(json.encode(message), window.location.href);
     return null;
   }
diff --git a/runtime/observatory/lib/src/elements/tree_map.dart b/runtime/observatory/lib/src/elements/tree_map.dart
index 5bf1d5f143a..f5e5de61251 100644
--- a/runtime/observatory/lib/src/elements/tree_map.dart
+++ b/runtime/observatory/lib/src/elements/tree_map.dart
@@ -9,7 +9,7 @@ abstract class TreeMap<T> {
   int getArea(T node);
   String getBackground(T node);
   String getLabel(T node);
-  T? getParent(T node);
+  T getParent(T node);
   Iterable<T> getChildren(T node);
   void onSelect(T node);
   void onDetails(T node);
@@ -32,8 +32,7 @@ abstract class TreeMap<T> {
     div.onDoubleClick.listen((event) {
       event.stopPropagation();
       if (depth == 0) {
-        var p = getParent(node);
-        onSelect(p ?? node); // Zoom out.
+        onSelect(getParent(node)); // Zoom out.
       } else {
         onSelect(node); // Zoom in.
       }
diff --git a/runtime/observatory/lib/src/elements/type_arguments_ref.dart b/runtime/observatory/lib/src/elements/type_arguments_ref.dart
index 5ab489d0c0b..c0c7ed1d67c 100644
--- a/runtime/observatory/lib/src/elements/type_arguments_ref.dart
+++ b/runtime/observatory/lib/src/elements/type_arguments_ref.dart
@@ -12,19 +12,19 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class TypeArgumentsRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<TypeArgumentsRefElement>('type-arguments-ref');
 
-  late RenderingScheduler<TypeArgumentsRefElement> _r;
+  RenderingScheduler<TypeArgumentsRefElement> _r;
 
   Stream<RenderedEvent<TypeArgumentsRefElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.TypeArgumentsRef _arguments;
+  M.IsolateRef _isolate;
+  M.TypeArgumentsRef _arguments;
 
   M.IsolateRef get isolate => _isolate;
   M.TypeArgumentsRef get arguments => _arguments;
 
   factory TypeArgumentsRefElement(M.IsolateRef isolate, M.TypeArgumentsRef args,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(args != null);
     TypeArgumentsRefElement e = new TypeArgumentsRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/unknown_ref.dart b/runtime/observatory/lib/src/elements/unknown_ref.dart
index ec0833f5000..cad113682a9 100644
--- a/runtime/observatory/lib/src/elements/unknown_ref.dart
+++ b/runtime/observatory/lib/src/elements/unknown_ref.dart
@@ -12,19 +12,19 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class UnknownObjectRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<UnknownObjectRefElement>('unknown-ref');
 
-  late RenderingScheduler<UnknownObjectRefElement> _r;
+  RenderingScheduler<UnknownObjectRefElement> _r;
 
   Stream<RenderedEvent<UnknownObjectRefElement>> get onRendered =>
       _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.UnknownObjectRef _obj;
+  M.IsolateRef _isolate;
+  M.UnknownObjectRef _obj;
 
   M.IsolateRef get isolate => _isolate;
   M.UnknownObjectRef get obj => _obj;
 
   factory UnknownObjectRefElement(M.IsolateRef isolate, M.UnknownObjectRef obj,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(obj != null);
     UnknownObjectRefElement e = new UnknownObjectRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/unlinkedcall_ref.dart b/runtime/observatory/lib/src/elements/unlinkedcall_ref.dart
index 474ef68d697..9a1357547eb 100644
--- a/runtime/observatory/lib/src/elements/unlinkedcall_ref.dart
+++ b/runtime/observatory/lib/src/elements/unlinkedcall_ref.dart
@@ -12,19 +12,19 @@ import 'package:observatory/src/elements/helpers/uris.dart';
 class UnlinkedCallRefElement extends CustomElement implements Renderable {
   static const tag = const Tag<UnlinkedCallRefElement>('unlinkedcall-ref');
 
-  late RenderingScheduler<UnlinkedCallRefElement> _r;
+  RenderingScheduler<UnlinkedCallRefElement> _r;
 
   Stream<RenderedEvent<UnlinkedCallRefElement>> get onRendered => _r.onRendered;
 
-  late M.IsolateRef _isolate;
-  late M.UnlinkedCallRef _unlinkedcall;
+  M.IsolateRef _isolate;
+  M.UnlinkedCallRef _unlinkedcall;
 
   M.IsolateRef get isolate => _isolate;
   M.UnlinkedCallRef get unlinkedcall => _unlinkedcall;
 
   factory UnlinkedCallRefElement(
       M.IsolateRef isolate, M.UnlinkedCallRef unlinkedcall,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(isolate != null);
     assert(unlinkedcall != null);
     UnlinkedCallRefElement e = new UnlinkedCallRefElement.created();
diff --git a/runtime/observatory/lib/src/elements/unlinkedcall_view.dart b/runtime/observatory/lib/src/elements/unlinkedcall_view.dart
index f1542da51d9..315c58bc3cf 100644
--- a/runtime/observatory/lib/src/elements/unlinkedcall_view.dart
+++ b/runtime/observatory/lib/src/elements/unlinkedcall_view.dart
@@ -32,22 +32,22 @@ class UnlinkedCallViewElement extends CustomElement implements Renderable {
         ViewFooterElement.tag
       ]);
 
-  late RenderingScheduler<UnlinkedCallViewElement> _r;
+  RenderingScheduler<UnlinkedCallViewElement> _r;
 
   Stream<RenderedEvent<UnlinkedCallViewElement>> get onRendered =>
       _r.onRendered;
 
-  late M.VM _vm;
-  late M.IsolateRef _isolate;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.UnlinkedCall _unlinkedcall;
-  late M.UnlinkedCallRepository _unlinkedcalls;
-  late M.RetainedSizeRepository _retainedSizes;
-  late M.ReachableSizeRepository _reachableSizes;
-  late M.InboundReferencesRepository _references;
-  late M.RetainingPathRepository _retainingPaths;
-  late M.ObjectRepository _objects;
+  M.VM _vm;
+  M.IsolateRef _isolate;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.UnlinkedCall _unlinkedcall;
+  M.UnlinkedCallRepository _unlinkedcalls;
+  M.RetainedSizeRepository _retainedSizes;
+  M.ReachableSizeRepository _reachableSizes;
+  M.InboundReferencesRepository _references;
+  M.RetainingPathRepository _retainingPaths;
+  M.ObjectRepository _objects;
 
   M.VMRef get vm => _vm;
   M.IsolateRef get isolate => _isolate;
@@ -66,7 +66,7 @@ class UnlinkedCallViewElement extends CustomElement implements Renderable {
       M.InboundReferencesRepository references,
       M.RetainingPathRepository retainingPaths,
       M.ObjectRepository objects,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(isolate != null);
     assert(events != null);
@@ -120,7 +120,7 @@ class UnlinkedCallViewElement extends CustomElement implements Renderable {
               ..onRefresh.listen((e) async {
                 e.element.disabled = true;
                 _unlinkedcall =
-                    await _unlinkedcalls.get(_isolate, _unlinkedcall.id!);
+                    await _unlinkedcalls.get(_isolate, _unlinkedcall.id);
                 _r.dirty();
               }))
             .element,
diff --git a/runtime/observatory/lib/src/elements/view_footer.dart b/runtime/observatory/lib/src/elements/view_footer.dart
index 41054b20031..2115632e650 100644
--- a/runtime/observatory/lib/src/elements/view_footer.dart
+++ b/runtime/observatory/lib/src/elements/view_footer.dart
@@ -12,11 +12,11 @@ import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 class ViewFooterElement extends CustomElement implements Renderable {
   static const tag = const Tag<ViewFooterElement>('view-footer');
 
-  late RenderingScheduler<ViewFooterElement> _r;
+  RenderingScheduler<ViewFooterElement> _r;
 
   Stream<RenderedEvent<ViewFooterElement>> get onRendered => _r.onRendered;
 
-  factory ViewFooterElement({RenderingQueue? queue}) {
+  factory ViewFooterElement({RenderingQueue queue}) {
     ViewFooterElement e = new ViewFooterElement.created();
     e._r = new RenderingScheduler<ViewFooterElement>(e, queue: queue);
     return e;
diff --git a/runtime/observatory/lib/src/elements/vm_connect.dart b/runtime/observatory/lib/src/elements/vm_connect.dart
index 9cdc72f10e0..926b2c9677c 100644
--- a/runtime/observatory/lib/src/elements/vm_connect.dart
+++ b/runtime/observatory/lib/src/elements/vm_connect.dart
@@ -25,19 +25,19 @@ class VMConnectElement extends CustomElement implements Renderable {
     VMConnectTargetElement.tag
   ]);
 
-  late RenderingScheduler<VMConnectElement> _r;
+  RenderingScheduler<VMConnectElement> _r;
 
   Stream<RenderedEvent<VMConnectElement>> get onRendered => _r.onRendered;
 
-  late M.NotificationRepository _notifications;
-  late M.TargetRepository _targets;
-  late StreamSubscription _targetsSubscription;
+  M.NotificationRepository _notifications;
+  M.TargetRepository _targets;
+  StreamSubscription _targetsSubscription;
 
-  late String _address;
+  String _address;
 
   factory VMConnectElement(
       M.TargetRepository targets, M.NotificationRepository notifications,
-      {String address: '', RenderingQueue? queue}) {
+      {String address: '', RenderingQueue queue}) {
     assert(address != null);
     assert(notifications != null);
     assert(targets != null);
@@ -147,7 +147,7 @@ class VMConnectElement extends CustomElement implements Renderable {
     _targets.add(normalizedNetworkAddress);
     var target = _targets.find(normalizedNetworkAddress);
     assert(target != null);
-    _targets.setCurrent(target!);
+    _targets.setCurrent(target);
     // the navigation to the VM page is done in the ObservatoryApplication
   }
 
diff --git a/runtime/observatory/lib/src/elements/vm_connect_target.dart b/runtime/observatory/lib/src/elements/vm_connect_target.dart
index 4cc2d88391e..0f6575b7c38 100644
--- a/runtime/observatory/lib/src/elements/vm_connect_target.dart
+++ b/runtime/observatory/lib/src/elements/vm_connect_target.dart
@@ -17,7 +17,7 @@ class TargetEvent {
 class VMConnectTargetElement extends CustomElement implements Renderable {
   static const tag = const Tag<VMConnectTargetElement>('vm-connect-target');
 
-  late RenderingScheduler<VMConnectTargetElement> _r;
+  RenderingScheduler<VMConnectTargetElement> _r;
 
   Stream<RenderedEvent<VMConnectTargetElement>> get onRendered => _r.onRendered;
 
@@ -28,14 +28,14 @@ class VMConnectTargetElement extends CustomElement implements Renderable {
       new StreamController<TargetEvent>.broadcast();
   Stream<TargetEvent> get onDelete => _onDelete.stream;
 
-  late M.Target _target;
-  late bool _current;
+  M.Target _target;
+  bool _current;
 
   M.Target get target => _target;
   bool get current => _current;
 
   factory VMConnectTargetElement(M.Target target,
-      {bool current: false, RenderingQueue? queue}) {
+      {bool current: false, RenderingQueue queue}) {
     assert(target != null);
     assert(current != null);
     VMConnectTargetElement e = new VMConnectTargetElement.created();
diff --git a/runtime/observatory/lib/src/elements/vm_view.dart b/runtime/observatory/lib/src/elements/vm_view.dart
index 6b3163e421e..100d0b9cddc 100644
--- a/runtime/observatory/lib/src/elements/vm_view.dart
+++ b/runtime/observatory/lib/src/elements/vm_view.dart
@@ -29,20 +29,20 @@ class VMViewElement extends CustomElement implements Renderable {
     ViewFooterElement.tag
   ]);
 
-  late RenderingScheduler<VMViewElement> _r;
+  RenderingScheduler<VMViewElement> _r;
 
   Stream<RenderedEvent<VMViewElement>> get onRendered => _r.onRendered;
 
-  late M.VM _vm;
-  late M.VMRepository _vms;
-  late M.EventRepository _events;
-  late M.NotificationRepository _notifications;
-  late M.IsolateRepository _isolates;
-  late M.IsolateGroupRepository _isolateGroups;
-  late M.ScriptRepository _scripts;
-  late StreamSubscription _vmSubscription;
-  late StreamSubscription _startSubscription;
-  late StreamSubscription _exitSubscription;
+  M.VM _vm;
+  M.VMRepository _vms;
+  M.EventRepository _events;
+  M.NotificationRepository _notifications;
+  M.IsolateRepository _isolates;
+  M.IsolateGroupRepository _isolateGroups;
+  M.ScriptRepository _scripts;
+  StreamSubscription _vmSubscription;
+  StreamSubscription _startSubscription;
+  StreamSubscription _exitSubscription;
 
   M.VMRef get vm => _vm;
   M.NotificationRepository get notifications => _notifications;
@@ -55,7 +55,7 @@ class VMViewElement extends CustomElement implements Renderable {
       M.IsolateRepository isolates,
       M.IsolateGroupRepository isolateGroups,
       M.ScriptRepository scripts,
-      {RenderingQueue? queue}) {
+      {RenderingQueue queue}) {
     assert(vm != null);
     assert(vms != null);
     assert(events != null);
@@ -81,7 +81,7 @@ class VMViewElement extends CustomElement implements Renderable {
     super.attached();
     _r.enable();
     _vmSubscription = _events.onVMUpdate.listen((e) {
-      _vm = e.vm as M.VM;
+      _vm = e.vm;
       _r.dirty();
     });
     _startSubscription = _events.onIsolateStart.listen((_) => _r.dirty());
@@ -128,7 +128,7 @@ class VMViewElement extends CustomElement implements Renderable {
   }
 
   Element describeVM() {
-    final uptime = new DateTime.now().difference(_vm.startTime!);
+    final uptime = new DateTime.now().difference(_vm.startTime);
     return new DivElement()
       ..classes = ['content-centered-big']
       ..children = <HtmlElement>[
@@ -164,7 +164,7 @@ class VMViewElement extends CustomElement implements Renderable {
                   ..text = 'embedder',
                 new DivElement()
                   ..classes = ['memberValue']
-                  ..text = _vm.embedder
+                  ..text = _vm.embedder ?? "UNKNOWN"
               ],
             new DivElement()
               ..classes = ['memberItem']
@@ -254,7 +254,7 @@ class VMViewElement extends CustomElement implements Renderable {
                       : 'unavailable'
                   ..title = _vm.heapAllocatedMemoryUsage != null
                       ? '${_vm.heapAllocatedMemoryUsage} bytes'
-                      : 'unavailable'
+                      : null
               ],
             new DivElement()
               ..classes = ['memberItem']
@@ -308,7 +308,7 @@ class VMViewElement extends CustomElement implements Renderable {
   }
 
   Element describeIsolateGroup(M.IsolateGroupRef group) {
-    final isolates = (group as M.IsolateGroup).isolates!;
+    final isolates = (group as M.IsolateGroup).isolates;
     return new DivElement()
       ..classes = ['content-centered-big']
       ..children = <Element>[
diff --git a/runtime/observatory/lib/src/models/objects/allocation_profile.dart b/runtime/observatory/lib/src/models/objects/allocation_profile.dart
index b8b3f2418cc..f9aa5a6af9e 100644
--- a/runtime/observatory/lib/src/models/objects/allocation_profile.dart
+++ b/runtime/observatory/lib/src/models/objects/allocation_profile.dart
@@ -5,8 +5,8 @@
 part of models;
 
 abstract class AllocationProfile {
-  DateTime? get lastServiceGC;
-  DateTime? get lastAccumulatorReset;
+  DateTime get lastServiceGC;
+  DateTime get lastAccumulatorReset;
   HeapSpace get newSpace;
   HeapSpace get oldSpace;
   HeapSpace get totalSpace;
@@ -15,10 +15,10 @@ abstract class AllocationProfile {
 
 abstract class ClassHeapStats {
   /// [Optional] at least one between clazz and displayName should be non null
-  ClassRef? get clazz;
+  ClassRef get clazz;
 
   /// [Optional] at least one between clazz and displayName should be non null
-  String? get displayName;
+  String get displayName;
   Allocations get newSpace;
   Allocations get oldSpace;
 }
diff --git a/runtime/observatory/lib/src/models/objects/breakpoint.dart b/runtime/observatory/lib/src/models/objects/breakpoint.dart
index 6417e12f1cc..28ed56c91e5 100644
--- a/runtime/observatory/lib/src/models/objects/breakpoint.dart
+++ b/runtime/observatory/lib/src/models/objects/breakpoint.dart
@@ -6,16 +6,16 @@ part of models;
 
 abstract class Breakpoint extends Object {
   /// A number identifying this breakpoint to the user.
-  int? get number;
+  int get number;
 
   /// Has this breakpoint been assigned to a specific program location?
-  bool? get resolved;
+  bool get resolved;
 
   /// [optional]Is this a breakpoint that was added synthetically as part of a
   /// step OverAsyncSuspension resume command?
-  bool? get isSyntheticAsyncContinuation;
+  bool get isSyntheticAsyncContinuation;
 
   /// SourceLocation when breakpoint is resolved, UnresolvedSourceLocation
   /// when a breakpoint is not resolved.
-  Location? get location;
+  Location get location;
 }
diff --git a/runtime/observatory/lib/src/models/objects/class.dart b/runtime/observatory/lib/src/models/objects/class.dart
index 92d946e3abc..47a1a3429fe 100644
--- a/runtime/observatory/lib/src/models/objects/class.dart
+++ b/runtime/observatory/lib/src/models/objects/class.dart
@@ -6,68 +6,68 @@ part of models;
 
 abstract class ClassRef extends ObjectRef {
   /// The name of this class.
-  String? get name;
+  String get name;
 }
 
 abstract class Class extends Object implements ClassRef {
   /// The error which occurred during class finalization, if it exists.
   /// [optional]
-  ErrorRef? get error;
+  ErrorRef get error;
 
   /// Is this an abstract class?
-  bool? get isAbstract;
+  bool get isAbstract;
 
   /// Is this a const class?
-  bool? get isConst;
+  bool get isConst;
 
   /// [internal]
-  bool? get isPatch;
+  bool get isPatch;
 
   /// [optional] The library which contains this class.
-  LibraryRef? get library;
+  LibraryRef get library;
 
   /// [optional] The location of this class in the source code.
-  SourceLocation? get location;
+  SourceLocation get location;
 
   /// [optional] The superclass of this class, if any.
-  ClassRef? get superclass;
+  ClassRef get superclass;
 
   /// [optional]The supertype for this class, if any.
   ///
   /// The value will be of the kind: Type.
-  InstanceRef? get superType;
+  InstanceRef get superType;
 
   /// A list of interface types for this class.
   ///
   /// The values will be of the kind: Type.
-  Iterable<InstanceRef>? get interfaces;
+  Iterable<InstanceRef> get interfaces;
 
   /// The mixin type for this class, if any.
   ///
   /// [optional] The value will be of the kind: Type.
-  InstanceRef? get mixin;
+  InstanceRef get mixin;
 
   /// A list of fields in this class. Does not include fields from
   /// superclasses.
-  Iterable<FieldRef>? get fields;
+  Iterable<FieldRef> get fields;
 
   /// A list of functions in this class. Does not include functions
   /// from superclasses.
-  Iterable<FunctionRef>? get functions;
+  Iterable<FunctionRef> get functions;
 
   // A list of subclasses of this class.
-  Iterable<ClassRef>? get subclasses;
+  Iterable<ClassRef> get subclasses;
 
-  bool? get hasAllocations;
-  bool? get hasNoAllocations;
+  bool get hasAllocations;
+  bool get hasNoAllocations;
 
-  Allocations? get newSpace;
-  Allocations? get oldSpace;
+  Allocations get newSpace;
+  Allocations get oldSpace;
 
-  bool? get traceAllocations;
+  bool get traceAllocations;
 }
 
 abstract class InstanceSet {
-  int? get count;
-  Iterable<ObjectRef>? get instances;
+  int get count;
+  Iterable<ObjectRef> get instances;
 }
diff --git a/runtime/observatory/lib/src/models/objects/code.dart b/runtime/observatory/lib/src/models/objects/code.dart
index 6e4a9153c70..12d84df2c86 100644
--- a/runtime/observatory/lib/src/models/objects/code.dart
+++ b/runtime/observatory/lib/src/models/objects/code.dart
@@ -6,10 +6,7 @@ part of models;
 
 enum CodeKind { dart, native, stub, tag, collected }
 
-bool isSyntheticCode(CodeKind? kind) {
-  if (kind == null) {
-    return false;
-  }
+bool isSyntheticCode(CodeKind kind) {
   switch (kind) {
     case CodeKind.collected:
     case CodeKind.native:
@@ -20,25 +17,20 @@ bool isSyntheticCode(CodeKind? kind) {
   }
 }
 
-bool isDartCode(CodeKind? kind) {
-  if (kind == null) {
-    return false;
-  }
-  return !isSyntheticCode(kind);
-}
+bool isDartCode(CodeKind kind) => !isSyntheticCode(kind);
 
 abstract class CodeRef extends ObjectRef {
   /// The name of this class.
-  String? get name;
+  String get name;
 
   // What kind of code object is this?
-  CodeKind? get kind;
+  CodeKind get kind;
 
-  bool? get isOptimized;
+  bool get isOptimized;
 }
 
 abstract class Code extends Object implements CodeRef {
-  FunctionRef? get function;
-  ObjectPoolRef? get objectPool;
-  Iterable<FunctionRef>? get inlinedFunctions;
+  FunctionRef get function;
+  ObjectPoolRef get objectPool;
+  Iterable<FunctionRef> get inlinedFunctions;
 }
diff --git a/runtime/observatory/lib/src/models/objects/context.dart b/runtime/observatory/lib/src/models/objects/context.dart
index a9e9fd82513..73780868552 100644
--- a/runtime/observatory/lib/src/models/objects/context.dart
+++ b/runtime/observatory/lib/src/models/objects/context.dart
@@ -6,17 +6,17 @@ part of models;
 
 abstract class ContextRef extends ObjectRef {
   /// The number of variables in this context.
-  int? get length;
+  int get length;
 }
 
 abstract class Context extends Object implements ContextRef {
   /// [optional] The enclosing context for this context.
-  Context? get parentContext;
+  Context get parentContext;
 
   // The variables in this context object.
-  Iterable<ContextElement>? get variables;
+  Iterable<ContextElement> get variables;
 }
 
 abstract class ContextElement {
-  Guarded<InstanceRef>? get value;
+  Guarded<InstanceRef> get value;
 }
diff --git a/runtime/observatory/lib/src/models/objects/error.dart b/runtime/observatory/lib/src/models/objects/error.dart
index cdf3e890637..56eff8b1231 100644
--- a/runtime/observatory/lib/src/models/objects/error.dart
+++ b/runtime/observatory/lib/src/models/objects/error.dart
@@ -20,8 +20,8 @@ enum ErrorKind {
 }
 
 abstract class ErrorRef extends ObjectRef {
-  ErrorKind? get kind;
-  String? get message;
+  ErrorKind get kind;
+  String get message;
 }
 
 abstract class Error extends Object implements ErrorRef {}
diff --git a/runtime/observatory/lib/src/models/objects/event.dart b/runtime/observatory/lib/src/models/objects/event.dart
index 5d540ed648a..24ed7e3c571 100644
--- a/runtime/observatory/lib/src/models/objects/event.dart
+++ b/runtime/observatory/lib/src/models/objects/event.dart
@@ -75,7 +75,7 @@ abstract class PauseExitEvent extends PauseEvent {}
 
 abstract class PauseBreakpointEvent extends AsyncSuspensionEvent {
   /// [optional] The breakpoint at which we are currently paused.
-  Breakpoint? get breakpoint;
+  Breakpoint get breakpoint;
 
   /// The list of breakpoints at which we are currently paused
   /// for a PauseBreakpoint event.
@@ -94,13 +94,13 @@ abstract class PauseBreakpointEvent extends AsyncSuspensionEvent {
 abstract class PauseInterruptedEvent extends AsyncSuspensionEvent {
   /// [optional] The top stack frame associated with this event. There will be
   /// no top frame if the isolate is idle (waiting in the message loop).
-  Frame? get topFrame;
+  Frame get topFrame;
 }
 
 abstract class PausePostRequestEvent extends AsyncSuspensionEvent {
   /// [optional] The top stack frame associated with this event. There will be
   /// no top frame if the isolate is idle (waiting in the message loop).
-  Frame? get topFrame;
+  Frame get topFrame;
 }
 
 abstract class PauseExceptionEvent extends PauseEvent {
@@ -115,7 +115,7 @@ abstract class ResumeEvent extends DebugEvent {
   /// [optional] The top stack frame associated with this event. It is provided
   /// at all times except for the initial resume event that is delivered when an
   /// isolate begins execution.
-  Frame? get topFrame;
+  Frame get topFrame;
 }
 
 abstract class BreakpointAddedEvent extends BreakpointEvent {}
@@ -168,7 +168,7 @@ abstract class ConnectionClosedEvent extends Event {
   String get reason;
 }
 
-Frame? topFrame(Event? event) {
+Frame topFrame(DebugEvent event) {
   if (event is PauseBreakpointEvent) {
     return event.topFrame;
   }
@@ -184,7 +184,7 @@ Frame? topFrame(Event? event) {
   return null;
 }
 
-bool isAtAsyncSuspension(DebugEvent? event) {
+bool isAtAsyncSuspension(DebugEvent event) {
   if (event is PauseBreakpointEvent) {
     return event.atAsyncSuspension;
   }
diff --git a/runtime/observatory/lib/src/models/objects/field.dart b/runtime/observatory/lib/src/models/objects/field.dart
index c063238468a..ca59328ca6e 100644
--- a/runtime/observatory/lib/src/models/objects/field.dart
+++ b/runtime/observatory/lib/src/models/objects/field.dart
@@ -6,38 +6,38 @@ part of models;
 
 abstract class FieldRef extends ObjectRef {
   /// The name of this field.
-  String? get name;
+  String get name;
 
   /// The owner of this field, which can be either a Library or a
   /// Class.
-  ObjectRef? get dartOwner;
+  ObjectRef get dartOwner;
 
   /// The declared type of this field.
   ///
   /// The value will always be of one of the kinds:
   /// Type, TypeRef, TypeParameter.
-  InstanceRef? get declaredType;
+  InstanceRef get declaredType;
 
   /// Is this field const?
-  bool? get isConst;
+  bool get isConst;
 
   /// Is this field final?
-  bool? get isFinal;
+  bool get isFinal;
 
   /// Is this field static?
-  bool? get isStatic;
+  bool get isStatic;
 }
 
 enum GuardClassKind { unknown, single, dynamic }
 
 abstract class Field extends Object implements FieldRef {
   /// [optional] The value of this field, if the field is static.
-  ObjectRef? get staticValue;
+  ObjectRef get staticValue;
 
   /// [optional] The location of this field in the source code.
-  SourceLocation? get location;
+  SourceLocation get location;
 
-  GuardClassKind? get guardClassKind;
-  ClassRef? get guardClass;
-  bool? get guardNullable;
+  GuardClassKind get guardClassKind;
+  ClassRef get guardClass;
+  bool get guardNullable;
 }
diff --git a/runtime/observatory/lib/src/models/objects/frame.dart b/runtime/observatory/lib/src/models/objects/frame.dart
index 694a7f5d03f..12140f67e8a 100644
--- a/runtime/observatory/lib/src/models/objects/frame.dart
+++ b/runtime/observatory/lib/src/models/objects/frame.dart
@@ -7,8 +7,8 @@ part of models;
 enum FrameKind { regular, asyncCausal, asyncSuspensionMarker, asyncActivation }
 
 abstract class Frame {
-  FrameKind? get kind;
-  String? get marker;
-  FunctionRef? get function;
-  SourceLocation? get location;
+  FrameKind get kind;
+  String get marker;
+  FunctionRef get function;
+  SourceLocation get location;
 }
diff --git a/runtime/observatory/lib/src/models/objects/function.dart b/runtime/observatory/lib/src/models/objects/function.dart
index 3dbe6535f41..82d3d7f5516 100644
--- a/runtime/observatory/lib/src/models/objects/function.dart
+++ b/runtime/observatory/lib/src/models/objects/function.dart
@@ -28,7 +28,7 @@ enum FunctionKind {
   dynamicInvocationForwarder
 }
 
-bool isSyntheticFunction(FunctionKind? kind) {
+bool isSyntheticFunction(FunctionKind kind) {
   switch (kind) {
     case FunctionKind.collected:
     case FunctionKind.native:
@@ -40,64 +40,64 @@ bool isSyntheticFunction(FunctionKind? kind) {
   }
 }
 
-bool isDartFunction(FunctionKind? kind) => !isSyntheticFunction(kind);
-bool isStubFunction(FunctionKind? kind) => kind == FunctionKind.stub;
-bool hasDartCode(FunctionKind? kind) =>
+bool isDartFunction(FunctionKind kind) => !isSyntheticFunction(kind);
+bool isStubFunction(FunctionKind kind) => kind == FunctionKind.stub;
+bool hasDartCode(FunctionKind kind) =>
     isDartFunction(kind) || isStubFunction(kind);
 
 String getFunctionFullName(FunctionRef function) {
-  String content = function.name!;
-  ObjectRef? owner = function.dartOwner;
+  var content = <String>[function.name];
+  ObjectRef owner = function.dartOwner;
   while (owner is FunctionRef) {
     FunctionRef function = (owner as FunctionRef);
-    content = "${function.name!}.${content}";
-    owner = function.dartOwner!;
+    content.add(function.name);
+    owner = function.dartOwner;
   }
   if (owner is ClassRef) {
-    content = "${owner.name!}.${content}";
+    content.add(owner.name);
   }
-  return content;
+  return content.reversed.join('.');
 }
 
 abstract class FunctionRef extends ObjectRef {
   /// The name of this class.
-  String? get name;
+  String get name;
 
   /// The owner of this function, which can be a LibraryRef, ClassRef,
   /// or a FunctionRef.
-  ObjectRef? get dartOwner; // owner
+  ObjectRef get dartOwner; // owner
 
   /// Is this function static?
-  bool? get isStatic;
+  bool get isStatic;
 
   /// Is this function const?
-  bool? get isConst;
+  bool get isConst;
 
   /// The kind of the function.
-  FunctionKind? get kind;
+  FunctionKind get kind;
 }
 
 abstract class ServiceFunction extends Object implements FunctionRef {
   /// The location of this function in the source code. [optional]
-  SourceLocation? get location;
+  SourceLocation get location;
 
   /// The compiled code associated with this function. [optional]
-  CodeRef? get code;
+  CodeRef get code;
 
   /// [optional]
-  CodeRef? get unoptimizedCode;
+  CodeRef get unoptimizedCode;
 
   /// [optional]
-  CodeRef? get bytecode;
+  CodeRef get bytecode;
 
   /// [optional]
-  FieldRef? get field;
-  int? get usageCounter;
-  InstanceRef? get icDataArray;
-  int? get deoptimizations;
-  bool? get isOptimizable;
-  bool? get isInlinable;
-  bool? get hasIntrinsic;
-  bool? get isRecognized;
-  bool? get isNative;
+  FieldRef get field;
+  int get usageCounter;
+  InstanceRef get icDataArray;
+  int get deoptimizations;
+  bool get isOptimizable;
+  bool get isInlinable;
+  bool get hasIntrinsic;
+  bool get isRecognized;
+  bool get isNative;
 }
diff --git a/runtime/observatory/lib/src/models/objects/guarded.dart b/runtime/observatory/lib/src/models/objects/guarded.dart
index 46175411517..82bf1eeee8d 100644
--- a/runtime/observatory/lib/src/models/objects/guarded.dart
+++ b/runtime/observatory/lib/src/models/objects/guarded.dart
@@ -7,6 +7,6 @@ part of models;
 abstract class Guarded<T> {
   bool get isValue;
   bool get isSentinel;
-  Sentinel? get asSentinel;
-  T? get asValue;
+  Sentinel get asSentinel;
+  T get asValue;
 }
diff --git a/runtime/observatory/lib/src/models/objects/icdata.dart b/runtime/observatory/lib/src/models/objects/icdata.dart
index 26e53351df1..d975301374b 100644
--- a/runtime/observatory/lib/src/models/objects/icdata.dart
+++ b/runtime/observatory/lib/src/models/objects/icdata.dart
@@ -5,11 +5,11 @@
 part of models;
 
 abstract class ICDataRef extends ObjectRef {
-  String? get selector;
+  String get selector;
 }
 
 abstract class ICData extends Object implements ICDataRef {
-  ObjectRef? get dartOwner;
-  InstanceRef? get argumentsDescriptor;
-  InstanceRef? get entries;
+  ObjectRef get dartOwner;
+  InstanceRef get argumentsDescriptor;
+  InstanceRef get entries;
 }
diff --git a/runtime/observatory/lib/src/models/objects/instance.dart b/runtime/observatory/lib/src/models/objects/instance.dart
index ab8d6d69291..66965e9889a 100644
--- a/runtime/observatory/lib/src/models/objects/instance.dart
+++ b/runtime/observatory/lib/src/models/objects/instance.dart
@@ -122,10 +122,7 @@ enum InstanceKind {
   typeRef,
 }
 
-bool isTypedData(InstanceKind? kind) {
-  if (kind == null) {
-    return false;
-  }
+bool isTypedData(InstanceKind kind) {
   switch (kind) {
     case InstanceKind.uint8ClampedList:
     case InstanceKind.uint8List:
@@ -147,10 +144,7 @@ bool isTypedData(InstanceKind? kind) {
   }
 }
 
-bool isSimdValue(InstanceKind? kind) {
-  if (kind == null) {
-    return false;
-  }
+bool isSimdValue(InstanceKind kind) {
   switch (kind) {
     case InstanceKind.float32x4:
     case InstanceKind.float64x2:
@@ -161,10 +155,7 @@ bool isSimdValue(InstanceKind? kind) {
   }
 }
 
-bool isAbstractType(InstanceKind? kind) {
-  if (kind == null) {
-    return false;
-  }
+bool isAbstractType(InstanceKind kind) {
   switch (kind) {
     case InstanceKind.type:
     case InstanceKind.typeRef:
@@ -177,10 +168,10 @@ bool isAbstractType(InstanceKind? kind) {
 
 abstract class InstanceRef extends ObjectRef {
   /// What kind of instance is this?
-  InstanceKind? get kind;
+  InstanceKind get kind;
 
   /// Instance references always include their class.
-  ClassRef? get clazz;
+  ClassRef get clazz;
 
   /// [optional] The value of this instance as a string.
   ///
@@ -194,13 +185,13 @@ abstract class InstanceRef extends ObjectRef {
   ///   Float64x2
   ///   Int32x4
   ///   StackTrace
-  String? get valueAsString;
+  String get valueAsString;
 
   /// [optional] The valueAsString for String references may be truncated. If so,
   /// this property is added with the value 'true'.
   ///
   /// New code should use 'length' and 'count' instead.
-  bool? get valueAsStringIsTruncated;
+  bool get valueAsStringIsTruncated;
 
   /// [optional] The length of a List or the number of associations in a Map or
   /// the number of codeunits in a String.
@@ -223,25 +214,25 @@ abstract class InstanceRef extends ObjectRef {
   ///   Int32x4List
   ///   Float32x4List
   ///   Float64x2List
-  int? get length;
+  int get length;
 
   /// [optional] The name of a Type instance.
   ///
   /// Provided for instance kinds:
   ///   Type
-  String? get name;
+  String get name;
 
   /// [optional] The corresponding Class if this Type is canonical.
   ///
   /// Provided for instance kinds:
   ///   Type
-  ClassRef? get typeClass;
+  ClassRef get typeClass;
 
   /// [optional] The parameterized class of a type parameter:
   ///
   /// Provided for instance kinds:
   ///   TypeParameter
-  ClassRef? get parameterizedClass;
+  ClassRef get parameterizedClass;
 
   /// [optional] The pattern of a RegExp instance.
   ///
@@ -249,19 +240,19 @@ abstract class InstanceRef extends ObjectRef {
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  InstanceRef? get pattern;
+  InstanceRef get pattern;
 
   /// [optional] The function associated with a Closure instance.
   ///
   /// Provided for instance kinds:
   ///   Closure
-  FunctionRef? get closureFunction;
+  FunctionRef get closureFunction;
 
   /// [optional] The context associated with a Closure instance.
   ///
   /// Provided for instance kinds:
   ///   Closure
-  ContextRef? get closureContext;
+  ContextRef get closureContext;
 }
 
 abstract class Instance extends Object implements InstanceRef {
@@ -286,7 +277,7 @@ abstract class Instance extends Object implements InstanceRef {
   ///   Int32x4List
   ///   Float32x4List
   ///   Float64x2List
-  int? get offset;
+  int get offset;
 
   /// [optional] The number of elements or associations or codeunits returned.
   /// This is only provided when it is less than length.
@@ -309,7 +300,7 @@ abstract class Instance extends Object implements InstanceRef {
   ///   Int32x4List
   ///   Float32x4List
   ///   Float64x2List
-  int? get count;
+  int get count;
 
   /// [optional] The elements of a TypedData instance.
   ///
@@ -328,19 +319,19 @@ abstract class Instance extends Object implements InstanceRef {
   ///   Int32x4List
   ///   Float32x4List
   ///   Float64x2List
-  List<dynamic>? get typedElements;
+  List<dynamic> get typedElements;
 
   /// [optional] The native fields of this Instance.
-  Iterable<NativeField>? get nativeFields;
+  Iterable<NativeField> get nativeFields;
 
   /// [optional] The fields of this Instance.
-  Iterable<BoundField>? get fields;
+  Iterable<BoundField> get fields;
 
   /// [optional] The elements of a List instance.
   ///
   /// Provided for instance kinds:
   ///   List
-  Iterable<Guarded<ObjectRef>>? get elements;
+  Iterable<Guarded<ObjectRef>> get elements;
   // It should be:
   // Iterable<Guarded<InstanceRef>> get elements;
   // In some situations we obtain lists of non Instances
@@ -349,37 +340,37 @@ abstract class Instance extends Object implements InstanceRef {
   ///
   /// Provided for instance kinds:
   ///   Map
-  Iterable<MapAssociation>? get associations;
+  Iterable<MapAssociation> get associations;
 
   /// [optional] The key for a WeakProperty instance.
   ///
   /// Provided for instance kinds:
   ///   WeakProperty
-  InstanceRef? get key;
+  InstanceRef get key;
 
   /// [optional] The key for a WeakProperty instance.
   ///
   /// Provided for instance kinds:
   ///   WeakProperty
-  InstanceRef? get value;
+  InstanceRef get value;
 
   /// [optional] The referent of a MirrorReference instance.
   ///
   /// Provided for instance kinds:
   ///   MirrorReference
-  ObjectRef? get referent;
+  ObjectRef get referent;
 
   /// [optional] The type arguments for this type.
   ///
   /// Provided for instance kinds:
   ///   Type
-  TypeArgumentsRef? get typeArguments;
+  TypeArgumentsRef get typeArguments;
 
   /// [optional] The index of a TypeParameter instance.
   ///
   /// Provided for instance kinds:
   ///   TypeParameter
-  int? get parameterIndex;
+  int get parameterIndex;
 
   /// [optional] The referent of a TypeRef instance.
   ///
@@ -388,7 +379,7 @@ abstract class Instance extends Object implements InstanceRef {
   ///
   /// Provided for instance kinds:
   ///   TypeRef
-  InstanceRef? get targetType;
+  InstanceRef get targetType;
 
   /// [optional] The bound of a TypeParameter.
   ///
@@ -397,66 +388,66 @@ abstract class Instance extends Object implements InstanceRef {
   ///
   /// Provided for instance kinds:
   ///   TypeParameter
-  InstanceRef? get bound;
+  InstanceRef get bound;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   Closure
-  Breakpoint? get activationBreakpoint;
+  Breakpoint get activationBreakpoint;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  bool? get isCaseSensitive;
+  bool get isCaseSensitive;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  bool? get isMultiLine;
+  bool get isMultiLine;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  FunctionRef? get oneByteFunction;
+  FunctionRef get oneByteFunction;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  FunctionRef? get twoByteFunction;
+  FunctionRef get twoByteFunction;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  FunctionRef? get externalOneByteFunction;
+  FunctionRef get externalOneByteFunction;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  FunctionRef? get externalTwoByteFunction;
+  FunctionRef get externalTwoByteFunction;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  InstanceRef? get oneByteBytecode;
+  InstanceRef get oneByteBytecode;
 
   /// [optional]
   ///
   /// Provided for instance kinds:
   ///   RegExp
-  InstanceRef? get twoByteBytecode;
+  InstanceRef get twoByteBytecode;
 }
 
 abstract class BoundField {
-  FieldRef? get decl;
-  Guarded<InstanceRef>? get value;
+  FieldRef get decl;
+  Guarded<InstanceRef> get value;
 }
 
 abstract class NativeField {
diff --git a/runtime/observatory/lib/src/models/objects/isolate.dart b/runtime/observatory/lib/src/models/objects/isolate.dart
index 9fbc26bf8cc..039e0bdc888 100644
--- a/runtime/observatory/lib/src/models/objects/isolate.dart
+++ b/runtime/observatory/lib/src/models/objects/isolate.dart
@@ -7,13 +7,13 @@ part of models;
 abstract class IsolateRef {
   /// The id which is passed to the getIsolate RPC to reload this
   /// isolate.
-  String? get id;
+  String get id;
 
   /// A numeric id for this isolate, represented as a string. Unique.
-  int? get number;
+  int get number;
 
   /// A name identifying this isolate. Not guaranteed to be unique.
-  String? get name;
+  String get name;
 
   /// Trigger a full GC, collecting all unreachable or weakly reachable objects.
   Future collectAllGarbage();
@@ -23,7 +23,7 @@ enum IsolateStatus { loading, idle, running, paused }
 
 abstract class Isolate extends IsolateRef {
   /// The time that the VM started in milliseconds since the epoch.
-  DateTime? get startTime;
+  DateTime get startTime;
 
   /// Is the isolate in a runnable state?
   bool get runnable;
@@ -36,51 +36,51 @@ abstract class Isolate extends IsolateRef {
 
   /// The last pause event delivered to the isolate. If the isolate is
   /// running, this will be a resume event.
-  Event? get pauseEvent;
+  Event get pauseEvent;
 
   /// [optional] The root library for this isolate.
   ///
   /// Guaranteed to be initialized when the IsolateRunnable event fires.
-  LibraryRef? get rootLibrary;
+  LibraryRef get rootLibrary;
 
   /// A list of all libraries for this isolate.
   ///
   /// Guaranteed to be initialized when the IsolateRunnable event fires.
-  Iterable<LibraryRef>? get libraries;
+  Iterable<LibraryRef> get libraries;
 
   /// A list of all breakpoints for this isolate.
   //Iterable<Breakpoint> get breakpoints;
 
   /// [optional] The error that is causing this isolate to exit, if applicable.
-  Error? get error;
+  Error get error;
 
   /// The list of threads associated with this isolate.
-  Iterable<Thread>? get threads;
+  Iterable<Thread> get threads;
 
   /// The maximum amount of zone memory in bytes allocated by the isolate in
   /// all threads at a given time. Calculated using the high watermarks of each
   /// thread alive when a thread is unscheduled.
-  int? get zoneHighWatermark;
+  int get zoneHighWatermark;
 
   /// The number of zone handles currently held by this isolate.
-  int? get numZoneHandles;
+  int get numZoneHandles;
 
   /// The number of scoped handles currently held by this isolate.
-  int? get numScopedHandles;
+  int get numScopedHandles;
 
   /// The current pause on exception mode for this isolate.
   //ExceptionPauseMode get exceptionPauseMode;
 
   /// [optional] The list of service extension RPCs that are registered for this
   /// isolate, if any.
-  Iterable<String>? get extensionRPCs;
+  Iterable<String> get extensionRPCs;
 
-  Map? get counters;
-  HeapSpace? get newSpace;
-  HeapSpace? get oldSpace;
+  Map get counters;
+  HeapSpace get newSpace;
+  HeapSpace get oldSpace;
 
-  IsolateStatus? get status;
+  IsolateStatus get status;
 
   /// [optional]
-  FunctionRef? get entry;
+  FunctionRef get entry;
 }
diff --git a/runtime/observatory/lib/src/models/objects/isolate_group.dart b/runtime/observatory/lib/src/models/objects/isolate_group.dart
index 7aff55073be..b18bbf72fe9 100644
--- a/runtime/observatory/lib/src/models/objects/isolate_group.dart
+++ b/runtime/observatory/lib/src/models/objects/isolate_group.dart
@@ -7,16 +7,16 @@ part of models;
 abstract class IsolateGroupRef {
   /// The id which is passed to the getIsolateGroup RPC to reload this
   /// isolate group.
-  String? get id;
+  String get id;
 
   /// A numeric id for this isolate group, represented as a string. Unique.
-  int? get number;
+  int get number;
 
   /// A name identifying this isolate group. Not guaranteed to be unique.
-  String? get name;
+  String get name;
 }
 
 abstract class IsolateGroup extends IsolateGroupRef {
   /// A list of all isolates in this isolate group.
-  Iterable<IsolateRef>? get isolates;
+  Iterable<IsolateRef> get isolates;
 }
diff --git a/runtime/observatory/lib/src/models/objects/library.dart b/runtime/observatory/lib/src/models/objects/library.dart
index b02a97dab82..83bab5449af 100644
--- a/runtime/observatory/lib/src/models/objects/library.dart
+++ b/runtime/observatory/lib/src/models/objects/library.dart
@@ -6,10 +6,10 @@ part of models;
 
 abstract class LibraryRef extends ObjectRef {
   /// The name of this library.
-  String? get name;
+  String get name;
 
   /// The uri of this library.
-  String? get uri;
+  String get uri;
 }
 
 abstract class Library extends Object implements LibraryRef {
@@ -17,22 +17,22 @@ abstract class Library extends Object implements LibraryRef {
   //bool get debuggable;
 
   /// A list of the imports for this library.
-  Iterable<LibraryDependency>? get dependencies;
+  Iterable<LibraryDependency> get dependencies;
 
   /// A list of the scripts which constitute this library.
-  Iterable<ScriptRef>? get scripts;
+  Iterable<ScriptRef> get scripts;
 
   /// A list of the top-level variables in this library.
-  Iterable<FieldRef>? get variables;
+  Iterable<FieldRef> get variables;
 
   /// A list of the top-level functions in this library.
-  Iterable<FunctionRef>? get functions;
+  Iterable<FunctionRef> get functions;
 
   /// A list of all classes in this library.
-  Iterable<ClassRef>? get classes;
+  Iterable<ClassRef> get classes;
 
-  ScriptRef? get rootScript;
-  String? get vmName;
+  ScriptRef get rootScript;
+  String get vmName;
 }
 
 abstract class LibraryDependency {
diff --git a/runtime/observatory/lib/src/models/objects/local_var_descriptors.dart b/runtime/observatory/lib/src/models/objects/local_var_descriptors.dart
index a8e6320167f..5643b8084e0 100644
--- a/runtime/observatory/lib/src/models/objects/local_var_descriptors.dart
+++ b/runtime/observatory/lib/src/models/objects/local_var_descriptors.dart
@@ -6,5 +6,5 @@ part of models;
 
 abstract class LocalVarDescriptorsRef extends ObjectRef {
   /// [optional]
-  String? get name;
+  String get name;
 }
diff --git a/runtime/observatory/lib/src/models/objects/megamorphiccache.dart b/runtime/observatory/lib/src/models/objects/megamorphiccache.dart
index 5618a7ee8d0..74657de0013 100644
--- a/runtime/observatory/lib/src/models/objects/megamorphiccache.dart
+++ b/runtime/observatory/lib/src/models/objects/megamorphiccache.dart
@@ -5,12 +5,12 @@
 part of models;
 
 abstract class MegamorphicCacheRef extends ObjectRef {
-  String? get selector;
+  String get selector;
 }
 
 abstract class MegamorphicCache extends Object implements MegamorphicCacheRef {
-  String? get selector;
-  int? get mask;
-  InstanceRef? get buckets;
-  InstanceRef? get argumentsDescriptor;
+  String get selector;
+  int get mask;
+  InstanceRef get buckets;
+  InstanceRef get argumentsDescriptor;
 }
diff --git a/runtime/observatory/lib/src/models/objects/metric.dart b/runtime/observatory/lib/src/models/objects/metric.dart
index 13c1c784fb7..fd8f7ace643 100644
--- a/runtime/observatory/lib/src/models/objects/metric.dart
+++ b/runtime/observatory/lib/src/models/objects/metric.dart
@@ -5,12 +5,12 @@
 part of models;
 
 abstract class Metric {
-  String? get id;
-  String? get name;
-  String? get description;
+  String get id;
+  String get name;
+  String get description;
 }
 
 abstract class MetricSample {
-  double? get value;
-  DateTime? get time;
+  double get value;
+  DateTime get time;
 }
diff --git a/runtime/observatory/lib/src/models/objects/notification.dart b/runtime/observatory/lib/src/models/objects/notification.dart
index 54e8c61d737..1f46766ceed 100644
--- a/runtime/observatory/lib/src/models/objects/notification.dart
+++ b/runtime/observatory/lib/src/models/objects/notification.dart
@@ -10,7 +10,7 @@ abstract class ExceptionNotification implements Notification {
   get exception;
 
   /// [optional]
-  StackTrace? get stacktrace;
+  StackTrace get stacktrace;
 }
 
 abstract class EventNotification implements Notification {
diff --git a/runtime/observatory/lib/src/models/objects/object.dart b/runtime/observatory/lib/src/models/objects/object.dart
index 6f258f601e4..044072fde44 100644
--- a/runtime/observatory/lib/src/models/objects/object.dart
+++ b/runtime/observatory/lib/src/models/objects/object.dart
@@ -6,7 +6,7 @@ part of models;
 
 abstract class ObjectRef {
   /// A unique identifier for an Object.
-  String? get id;
+  String get id;
 }
 
 abstract class Object implements ObjectRef {
@@ -18,7 +18,7 @@ abstract class Object implements ObjectRef {
   ///
   /// Moving an Object into or out of the heap is considered a
   /// backwards compatible change for types other than Instance.
-  ClassRef? get clazz;
+  ClassRef get clazz;
 
   /// [optional] The size of this object in the heap.
   ///
@@ -27,9 +27,9 @@ abstract class Object implements ObjectRef {
   /// Note that the size can be zero for some objects. In the current
   /// VM implementation, this occurs for small integers, which are
   /// stored entirely within their object pointers.
-  int? get size;
+  int get size;
 
-  String? get vmName;
+  String get vmName;
 }
 
 abstract class RetainingObject {
diff --git a/runtime/observatory/lib/src/models/objects/objectpool.dart b/runtime/observatory/lib/src/models/objects/objectpool.dart
index da7fd76c49c..ba659611804 100644
--- a/runtime/observatory/lib/src/models/objects/objectpool.dart
+++ b/runtime/observatory/lib/src/models/objects/objectpool.dart
@@ -5,11 +5,11 @@
 part of models;
 
 abstract class ObjectPoolRef extends ObjectRef {
-  int? get length;
+  int get length;
 }
 
 abstract class ObjectPool extends Object implements ObjectPoolRef {
-  Iterable<ObjectPoolEntry>? get entries;
+  Iterable<ObjectPoolEntry> get entries;
 }
 
 enum ObjectPoolEntryKind { object, immediate, nativeEntryData, nativeEntry }
@@ -17,6 +17,6 @@ enum ObjectPoolEntryKind { object, immediate, nativeEntryData, nativeEntry }
 abstract class ObjectPoolEntry {
   int get offset;
   ObjectPoolEntryKind get kind;
-  ObjectRef? get asObject;
-  int? get asInteger;
+  ObjectRef get asObject;
+  int get asInteger;
 }
diff --git a/runtime/observatory/lib/src/models/objects/pc_descriptors.dart b/runtime/observatory/lib/src/models/objects/pc_descriptors.dart
index 256a9dd5998..fea4ce66d93 100644
--- a/runtime/observatory/lib/src/models/objects/pc_descriptors.dart
+++ b/runtime/observatory/lib/src/models/objects/pc_descriptors.dart
@@ -6,5 +6,5 @@ part of models;
 
 abstract class PcDescriptorsRef extends ObjectRef {
   /// [optional]
-  String? get name;
+  String get name;
 }
diff --git a/runtime/observatory/lib/src/models/objects/sample_profile.dart b/runtime/observatory/lib/src/models/objects/sample_profile.dart
index 3dcd394523e..582e571b89a 100644
--- a/runtime/observatory/lib/src/models/objects/sample_profile.dart
+++ b/runtime/observatory/lib/src/models/objects/sample_profile.dart
@@ -26,14 +26,14 @@ abstract class Profile {
 }
 
 abstract class ProfileCode extends Profile {
-  CodeRef? get code;
+  CodeRef get code;
   Map<ProfileCode, int> get callers;
   Map<ProfileCode, int> get callees;
 }
 
 abstract class ProfileFunction extends Profile {
-  FunctionRef? get function;
-  String? get resolvedUrl;
+  FunctionRef get function;
+  String get resolvedUrl;
   Map<ProfileFunction, int> get callers;
   Map<ProfileFunction, int> get callees;
 }
diff --git a/runtime/observatory/lib/src/models/objects/script.dart b/runtime/observatory/lib/src/models/objects/script.dart
index a6ce8ee0865..df4b63be811 100644
--- a/runtime/observatory/lib/src/models/objects/script.dart
+++ b/runtime/observatory/lib/src/models/objects/script.dart
@@ -6,23 +6,23 @@ part of models;
 
 abstract class ScriptRef extends ObjectRef {
   /// The uri from which this script was loaded.
-  String? get uri;
+  String get uri;
 }
 
 abstract class Script extends Object implements ScriptRef {
   /// The library which owns this script.
-  LibraryRef? get library;
+  LibraryRef get library;
 
   /// The source code for this script. For certain built-in scripts,
   /// this may be reconstructed without source comments.
-  String? get source;
+  String get source;
 
-  DateTime? get loadTime;
-  int? get firstTokenPos;
-  int? get lastTokenPos;
-  int? get lineOffset;
-  int? get columnOffset;
+  DateTime get loadTime;
+  int get firstTokenPos;
+  int get lastTokenPos;
+  int get lineOffset;
+  int get columnOffset;
 
-  int? tokenToLine(int? token);
-  int? tokenToCol(int? token);
+  int tokenToLine(int token);
+  int tokenToCol(int token);
 }
diff --git a/runtime/observatory/lib/src/models/objects/single_target_cache.dart b/runtime/observatory/lib/src/models/objects/single_target_cache.dart
index 185df81e14e..5178e7ada4f 100644
--- a/runtime/observatory/lib/src/models/objects/single_target_cache.dart
+++ b/runtime/observatory/lib/src/models/objects/single_target_cache.dart
@@ -5,11 +5,11 @@
 part of models;
 
 abstract class SingleTargetCacheRef extends ObjectRef {
-  Code? get target;
+  Code get target;
 }
 
 abstract class SingleTargetCache extends Object
     implements SingleTargetCacheRef {
-  int? get lowerLimit;
-  int? get upperLimit;
+  int get lowerLimit;
+  int get upperLimit;
 }
diff --git a/runtime/observatory/lib/src/models/objects/source_location.dart b/runtime/observatory/lib/src/models/objects/source_location.dart
index 27a7c5924ec..8a03f13eff0 100644
--- a/runtime/observatory/lib/src/models/objects/source_location.dart
+++ b/runtime/observatory/lib/src/models/objects/source_location.dart
@@ -9,7 +9,7 @@ abstract class Location {
   ScriptRef get script;
 
   /// [optional] The first token of the location.
-  int? get tokenPos;
+  int get tokenPos;
 }
 
 abstract class SourceLocation implements Location {
@@ -17,19 +17,19 @@ abstract class SourceLocation implements Location {
   int get tokenPos;
 
   /// [optional] The last token of the location if this is a range.
-  int? get endTokenPos;
+  int get endTokenPos;
 }
 
 abstract class UnresolvedSourceLocation implements Location {
   /// [optional] The uri of the script containing the source location if the
   /// script has yet to be loaded.
-  String? get scriptUri;
+  String get scriptUri;
 
   /// [optional] An approximate line number for the source location. This may
   /// change when the location is resolved.
-  int? get line;
+  int get line;
 
   /// [optional] An approximate column number for the source location. This may
   /// change when the location is resolved.
-  int? get column;
+  int get column;
 }
diff --git a/runtime/observatory/lib/src/models/objects/subtype_test_cache.dart b/runtime/observatory/lib/src/models/objects/subtype_test_cache.dart
index 99ab1b1fd75..cf569392396 100644
--- a/runtime/observatory/lib/src/models/objects/subtype_test_cache.dart
+++ b/runtime/observatory/lib/src/models/objects/subtype_test_cache.dart
@@ -7,5 +7,5 @@ part of models;
 abstract class SubtypeTestCacheRef extends ObjectRef {}
 
 abstract class SubtypeTestCache extends Object implements SubtypeTestCacheRef {
-  InstanceRef? get cache;
+  InstanceRef get cache;
 }
diff --git a/runtime/observatory/lib/src/models/objects/thread.dart b/runtime/observatory/lib/src/models/objects/thread.dart
index b29d3e63174..8d16a8ce56c 100644
--- a/runtime/observatory/lib/src/models/objects/thread.dart
+++ b/runtime/observatory/lib/src/models/objects/thread.dart
@@ -15,17 +15,17 @@ enum ThreadKind {
 
 abstract class Thread {
   /// The id associated with the thread on creation.
-  String? get id;
+  String get id;
 
   /// The task type associated with the thread.
-  ThreadKind? get kind;
+  ThreadKind get kind;
 
-  String? get kindString;
+  String get kindString;
 
   /// The maximum amount of zone memory in bytes allocated by a thread at a
   /// given time throughout the entire life of the thread.
-  int? get zoneHighWatermark;
+  int get zoneHighWatermark;
 
   /// The current Zone capacity available to this thread.
-  int? get zoneCapacity;
+  int get zoneCapacity;
 }
diff --git a/runtime/observatory/lib/src/models/objects/type_arguments.dart b/runtime/observatory/lib/src/models/objects/type_arguments.dart
index ffe2186cdc0..a3614ddce44 100644
--- a/runtime/observatory/lib/src/models/objects/type_arguments.dart
+++ b/runtime/observatory/lib/src/models/objects/type_arguments.dart
@@ -6,7 +6,7 @@ part of models;
 
 abstract class TypeArgumentsRef extends ObjectRef {
   /// A name for this type argument list.
-  String? get name;
+  String get name;
 }
 
 abstract class TypeArguments extends Object implements TypeArgumentsRef {
@@ -14,5 +14,5 @@ abstract class TypeArguments extends Object implements TypeArgumentsRef {
   ///
   /// The value will always be one of the kinds:
   /// Type, TypeRef, TypeParameter.
-  Iterable<InstanceRef>? get types;
+  Iterable<InstanceRef> get types;
 }
diff --git a/runtime/observatory/lib/src/models/objects/unknown.dart b/runtime/observatory/lib/src/models/objects/unknown.dart
index 9228c8e47ed..3faeed0c92c 100644
--- a/runtime/observatory/lib/src/models/objects/unknown.dart
+++ b/runtime/observatory/lib/src/models/objects/unknown.dart
@@ -5,5 +5,5 @@
 part of models;
 
 abstract class UnknownObjectRef extends ObjectRef {
-  String? get vmType;
+  String get vmType;
 }
diff --git a/runtime/observatory/lib/src/models/objects/unlinked_call.dart b/runtime/observatory/lib/src/models/objects/unlinked_call.dart
index 37980ecbe40..ed07750425c 100644
--- a/runtime/observatory/lib/src/models/objects/unlinked_call.dart
+++ b/runtime/observatory/lib/src/models/objects/unlinked_call.dart
@@ -5,9 +5,9 @@
 part of models;
 
 abstract class UnlinkedCallRef extends ObjectRef {
-  String? get selector;
+  String get selector;
 }
 
 abstract class UnlinkedCall extends Object implements UnlinkedCallRef {
-  InstanceRef? get argumentsDescriptor;
+  InstanceRef get argumentsDescriptor;
 }
diff --git a/runtime/observatory/lib/src/models/objects/vm.dart b/runtime/observatory/lib/src/models/objects/vm.dart
index 38952576b5b..fe25c846790 100644
--- a/runtime/observatory/lib/src/models/objects/vm.dart
+++ b/runtime/observatory/lib/src/models/objects/vm.dart
@@ -6,11 +6,11 @@ part of models;
 
 abstract class VMRef {
   /// A name identifying this vm. Not guaranteed to be unique.
-  String? get name;
+  String get name;
 
   /// [Not actually from the apis]
   /// A name used to identify the VM in the UI.
-  String? get displayName;
+  String get displayName;
 }
 
 abstract class VM implements VMRef {
@@ -46,7 +46,7 @@ abstract class VM implements VMRef {
   /// The time that the VM started in milliseconds since the epoch.
   ///
   /// Suitable to pass to DateTime.fromMillisecondsSinceEpoch.
-  DateTime? get startTime;
+  DateTime get startTime;
 
   // A list of isolates running in the VM.
   Iterable<IsolateRef> get isolates;
diff --git a/runtime/observatory/lib/src/models/repositories/metric.dart b/runtime/observatory/lib/src/models/repositories/metric.dart
index ed90d505d61..ee10fdbe160 100644
--- a/runtime/observatory/lib/src/models/repositories/metric.dart
+++ b/runtime/observatory/lib/src/models/repositories/metric.dart
@@ -14,7 +14,7 @@ abstract class MetricRepository {
   MetricSamplingRate getSamplingRate(IsolateRef isolate, Metric metric);
   void setBufferSize(IsolateRef isolate, Metric metric, MetricBufferSize r);
   MetricBufferSize getBufferSize(IsolateRef isolate, Metric metric);
-  Iterable<MetricSample>? getSamples(IsolateRef isolate, Metric metric);
+  Iterable<MetricSample> getSamples(IsolateRef isolate, Metric metric);
   double getMinValue(IsolateRef isolate, Metric metric);
   double getMaxValue(IsolateRef isolate, Metric metric);
 }
diff --git a/runtime/observatory/lib/src/models/repositories/target.dart b/runtime/observatory/lib/src/models/repositories/target.dart
index a443a54f3a7..2c2db92e366 100644
--- a/runtime/observatory/lib/src/models/repositories/target.dart
+++ b/runtime/observatory/lib/src/models/repositories/target.dart
@@ -11,11 +11,11 @@ abstract class TargetChangeEvent {
 abstract class TargetRepository {
   Stream<TargetChangeEvent> get onChange;
 
-  Target? get current;
+  Target get current;
   Iterable<Target> list();
   void add(String address);
   void setCurrent(Target t);
   void delete(Target o);
-  Target? find(String networkAddress);
+  Target find(String networkAddress);
   bool isConnectedVMTarget(Target target);
 }
diff --git a/runtime/observatory/lib/src/repositories/allocation_profile.dart b/runtime/observatory/lib/src/repositories/allocation_profile.dart
index 5dc86671e8b..365a6fc7fa4 100644
--- a/runtime/observatory/lib/src/repositories/allocation_profile.dart
+++ b/runtime/observatory/lib/src/repositories/allocation_profile.dart
@@ -22,7 +22,7 @@ class AllocationProfileRepository implements M.AllocationProfileRepository {
       params['reset'] = true;
     }
     final dynamic response = await isolate.invokeRpc(_api, params);
-    Map? defaults;
+    Map defaults;
     if (combine) {
       defaults = await isolate.vm.invokeRpcNoUpgrade(_defaultsApi, {});
       defaults = defaults['map'];
diff --git a/runtime/observatory/lib/src/repositories/breakpoint.dart b/runtime/observatory/lib/src/repositories/breakpoint.dart
index 7465cb0604f..99a66ca8753 100644
--- a/runtime/observatory/lib/src/repositories/breakpoint.dart
+++ b/runtime/observatory/lib/src/repositories/breakpoint.dart
@@ -8,12 +8,12 @@ class BreakpointRepository extends M.BreakpointRepository {
   Future addOnActivation(M.IsolateRef i, M.Instance closure) async {
     S.Isolate isolate = i as S.Isolate;
     assert(isolate != null);
-    await isolate.addBreakOnActivation(closure as S.Instance);
+    await isolate.addBreakOnActivation(closure);
   }
 
   Future remove(M.IsolateRef i, M.Breakpoint breakpoint) async {
     S.Isolate isolate = i as S.Isolate;
     assert(isolate != null);
-    await isolate.removeBreakpoint(breakpoint as S.Breakpoint);
+    await isolate.removeBreakpoint(breakpoint);
   }
 }
diff --git a/runtime/observatory/lib/src/repositories/editor.dart b/runtime/observatory/lib/src/repositories/editor.dart
index 2728b14f8ee..d58cb36246f 100644
--- a/runtime/observatory/lib/src/repositories/editor.dart
+++ b/runtime/observatory/lib/src/repositories/editor.dart
@@ -6,24 +6,24 @@ part of repositories;
 
 class EditorRepository extends M.EditorRepository {
   final S.VM _vm;
-  final String? _editor;
+  final String _editor;
 
   bool get isAvailable => _getService() != null;
 
-  EditorRepository(S.VM vm, {String? editor})
+  EditorRepository(S.VM vm, {String editor})
       : _vm = vm,
         _editor = editor {
     assert(_vm != null);
   }
 
-  S.Service? _getService() {
+  S.Service _getService() {
     Iterable<M.Service> services =
         _vm.services.where((s) => s.service == 'openSourceLocation');
     if (_editor != null) {
       services = services.where((s) => s.alias == _editor);
     }
     if (services.isNotEmpty) {
-      return services.first as S.Service;
+      return services.first;
     }
     return null;
   }
@@ -70,7 +70,7 @@ class EditorRepository extends M.EditorRepository {
       return await openClass(i, o);
     }
     if (o is M.InstanceRef) {
-      return await openClass(i, o.clazz!);
+      return await openClass(i, o.clazz);
     }
     if (o is M.FieldRef) {
       return await openField(i, o);
@@ -80,18 +80,18 @@ class EditorRepository extends M.EditorRepository {
     }
     if (o is M.InstanceRef) {
       if (o.closureFunction != null) {
-        return await openFunction(i, o.closureFunction!);
+        return await openFunction(i, o.closureFunction);
       }
-      return await openClass(i, o.clazz!);
+      return await openClass(i, o.clazz);
     }
     return new Future.value();
   }
 
-  Future openSourceLocation(M.IsolateRef i, M.SourceLocation? l) async {
+  Future openSourceLocation(M.IsolateRef i, M.SourceLocation l) async {
     final isolate = i as S.Isolate;
     assert(isolate != null);
     assert(l != null);
-    return await isolate.invokeRpc(_getService()!.method,
-        {'scriptId': l!.script.id!, 'tokenPos': l.tokenPos});
+    return await isolate.invokeRpc(_getService().method,
+        {'scriptId': l.script.id, 'tokenPos': l.tokenPos});
   }
 }
diff --git a/runtime/observatory/lib/src/repositories/inbound_references.dart b/runtime/observatory/lib/src/repositories/inbound_references.dart
index 18895fcd1ea..a9b63e04f62 100644
--- a/runtime/observatory/lib/src/repositories/inbound_references.dart
+++ b/runtime/observatory/lib/src/repositories/inbound_references.dart
@@ -10,6 +10,6 @@ class InboundReferencesRepository implements M.InboundReferencesRepository {
     assert(isolate != null);
     final response = await isolate
         .invokeRpc('getInboundReferences', {'targetId': id, 'limit': 100});
-    return new S.InboundReferences(response as S.ServiceMap);
+    return new S.InboundReferences(response);
   }
 }
diff --git a/runtime/observatory/lib/src/repositories/isolate.dart b/runtime/observatory/lib/src/repositories/isolate.dart
index e72431ad29e..68b8ae1551a 100644
--- a/runtime/observatory/lib/src/repositories/isolate.dart
+++ b/runtime/observatory/lib/src/repositories/isolate.dart
@@ -25,7 +25,7 @@ class IsolateRepository extends M.IsolateRepository {
     return isolate;
   }
 
-  Future reloadSources(M.IsolateRef i, {M.Service? service}) async {
+  Future reloadSources(M.IsolateRef i, {M.Service service}) async {
     if (service == null) {
       S.Isolate isolate = i as S.Isolate;
       assert(isolate != null);
diff --git a/runtime/observatory/lib/src/repositories/metric.dart b/runtime/observatory/lib/src/repositories/metric.dart
index e1b7a9afaab..2d901878470 100644
--- a/runtime/observatory/lib/src/repositories/metric.dart
+++ b/runtime/observatory/lib/src/repositories/metric.dart
@@ -26,14 +26,14 @@ class MetricRepository implements M.MetricRepository {
       <S.Isolate, Map<Metric, int>>{};
   final Map<S.Isolate, Map<Metric, int>> _sizes =
       <S.Isolate, Map<Metric, int>>{};
-  Timer? _timer;
+  Timer _timer;
   int count = 0;
 
   Future<Iterable<Metric>> list(M.IsolateRef i) async {
     S.Isolate isolate = i as S.Isolate;
     assert(isolate != null);
     if (_samples.containsKey(isolate)) {
-      return _samples[isolate]!.keys;
+      return _samples[isolate].keys;
     }
     return const [];
   }
@@ -71,14 +71,14 @@ class MetricRepository implements M.MetricRepository {
       _rates.remove(isolate);
       _sizes.remove(isolate);
       if (_samples.isEmpty) {
-        _timer!.cancel();
+        _timer.cancel();
       }
     }
   }
 
   M.MetricSamplingRate getSamplingRate(M.IsolateRef i, M.Metric m) {
     if (_rates.containsKey(i)) {
-      final metrics = _rates[i]!;
+      final metrics = _rates[i];
       if (metrics.containsKey(m)) {
         switch (metrics[m]) {
           case 0:
@@ -101,9 +101,9 @@ class MetricRepository implements M.MetricRepository {
 
   void setSamplingRate(M.IsolateRef i, M.Metric m, M.MetricSamplingRate r) {
     if (_rates.containsKey(i)) {
-      final metrics = _rates[i]!;
+      final metrics = _rates[i];
       if (metrics.containsKey(m)) {
-        metrics[m as Metric] = _rateToInteger(r);
+        metrics[m] = _rateToInteger(r);
       }
     } else {
       throw new Exception('Sampling for isolate ${i.id} is not started');
@@ -112,7 +112,7 @@ class MetricRepository implements M.MetricRepository {
 
   M.MetricBufferSize getBufferSize(M.IsolateRef i, M.Metric m) {
     if (_sizes.containsKey(i)) {
-      final metrics = _sizes[i]!;
+      final metrics = _sizes[i];
       if (metrics.containsKey(m)) {
         switch (metrics[m]) {
           case 10:
@@ -129,9 +129,9 @@ class MetricRepository implements M.MetricRepository {
 
   void setBufferSize(M.IsolateRef i, M.Metric m, M.MetricBufferSize s) {
     if (_sizes.containsKey(i)) {
-      final metrics = _sizes[i]!;
+      final metrics = _sizes[i];
       if (metrics.containsKey(m)) {
-        metrics[m as Metric] = _sizeToInteger(s);
+        metrics[m] = _sizeToInteger(s);
       }
     } else {
       throw new Exception('Sampling for isolate ${i.id} is not started');
@@ -168,9 +168,9 @@ class MetricRepository implements M.MetricRepository {
     throw new Exception('Unknown MetricBufferSize ($s)');
   }
 
-  Iterable<M.MetricSample>? getSamples(M.IsolateRef i, M.Metric m) {
+  Iterable<M.MetricSample> getSamples(M.IsolateRef i, M.Metric m) {
     if (_samples.containsKey(i)) {
-      final metrics = _samples[i]!;
+      final metrics = _samples[i];
       if (metrics.containsKey(m)) {
         return metrics[m];
       }
@@ -192,12 +192,12 @@ class MetricRepository implements M.MetricRepository {
 
   void _update(_) {
     for (final isolate in _rates.keys) {
-      final metrics = _rates[isolate]!;
+      final metrics = _rates[isolate];
       for (final metric in metrics.keys) {
-        final rate = metrics[metric]!;
+        final rate = metrics[metric];
         if (rate != 0 && count % rate == 0) {
-          final size = _sizes[isolate]![metric]!;
-          final samples = _samples[isolate]![metric]!;
+          final size = _sizes[isolate][metric];
+          final samples = _samples[isolate][metric];
           metric.internal.reload().then((m) {
             if (samples.length >= size) {
               samples.removeRange(0, samples.length - size + 1);
diff --git a/runtime/observatory/lib/src/repositories/notification.dart b/runtime/observatory/lib/src/repositories/notification.dart
index 22a891f2f89..19eb99eda9f 100644
--- a/runtime/observatory/lib/src/repositories/notification.dart
+++ b/runtime/observatory/lib/src/repositories/notification.dart
@@ -47,7 +47,7 @@ class NotificationRepository implements M.NotificationRepository {
     if (_list.length != length) _notify();
   }
 
-  void deletePauseEvents({M.Isolate? isolate}) {
+  void deletePauseEvents({M.Isolate isolate}) {
     if (isolate == null) {
       deleteWhere((notification) {
         return notification is M.EventNotification &&
diff --git a/runtime/observatory/lib/src/repositories/persistent_handles.dart b/runtime/observatory/lib/src/repositories/persistent_handles.dart
index 771652928da..db94cbc8a7d 100644
--- a/runtime/observatory/lib/src/repositories/persistent_handles.dart
+++ b/runtime/observatory/lib/src/repositories/persistent_handles.dart
@@ -9,6 +9,6 @@ class PersistentHandlesRepository implements M.PersistentHandlesRepository {
     S.Isolate isolate = i as S.Isolate;
     assert(isolate != null);
     final response = await isolate.invokeRpc('_getPersistentHandles', {});
-    return new S.PersistentHandles(response as S.ServiceMap);
+    return new S.PersistentHandles(response);
   }
 }
diff --git a/runtime/observatory/lib/src/repositories/ports.dart b/runtime/observatory/lib/src/repositories/ports.dart
index 21f56ad50bf..324ef7430e8 100644
--- a/runtime/observatory/lib/src/repositories/ports.dart
+++ b/runtime/observatory/lib/src/repositories/ports.dart
@@ -9,6 +9,6 @@ class PortsRepository implements M.PortsRepository {
     S.Isolate isolate = i as S.Isolate;
     assert(isolate != null);
     final response = await isolate.invokeRpc('_getPorts', {});
-    return new S.Ports(response as S.ServiceMap);
+    return new S.Ports(response);
   }
 }
diff --git a/runtime/observatory/lib/src/repositories/retaining_path.dart b/runtime/observatory/lib/src/repositories/retaining_path.dart
index 8d0f1b3d8d4..cfe4c381098 100644
--- a/runtime/observatory/lib/src/repositories/retaining_path.dart
+++ b/runtime/observatory/lib/src/repositories/retaining_path.dart
@@ -10,6 +10,6 @@ class RetainingPathRepository implements M.RetainingPathRepository {
     assert(isolate != null);
     final response = await isolate
         .invokeRpc('getRetainingPath', {'targetId': id, 'limit': 100});
-    return new S.RetainingPath(response as S.ServiceMap);
+    return new S.RetainingPath(response);
   }
 }
diff --git a/runtime/observatory/lib/src/repositories/sample_profile.dart b/runtime/observatory/lib/src/repositories/sample_profile.dart
index edb45cc8a54..944eabaf456 100644
--- a/runtime/observatory/lib/src/repositories/sample_profile.dart
+++ b/runtime/observatory/lib/src/repositories/sample_profile.dart
@@ -17,7 +17,7 @@ class SampleProfileLoadingProgress extends M.SampleProfileLoadingProgress {
       _onProgress.stream;
 
   final S.ServiceObjectOwner owner;
-  final S.Class? cls;
+  final S.Class cls;
   final M.SampleProfileTag tag;
   final bool clear;
   final M.SampleProfileType type;
@@ -26,13 +26,13 @@ class SampleProfileLoadingProgress extends M.SampleProfileLoadingProgress {
   double _progress = 0.0;
   final _fetchingTime = Stopwatch();
   final _loadingTime = Stopwatch();
-  SampleProfile? _profile;
+  SampleProfile _profile;
 
   M.SampleProfileLoadingStatus get status => _status;
   double get progress => _progress;
   Duration get fetchingTime => _fetchingTime.elapsed;
   Duration get loadingTime => _loadingTime.elapsed;
-  SampleProfile get profile => _profile!;
+  SampleProfile get profile => _profile;
 
   SampleProfileLoadingProgress(this.owner, this.tag, this.clear,
       {this.type: M.SampleProfileType.cpu, this.cls}) {
@@ -49,7 +49,7 @@ class SampleProfileLoadingProgress extends M.SampleProfileLoadingProgress {
       var response;
       if (type == M.SampleProfileType.cpu) {
         response = cls != null
-            ? await cls!.getAllocationSamples()
+            ? await cls.getAllocationSamples()
             : await owner.invokeRpc('getCpuSamples', {'_code': true});
       } else if (type == M.SampleProfileType.memory) {
         assert(owner is M.VM);
@@ -65,8 +65,7 @@ class SampleProfileLoadingProgress extends M.SampleProfileLoadingProgress {
       _triggerOnProgress();
 
       SampleProfile profile = SampleProfile();
-      Stream<double> progress =
-          profile.loadProgress(owner, response as S.ServiceMap);
+      Stream<double> progress = profile.loadProgress(owner, response);
       progress.listen((value) {
         _progress = value;
         _triggerOnProgress();
@@ -99,7 +98,7 @@ class SampleProfileLoadingProgress extends M.SampleProfileLoadingProgress {
   }
 
   void reuse(M.SampleProfileTag t) {
-    _profile!.tagOrder = t;
+    _profile.tagOrder = t;
     final onProgress =
         StreamController<SampleProfileLoadingProgressEvent>.broadcast();
     Timer.run(() {
@@ -112,7 +111,7 @@ class SampleProfileLoadingProgress extends M.SampleProfileLoadingProgress {
 
 class IsolateSampleProfileRepository
     implements M.IsolateSampleProfileRepository {
-  SampleProfileLoadingProgress? _last;
+  SampleProfileLoadingProgress _last;
 
   Stream<SampleProfileLoadingProgressEvent> get(
       M.IsolateRef i, M.SampleProfileTag t,
@@ -121,12 +120,12 @@ class IsolateSampleProfileRepository
     assert(forceFetch != null);
     S.Isolate isolate = i as S.Isolate;
     assert(isolate != null);
-    if ((_last != null) && !clear && !forceFetch && (_last!.owner == isolate)) {
-      _last!.reuse(t);
+    if ((_last != null) && !clear && !forceFetch && (_last.owner == isolate)) {
+      _last.reuse(t);
     } else {
       _last = SampleProfileLoadingProgress(isolate, t, clear);
     }
-    return _last!.onProgress;
+    return _last.onProgress;
   }
 }
 
@@ -156,7 +155,7 @@ class ClassSampleProfileRepository implements M.ClassSampleProfileRepository {
 
 class NativeMemorySampleProfileRepository
     implements M.NativeMemorySampleProfileRepository {
-  SampleProfileLoadingProgress? _last;
+  SampleProfileLoadingProgress _last;
 
   Stream<SampleProfileLoadingProgressEvent> get(M.VM vm, M.SampleProfileTag t,
       {bool forceFetch: false, bool clear: false}) {
@@ -164,12 +163,12 @@ class NativeMemorySampleProfileRepository
     S.VM owner = vm as S.VM;
     assert(owner != null);
 
-    if ((_last != null) && (_last!.profile != null) && !forceFetch) {
-      _last!.reuse(t);
+    if ((_last != null) && (_last.profile != null) && !forceFetch) {
+      _last.reuse(t);
     } else {
       _last = SampleProfileLoadingProgress(owner, t, false,
           type: M.SampleProfileType.memory);
     }
-    return _last!.onProgress;
+    return _last.onProgress;
   }
 }
diff --git a/runtime/observatory/lib/src/repositories/target.dart b/runtime/observatory/lib/src/repositories/target.dart
index eec61d0c160..4fb1fb1ef72 100644
--- a/runtime/observatory/lib/src/repositories/target.dart
+++ b/runtime/observatory/lib/src/repositories/target.dart
@@ -20,7 +20,7 @@ class TargetRepository implements M.TargetRepository {
   final SettingsRepository _settings = new SettingsRepository('targetManager');
 
   final List<SC.WebSocketVMTarget> _list = <SC.WebSocketVMTarget>[];
-  SC.WebSocketVMTarget? current;
+  SC.WebSocketVMTarget current;
   final IsConnectedVMTargetDelegate _isConnectedVMTarget;
 
   factory TargetRepository(IsConnectedVMTargetDelegate isConnectedVMTarget) {
@@ -59,7 +59,7 @@ class TargetRepository implements M.TargetRepository {
       return;
     }
     current = target;
-    current!.lastConnectionTime = new DateTime.now().millisecondsSinceEpoch;
+    current.lastConnectionTime = new DateTime.now().millisecondsSinceEpoch;
     _onChange.add(new TargetChangeEvent(this));
     _store();
   }
@@ -99,7 +99,7 @@ class TargetRepository implements M.TargetRepository {
   }
 
   /// Find by networkAddress.
-  SC.WebSocketVMTarget? find(String networkAddress) {
+  SC.WebSocketVMTarget find(String networkAddress) {
     for (SC.WebSocketVMTarget item in _list) {
       if (item.networkAddress == networkAddress) {
         return item;
@@ -114,8 +114,8 @@ class TargetRepository implements M.TargetRepository {
     // http://localhost:8080?override-port=8181
     // http://localhost:8080?override-port=8181&override-host=10.0.0.2
     final Uri serverAddress = Uri.parse(window.location.toString());
-    final String? port = serverAddress.queryParameters['override-port'];
-    final String? host = serverAddress.queryParameters['override-host'];
+    final String port = serverAddress.queryParameters['override-port'];
+    final String host = serverAddress.queryParameters['override-host'];
     final Uri wsAddress = new Uri(
       scheme: 'ws',
       host: host ?? serverAddress.host,
diff --git a/runtime/observatory/lib/src/repositories/timeline.dart b/runtime/observatory/lib/src/repositories/timeline.dart
index 834e6374fd3..7c0e4a74087 100644
--- a/runtime/observatory/lib/src/repositories/timeline.dart
+++ b/runtime/observatory/lib/src/repositories/timeline.dart
@@ -11,8 +11,7 @@ class TimelineRepository extends TimelineRepositoryBase
 
   Future<M.TimelineFlags> getFlags(M.VMRef ref) async {
     S.VM vm = ref as S.VM;
-    S.ServiceMap response =
-        await vm.invokeRpc('getVMTimelineFlags', {}) as S.ServiceMap;
+    S.ServiceMap response = await vm.invokeRpc('getVMTimelineFlags', {});
     return new S.TimelineFlags(response);
   }
 
@@ -37,7 +36,7 @@ class TimelineRepository extends TimelineRepositoryBase
     final trie = profile.loadFunctionTree(M.ProfileTreeDirection.inclusive);
     final root = trie.root;
     int nextId = kRootFrameId;
-    processFrame(FunctionCallTreeNode current, FunctionCallTreeNode? parent) {
+    processFrame(FunctionCallTreeNode current, FunctionCallTreeNode parent) {
       int id = nextId;
       ++nextId;
       current.frameId = id;
diff --git a/runtime/observatory/lib/src/repositories/timeline_base.dart b/runtime/observatory/lib/src/repositories/timeline_base.dart
index 24a5daf9de5..6f12bc00610 100644
--- a/runtime/observatory/lib/src/repositories/timeline_base.dart
+++ b/runtime/observatory/lib/src/repositories/timeline_base.dart
@@ -18,12 +18,11 @@ class TimelineRepositoryBase {
       Future<S.ServiceObject> cpuSamples) async {
     const kRootFrameId = 0;
     final profile = SampleProfile();
-    await profile.load(
-        isolate as S.ServiceObjectOwner, await cpuSamples as S.ServiceMap);
+    await profile.load(isolate as S.ServiceObjectOwner, await cpuSamples);
     final trie = profile.loadFunctionTree(M.ProfileTreeDirection.inclusive);
     final root = trie.root;
     int nextId = kRootFrameId;
-    processFrame(FunctionCallTreeNode current, FunctionCallTreeNode? parent) {
+    processFrame(FunctionCallTreeNode current, FunctionCallTreeNode parent) {
       int id = nextId;
       ++nextId;
       current.frameId = id;
@@ -65,7 +64,7 @@ class TimelineRepositoryBase {
   }
 
   Future<Map> getCpuProfileTimeline(M.VMRef ref,
-      {int? timeOriginMicros, int? timeExtentMicros}) async {
+      {int timeOriginMicros, int timeExtentMicros}) async {
     final S.VM vm = ref as S.VM;
     final traceObject = <String, dynamic>{
       _kStackFrames: {},
@@ -87,10 +86,10 @@ class TimelineRepositoryBase {
   Future<Map> getTimeline(M.VMRef ref) async {
     final S.VM vm = ref as S.VM;
     final S.ServiceMap vmTimelineResponse =
-        await vm.invokeRpc('getVMTimeline', {}) as S.ServiceMap;
+        await vm.invokeRpc('getVMTimeline', {});
     final timeOriginMicros = vmTimelineResponse[kTimeOriginMicros];
     final timeExtentMicros = vmTimelineResponse[kTimeExtentMicros];
-    var traceObject = <dynamic, dynamic>{
+    var traceObject = <String, dynamic>{
       _kStackFrames: {},
       _kTraceEvents: [],
     };
diff --git a/runtime/observatory/lib/src/sample_profile/sample_profile.dart b/runtime/observatory/lib/src/sample_profile/sample_profile.dart
index 06280ce7e40..48667cca923 100644
--- a/runtime/observatory/lib/src/sample_profile/sample_profile.dart
+++ b/runtime/observatory/lib/src/sample_profile/sample_profile.dart
@@ -16,8 +16,8 @@ abstract class CallTreeNode<NodeT extends M.CallTreeNode>
   final attributes = <String>{};
 
   // Used for building timeline
-  int? frameId = null;
-  int? parentId = null;
+  int frameId = null;
+  int parentId = null;
 
   // Either a ProfileCode or a ProfileFunction.
   Object get profileData;
@@ -38,8 +38,7 @@ abstract class CallTreeNode<NodeT extends M.CallTreeNode>
   void tick(Map sample, {bool exclusive = false}) {
     ++_count;
     if (SampleProfile._isNativeAllocationSample(sample)) {
-      final allocationSize =
-          sample[SampleProfile._kNativeAllocationSizeBytes] as int;
+      final allocationSize = sample[SampleProfile._kNativeAllocationSizeBytes];
       if (exclusive) {
         exclusiveNativeAllocations += allocationSize;
       }
@@ -55,7 +54,7 @@ class CodeCallTreeNode extends CallTreeNode<CodeCallTreeNode>
 
   Object get profileData => profileCode;
 
-  String get name => profileCode.code.name!;
+  String get name => profileCode.code.name;
 
   final attributes = <String>{};
   CodeCallTreeNode(this.profileCode, int count, int inclusiveNativeAllocations,
@@ -67,7 +66,7 @@ class CodeCallTreeNode extends CallTreeNode<CodeCallTreeNode>
   }
 
   CodeCallTreeNode.fromIndex(this.profile, int tableIndex)
-      : profileCode = profile.codes[tableIndex] as ProfileCode,
+      : profileCode = profile.codes[tableIndex],
         super(<CodeCallTreeNode>[]);
 
   CodeCallTreeNode getChild(int codeTableIndex) {
@@ -117,15 +116,14 @@ class CodeCallTree extends CallTree<CodeCallTreeNode>
     treeFilter.build();
     if ((treeFilter.filtered.root.inclusiveNativeAllocations != null) &&
         (treeFilter.filtered.root.inclusiveNativeAllocations != 0)) {
-      _setCodeMemoryPercentage(
-          null, treeFilter.filtered.root as CodeCallTreeNode);
+      _setCodeMemoryPercentage(null, treeFilter.filtered.root);
     } else {
-      _setCodePercentage(null, treeFilter.filtered.root as CodeCallTreeNode);
+      _setCodePercentage(null, treeFilter.filtered.root);
     }
-    return treeFilter.filtered as CodeCallTree;
+    return treeFilter.filtered;
   }
 
-  _setCodePercentage(CodeCallTreeNode? parent, CodeCallTreeNode node) {
+  _setCodePercentage(CodeCallTreeNode parent, CodeCallTreeNode node) {
     assert(node != null);
     var parentPercentage = 1.0;
     var parentCount = node.count;
@@ -143,7 +141,7 @@ class CodeCallTree extends CallTree<CodeCallTreeNode>
     }
   }
 
-  _setCodeMemoryPercentage(CodeCallTreeNode? parent, CodeCallTreeNode node) {
+  _setCodeMemoryPercentage(CodeCallTreeNode parent, CodeCallTreeNode node) {
     assert(node != null);
     var parentPercentage = 1.0;
     var parentMemory = node.inclusiveNativeAllocations;
@@ -166,7 +164,7 @@ class CodeCallTree extends CallTree<CodeCallTreeNode>
   }
 
   _recordCallerAndCalleesInner(
-      CodeCallTreeNode? caller, CodeCallTreeNode callee) {
+      CodeCallTreeNode caller, CodeCallTreeNode callee) {
     if (caller != null) {
       caller.profileCode._recordCallee(callee.profileCode, callee.count);
       callee.profileCode._recordCaller(caller.profileCode, caller.count);
@@ -210,7 +208,7 @@ class FunctionCallTreeNode extends CallTreeNode<FunctionCallTreeNode>
   }
 
   FunctionCallTreeNode.fromIndex(this.profile, int tableIndex)
-      : profileFunction = profile.functions[tableIndex] as ProfileFunction,
+      : profileFunction = profile.functions[tableIndex],
         super(<FunctionCallTreeNode>[]);
 
   FunctionCallTreeNode getChild(int functionTableIndex) {
@@ -265,7 +263,7 @@ abstract class _FilteredCallTreeBuilder<NodeT extends CallTreeNode> {
     _descend(_unfilteredTree.root);
   }
 
-  CallTreeNode? _findInChildren(CallTreeNode current, CallTreeNode needle) {
+  CallTreeNode _findInChildren(CallTreeNode current, CallTreeNode needle) {
     for (var child in current.children) {
       if ((child as CallTreeNode).profileData == needle.profileData) {
         return child;
@@ -278,7 +276,7 @@ abstract class _FilteredCallTreeBuilder<NodeT extends CallTreeNode> {
 
   /// Add all nodes in [_currentPath].
   FunctionCallTreeNode _addCurrentPath() {
-    FunctionCallTreeNode current = filtered.root as FunctionCallTreeNode;
+    FunctionCallTreeNode current = filtered.root;
     // Tree root is always the first element of the current path.
     assert(_unfilteredTree.root == _currentPath[0]);
     // Assert that unfiltered tree's root and filtered tree's root are different.
@@ -291,38 +289,38 @@ abstract class _FilteredCallTreeBuilder<NodeT extends CallTreeNode> {
       if (child == null) {
         // New node.
         child = _copyNode(toAdd);
-        current.children.add(child as FunctionCallTreeNode);
+        current.children.add(child);
       }
-      current = child as FunctionCallTreeNode;
+      current = child;
     }
     return current;
   }
 
   /// Starting at [current] append [next] and all of [next]'s sub-trees
-  _appendTree(CallTreeNode current, CallTreeNode? next) {
+  _appendTree(CallTreeNode current, CallTreeNode next) {
     if (next == null) {
       return;
     }
     var child = _findInChildren(current, next);
     if (child == null) {
-      child = _copyNode(next as NodeT);
+      child = _copyNode(next);
       current.children.add(child);
     }
     current = child;
     for (var nextChild in next.children) {
-      _appendTree(current, nextChild as CallTreeNode);
+      _appendTree(current, nextChild);
     }
   }
 
   /// Add path from root to [child], [child], and all of [child]'s sub-trees
   /// to filtered tree.
-  _addTree(CallTreeNode? child) {
+  _addTree(CallTreeNode child) {
     var current = _addCurrentPath();
     _appendTree(current, child);
   }
 
   /// Descend further into the tree. [current] is from the unfiltered tree.
-  _descend(CallTreeNode? current) {
+  _descend(CallTreeNode current) {
     if (current == null) {
       return;
     }
@@ -336,13 +334,13 @@ abstract class _FilteredCallTreeBuilder<NodeT extends CallTreeNode> {
       } else {
         // Add all child trees.
         for (var child in current.children) {
-          _addTree(child as CallTreeNode);
+          _addTree(child);
         }
       }
     } else {
       // Did not match, descend to each child.
       for (var child in current.children) {
-        _descend(child as CallTreeNode);
+        _descend(child);
       }
     }
 
@@ -361,13 +359,13 @@ class _FilteredFunctionCallTreeBuilder
             FunctionCallTree(
                 tree.inclusive,
                 FunctionCallTreeNode(
-                    tree.root.profileData as ProfileFunction,
+                    tree.root.profileData,
                     tree.root.count,
                     tree.root.inclusiveNativeAllocations,
                     tree.root.exclusiveNativeAllocations)));
 
   _copyNode(FunctionCallTreeNode node) {
-    return FunctionCallTreeNode(node.profileData as ProfileFunction, node.count,
+    return FunctionCallTreeNode(node.profileData, node.count,
         node.inclusiveNativeAllocations, node.exclusiveNativeAllocations);
   }
 }
@@ -381,13 +379,13 @@ class _FilteredCodeCallTreeBuilder
             CodeCallTree(
                 tree.inclusive,
                 CodeCallTreeNode(
-                    tree.root.profileData as ProfileCode,
+                    tree.root.profileData,
                     tree.root.count,
                     tree.root.inclusiveNativeAllocations,
                     tree.root.exclusiveNativeAllocations)));
 
   _copyNode(CodeCallTreeNode node) {
-    return CodeCallTreeNode(node.profileData as ProfileCode, node.count,
+    return CodeCallTreeNode(node.profileData, node.count,
         node.inclusiveNativeAllocations, node.exclusiveNativeAllocations);
   }
 }
@@ -409,17 +407,15 @@ class FunctionCallTree extends CallTree<FunctionCallTreeNode>
     treeFilter.build();
     if ((treeFilter.filtered.root.inclusiveNativeAllocations != null) &&
         (treeFilter.filtered.root.inclusiveNativeAllocations != 0)) {
-      _setFunctionMemoryPercentage(
-          null, treeFilter.filtered.root as FunctionCallTreeNode);
+      _setFunctionMemoryPercentage(null, treeFilter.filtered.root);
     } else {
-      _setFunctionPercentage(
-          null, treeFilter.filtered.root as FunctionCallTreeNode);
+      _setFunctionPercentage(null, treeFilter.filtered.root);
     }
-    return treeFilter.filtered as FunctionCallTree;
+    return treeFilter.filtered;
   }
 
   void _setFunctionPercentage(
-      FunctionCallTreeNode? parent, FunctionCallTreeNode node) {
+      FunctionCallTreeNode parent, FunctionCallTreeNode node) {
     assert(node != null);
     var parentPercentage = 1.0;
     var parentCount = node.count;
@@ -438,7 +434,7 @@ class FunctionCallTree extends CallTree<FunctionCallTreeNode>
   }
 
   void _setFunctionMemoryPercentage(
-      FunctionCallTreeNode? parent, FunctionCallTreeNode node) {
+      FunctionCallTreeNode parent, FunctionCallTreeNode node) {
     assert(node != null);
     var parentPercentage = 1.0;
     var parentMemory = node.inclusiveNativeAllocations;
@@ -461,7 +457,7 @@ class FunctionCallTree extends CallTree<FunctionCallTreeNode>
   }
 
   _markFunctionCallsInner(
-      FunctionCallTreeNode? caller, FunctionCallTreeNode callee) {
+      FunctionCallTreeNode caller, FunctionCallTreeNode callee) {
     if (caller != null) {
       caller.profileFunction
           ._recordCallee(callee.profileFunction, callee.count);
@@ -499,8 +495,8 @@ class ProfileCode implements M.ProfileCode {
   final int tableIndex;
   final SampleProfile profile;
   final Code code;
-  int exclusiveTicks = 0;
-  int inclusiveTicks = 0;
+  int exclusiveTicks;
+  int inclusiveTicks;
   double normalizedExclusiveTicks = 0.0;
   double normalizedInclusiveTicks = 0.0;
   final addressTicks = <int, CodeTick>{};
@@ -557,15 +553,15 @@ class ProfileCode implements M.ProfileCode {
     if (code.kind == M.CodeKind.stub) {
       attributes.add('stub');
     } else if (code.kind == M.CodeKind.dart) {
-      if (code.isNative!) {
+      if (code.isNative) {
         attributes.add('ffi'); // Not to be confused with a C function.
       } else {
         attributes.add('dart');
       }
-      if (code.hasIntrinsic!) {
+      if (code.hasIntrinsic) {
         attributes.add('intrinsic');
       }
-      if (code.isOptimized!) {
+      if (code.isOptimized) {
         attributes.add('optimized');
       } else {
         attributes.add('unoptimized');
@@ -684,7 +680,7 @@ class ProfileFunction implements M.ProfileFunction {
       attribs.add('native');
     } else if (M.isSyntheticFunction(function.kind)) {
       attribs.add('synthetic');
-    } else if (function.isNative!) {
+    } else if (function.isNative) {
       attribs.add('ffi'); // Not to be confused with a C function.
     } else {
       attribs.add('dart');
@@ -750,7 +746,7 @@ class ProfileFunction implements M.ProfileFunction {
 }
 
 class SampleProfile extends M.SampleProfile {
-  Isolate? isolate;
+  Isolate isolate;
 
   int sampleCount = 0;
   int samplePeriod = 0;
@@ -856,7 +852,7 @@ class SampleProfile extends M.SampleProfile {
       Future.delayed(duration);
 
   Future _loadCommon(ServiceObjectOwner owner, ServiceMap profile,
-      [StreamController<double>? progress]) async {
+      [StreamController<double> progress]) async {
     final watch = Stopwatch();
     watch.start();
     int count = 0;
@@ -887,7 +883,7 @@ class SampleProfile extends M.SampleProfile {
 
       if ((owner != null) && (owner is Isolate)) {
         isolate = owner;
-        isolate!.resetCachedProfileData();
+        isolate.resetCachedProfileData();
       }
 
       pid = profile[_kPid];
@@ -961,32 +957,32 @@ class SampleProfile extends M.SampleProfile {
 
   int _getProfileFunctionTagIndex(String tag) {
     if (_functionTagMapping.containsKey(tag)) {
-      return _functionTagMapping[tag]!;
+      return _functionTagMapping[tag];
     }
     throw ArgumentError('$tag is not a valid tag!');
   }
 
   int _getProfileCodeTagIndex(String tag) {
     if (_codeTagMapping.containsKey(tag)) {
-      return _codeTagMapping[tag]!;
+      return _codeTagMapping[tag];
     }
     throw ArgumentError('$tag is not a valid tag!');
   }
 
   void _buildFunctionTagMapping() {
     for (int i = 0; i < functions.length; ++i) {
-      final function = functions[i].function!;
+      final function = functions[i].function;
       if (function.kind == M.FunctionKind.tag) {
-        _functionTagMapping[function.name!] = i;
+        _functionTagMapping[function.name] = i;
       }
     }
   }
 
   void _buildCodeTagMapping() {
     for (int i = 0; i < codes.length; ++i) {
-      final code = codes[i].code!;
+      final code = codes[i].code;
       if (code.kind == M.CodeKind.tag) {
-        _codeTagMapping[code.name!] = i;
+        _codeTagMapping[code.name] = i;
       }
     }
   }
@@ -1014,7 +1010,7 @@ class SampleProfile extends M.SampleProfile {
       final tableIndex = isCode
           ? _getProfileCodeTagIndex(userTag)
           : _getProfileFunctionTagIndex(userTag);
-      current = current.getChild(tableIndex) as NodeT;
+      current = current.getChild(tableIndex);
       current.tick(sample);
     } catch (_) {/* invalid tag */} finally {
       return current;
@@ -1028,7 +1024,7 @@ class SampleProfile extends M.SampleProfile {
       final tableIndex = isCode
           ? _getProfileCodeTagIndex(kTruncatedTag)
           : _getProfileFunctionTagIndex(kTruncatedTag);
-      current = current.getChild(tableIndex) as NodeT;
+      current = current.getChild(tableIndex);
       current.tick(sample);
       // We don't need to tick the tag itself since this is done in the VM for
       // the truncated tag, unlike other VM and user tags.
@@ -1144,7 +1140,7 @@ class SampleProfile extends M.SampleProfile {
       Map sample, List<int> stack, int frameIndex, bool inclusive) {
     final child = parent.getChild(stack[frameIndex]);
     child.tick(sample, exclusive: (frameIndex == 0));
-    return child as NodeT;
+    return child;
   }
 
   FunctionCallTreeNode buildFunctionTrie(bool inclusive) {
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index 0e3b076e605..5badd86fa64 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -54,8 +54,8 @@ class ServerRpcException extends RpcException implements M.RequestException {
   static const kFileSystemDoesNotExist = 1002;
   static const kFileDoesNotExist = 1003;
 
-  int? code;
-  Map? data;
+  int code;
+  Map data;
 
   static _getMessage(Map errorMap) {
     Map data = errorMap['data'];
@@ -83,12 +83,12 @@ class NetworkRpcException extends RpcException
   String toString() => 'NetworkRpcException(${message})';
 }
 
-Future<ServiceObject?> ignoreNetworkErrors(Object error, StackTrace st,
-    [ServiceObject? resultOnNetworkError]) {
+Future<ServiceObject> ignoreNetworkErrors(Object error,
+    [ServiceObject resultOnNetworkError = null]) {
   if (error is NetworkRpcException) {
     return new Future.value(resultOnNetworkError);
   }
-  return new Future.error(error, st);
+  return new Future.error(error);
 }
 
 class MalformedResponseRpcException extends RpcException {
@@ -102,7 +102,7 @@ class MalformedResponseRpcException extends RpcException {
 /// A [ServiceObject] represents a persistent object within the vm.
 abstract class ServiceObject implements M.ObjectRef {
   static int LexicalSortName(ServiceObject o1, ServiceObject o2) {
-    return o1.name!.compareTo(o2.name!);
+    return o1.name.compareTo(o2.name);
   }
 
   List<T> removeDuplicatesAndSortLexical<T extends ServiceObject>(
@@ -112,26 +112,26 @@ abstract class ServiceObject implements M.ObjectRef {
 
   /// The owner of this [ServiceObject].  This can be an [Isolate], a
   /// [VM], or null.
-  ServiceObjectOwner? get owner => _owner;
-  ServiceObjectOwner? _owner;
+  ServiceObjectOwner get owner => _owner;
+  ServiceObjectOwner _owner;
 
   /// The [VM] which owns this [ServiceObject].
-  VM get vm => _owner!.vm;
+  VM get vm => _owner.vm;
 
   /// The [Isolate] which owns this [ServiceObject].  May be null.
-  Isolate? get isolate => _owner!.isolate;
+  Isolate get isolate => _owner.isolate;
 
   /// The id of this object.
-  String? get id => _id;
-  String? _id;
+  String get id => _id;
+  String _id;
 
   /// The user-level type of this object.
-  String? get type => _type;
-  String? _type;
+  String get type => _type;
+  String _type;
 
   /// The vm type of this object.
-  String? get vmType => _vmType;
-  String? _vmType;
+  String get vmType => _vmType;
+  String _vmType;
 
   bool get isICData => vmType == 'ICData';
   bool get isMegamorphicCache => vmType == 'MegamorphicCache';
@@ -169,20 +169,23 @@ abstract class ServiceObject implements M.ObjectRef {
 
   /// Is this object cacheable?  That is, is it impossible for the [id]
   /// of this object to change?
-  late bool _canCache;
+  bool _canCache;
   bool get canCache => _canCache;
 
   /// Is this object immutable after it is [loaded]?
   bool get immutable => false;
 
-  String? name;
-  String? vmName;
+  String name;
+  String vmName;
 
   /// Creates an empty [ServiceObject].
-  ServiceObject._empty(ServiceObjectOwner? this._owner);
+  ServiceObject._empty(this._owner);
 
   /// Creates a [ServiceObject] initialized from [map].
-  static ServiceObject _fromMap(ServiceObjectOwner? owner, Map map) {
+  factory ServiceObject._fromMap(ServiceObjectOwner owner, Map map) {
+    if (map == null) {
+      return null;
+    }
     if (!_isServiceMap(map)) {
       Logger.root.severe('Malformed service object: $map');
     }
@@ -224,10 +227,10 @@ abstract class ServiceObject implements M.ObjectRef {
         obj = new ServiceMetric._empty(owner);
         break;
       case 'Isolate':
-        obj = new Isolate._empty(owner!.vm);
+        obj = new Isolate._empty(owner.vm);
         break;
       case 'IsolateGroup':
-        obj = new IsolateGroup._empty(owner!.vm);
+        obj = new IsolateGroup._empty(owner.vm);
         break;
       case 'Library':
         obj = new Library._empty(owner);
@@ -313,14 +316,14 @@ abstract class ServiceObject implements M.ObjectRef {
     return reload();
   }
 
-  Future<ServiceObject>? _inProgressReload;
+  Future<ServiceObject> _inProgressReload;
 
   Future<Map> _fetchDirect({int count: kDefaultFieldLimit}) {
     Map params = {
       'objectId': id,
       'count': count,
     };
-    return isolate!.invokeRpcNoUpgrade('getObject', params);
+    return isolate.invokeRpcNoUpgrade('getObject', params);
   }
 
   /// Reload [this]. Returns a future which completes to [this] or
@@ -344,7 +347,7 @@ abstract class ServiceObject implements M.ObjectRef {
         var mapType = _stripRef(map['type']);
         if (mapType == 'Sentinel') {
           // An object may have been collected, etc.
-          completer.complete(ServiceObject._fromMap(owner, map));
+          completer.complete(new ServiceObject._fromMap(owner, map));
         } else {
           // TODO(turnidge): Check for vmType changing as well?
           assert(mapType == _type);
@@ -360,7 +363,7 @@ abstract class ServiceObject implements M.ObjectRef {
         _inProgressReload = null;
       });
     }
-    return _inProgressReload!;
+    return _inProgressReload;
   }
 
   /// Update [this] using [map] as a source. [map] can be a reference.
@@ -389,6 +392,7 @@ abstract class ServiceObject implements M.ObjectRef {
     } else {
       _vmType = _type;
     }
+
     _update(map, mapIsRef);
   }
 
@@ -402,11 +406,11 @@ abstract class ServiceObject implements M.ObjectRef {
 }
 
 abstract class HeapObject extends ServiceObject implements M.Object {
-  Class? clazz;
-  int? size;
-  int? retainedSize;
+  Class clazz;
+  int size;
+  int retainedSize;
 
-  HeapObject._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  HeapObject._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     if (map['class'] != null) {
@@ -417,12 +421,10 @@ abstract class HeapObject extends ServiceObject implements M.Object {
 
     // Load the full class object if the isolate is runnable.
     if (clazz != null) {
-      if (clazz!.isolate!.runnable) {
+      if (clazz.isolate.runnable) {
         // No one awaits on this request so we silence any network errors
         // that occur here but forward other errors.
-        clazz!
-            .load()
-            .catchError((error, st) => ignoreNetworkErrors(error, st, clazz));
+        clazz.load().catchError((error) => ignoreNetworkErrors(error, clazz));
       }
     }
 
@@ -434,14 +436,14 @@ abstract class HeapObject extends ServiceObject implements M.Object {
 }
 
 class RetainingObject implements M.RetainingObject {
-  int get retainedSize => object.retainedSize!;
+  int get retainedSize => object.retainedSize;
   final HeapObject object;
   RetainingObject(this.object);
 }
 
 abstract class ServiceObjectOwner extends ServiceObject {
   /// Creates an empty [ServiceObjectOwner].
-  ServiceObjectOwner._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ServiceObjectOwner._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   /// Builds a [ServiceObject] corresponding to the [id] from [map].
   /// The result may come from the cache.  The result will not necessarily
@@ -453,30 +455,30 @@ abstract class ServiceObjectOwner extends ServiceObject {
 
 abstract class Location implements M.Location {
   Script get script;
-  int? get tokenPos;
-  Future<int?> getLine();
-  Future<int?> getColumn();
+  int get tokenPos;
+  Future<int> getLine();
+  Future<int> getColumn();
   Future<String> toUserString();
 }
 
 /// A [SourceLocation] represents a location or range in the source code.
 class SourceLocation extends ServiceObject
     implements Location, M.SourceLocation {
-  late Script script;
-  late int tokenPos;
-  int? endTokenPos;
+  Script script;
+  int tokenPos;
+  int endTokenPos;
 
-  Future<int?> getLine() async {
+  Future<int> getLine() async {
     await script.load();
     return script.tokenToLine(tokenPos);
   }
 
-  Future<int?> getColumn() async {
+  Future<int> getColumn() async {
     await script.load();
     return script.tokenToCol(tokenPos);
   }
 
-  SourceLocation._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  SourceLocation._empty(ServiceObject owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     assert(!mapIsRef);
@@ -489,8 +491,8 @@ class SourceLocation extends ServiceObject
   }
 
   Future<String> toUserString() async {
-    int? line = await getLine();
-    int? column = await getColumn();
+    int line = await getLine();
+    int column = await getColumn();
     return '${script.name}:${line}:${column}';
   }
 
@@ -507,13 +509,13 @@ class SourceLocation extends ServiceObject
 // code which has not been precisely mapped to a token position.
 class UnresolvedSourceLocation extends ServiceObject
     implements Location, M.UnresolvedSourceLocation {
-  late Script script;
-  String? scriptUri;
-  int? line;
-  int? column;
-  int? tokenPos;
+  Script script;
+  String scriptUri;
+  int line;
+  int column;
+  int tokenPos;
 
-  Future<int?> getLine() async {
+  Future<int> getLine() async {
     if (tokenPos != null) {
       await script.load();
       return script.tokenToLine(tokenPos);
@@ -522,7 +524,7 @@ class UnresolvedSourceLocation extends ServiceObject
     }
   }
 
-  Future<int?> getColumn() async {
+  Future<int> getColumn() async {
     if (tokenPos != null) {
       await script.load();
       return script.tokenToCol(tokenPos);
@@ -531,8 +533,7 @@ class UnresolvedSourceLocation extends ServiceObject
     }
   }
 
-  UnresolvedSourceLocation._empty(ServiceObjectOwner? owner)
-      : super._empty(owner);
+  UnresolvedSourceLocation._empty(ServiceObject owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     assert(!mapIsRef);
@@ -550,8 +551,8 @@ class UnresolvedSourceLocation extends ServiceObject
   Future<String> toUserString() async {
     StringBuffer sb = new StringBuffer();
 
-    int? line = await getLine();
-    int? column = await getColumn();
+    int line = await getLine();
+    int column = await getColumn();
 
     if (script != null) {
       sb.write('${script.name}:');
@@ -595,10 +596,10 @@ class _EventStreamState {
   List _controllers = [];
 
   // Completes when the listen rpc is finished.
-  Future? _listenFuture;
+  Future _listenFuture;
 
   // Completes when then cancel rpc is finished.
-  Future? _cancelFuture;
+  Future _cancelFuture;
 
   _EventStreamState(this._vm, this.streamId, this._onDone);
 
@@ -608,7 +609,7 @@ class _EventStreamState {
       assert(_listenFuture != null);
       _listenFuture = null;
       _cancelFuture = _vm._streamCancel(streamId);
-      _cancelFuture!.then((_) {
+      _cancelFuture.then((_) {
         if (_controllers.isEmpty) {
           // No new listeners showed up during cancelation.
           _onDone();
@@ -622,7 +623,7 @@ class _EventStreamState {
   }
 
   Future<Stream<ServiceEvent>> addStream() async {
-    late StreamController<ServiceEvent> controller;
+    var controller;
     controller = new StreamController<ServiceEvent>(
         onCancel: () => _cancelController(controller));
     _controllers.add(controller);
@@ -650,7 +651,7 @@ class _EventStreamState {
 /// State for a VM being inspected.
 abstract class VM extends ServiceObjectOwner implements M.VM {
   VM get vm => this;
-  Isolate? get isolate => null;
+  Isolate get isolate => null;
   WebSocketVMTarget get target;
 
   // TODO(turnidge): The connection should not be stored in the VM object.
@@ -673,33 +674,33 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
   final List<Service> services = <Service>[];
 
   String version = 'unknown';
-  String hostCPU = 'unknown';
-  String targetCPU = 'unknown';
-  String embedder = 'unknown';
-  int architectureBits = 0;
+  String hostCPU;
+  String targetCPU;
+  String embedder;
+  int architectureBits;
   bool assertsEnabled = false;
   bool typeChecksEnabled = false;
   int nativeZoneMemoryUsage = 0;
   int pid = 0;
   int heapAllocatedMemoryUsage = 0;
   int heapAllocationCount = 0;
-  int maxRSS = 0;
-  int currentRSS = 0;
+  int maxRSS;
+  int currentRSS;
   bool profileVM = false;
-  DateTime? startTime;
-  DateTime? refreshTime;
-  Duration? get upTime {
+  DateTime startTime;
+  DateTime refreshTime;
+  Duration get upTime {
     if (startTime == null) {
       return null;
     }
-    return (new DateTime.now().difference(startTime!));
+    return (new DateTime.now().difference(startTime));
   }
 
   VM() : super._empty(null) {
     updateFromServiceMap({'name': 'vm', 'type': '@VM'});
   }
 
-  void postServiceEvent(String streamId, Map response, Uint8List? data) {
+  void postServiceEvent(String streamId, Map response, Uint8List data) {
     var map = response;
     assert(!map.containsKey('_data'));
     if (data != null) {
@@ -711,13 +712,13 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
     }
 
     var eventIsolate = map['isolate'];
-    ServiceEvent event;
+    var event;
     if (eventIsolate == null) {
-      event = ServiceObject._fromMap(vm, map) as ServiceEvent;
+      event = new ServiceObject._fromMap(vm, map);
     } else {
       // getFromMap creates the Isolate if it hasn't been seen already.
-      var isolate = getFromMap(map['isolate']) as Isolate;
-      event = ServiceObject._fromMap(isolate, map) as ServiceEvent;
+      var isolate = getFromMap(map['isolate']);
+      event = new ServiceObject._fromMap(isolate, map);
       if (event.kind == ServiceEvent.kIsolateExit) {
         _isolateCache.remove(isolate.id);
         _buildIsolateList();
@@ -779,6 +780,9 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
   static final String _isolateGroupIdPrefix = 'isolateGroups/';
 
   ServiceObject getFromMap(Map map) {
+    if (map == null) {
+      return null;
+    }
     var type = _stripRef(map['type']);
     if (type == 'VM') {
       // Update this VM object.
@@ -793,7 +797,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
         var isolate = _isolateCache[id];
         if (isolate == null) {
           // Add new isolate to the cache.
-          isolate = ServiceObject._fromMap(this, map) as Isolate;
+          isolate = ServiceObject._fromMap(this, map);
           _isolateCache[id] = isolate;
           _buildIsolateList();
 
@@ -811,7 +815,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
         var isolateGroup = _isolateGroupCache[id];
         if (isolateGroup == null) {
           // Add new isolate to the cache.
-          isolateGroup = ServiceObject._fromMap(this, map) as IsolateGroup;
+          isolateGroup = new ServiceObject._fromMap(this, map);
           _isolateGroupCache[id] = isolateGroup;
           _buildIsolateGroupList();
 
@@ -828,7 +832,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
     }
 
     // Build the object from the map directly.
-    return ServiceObject._fromMap(this, map);
+    return new ServiceObject._fromMap(this, map);
   }
 
   // Note that this function does not reload the isolate if it found
@@ -842,7 +846,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
   }
 
   int _compareIsolateGroups(IsolateGroup a, IsolateGroup b) {
-    return a.id!.compareTo(b.id!);
+    return a.id.compareTo(b.id);
   }
 
   void _buildIsolateGroupList() {
@@ -868,9 +872,9 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
   Future<Map> invokeRpcNoUpgrade(String method, Map params) {
     return invokeRpcRaw(method, params).then<Map>((Map response) {
       var map = response;
-      var tracer = Tracer.current;
-      if (tracer != null) {
-        tracer.trace("Received response for ${method}/${params}}", map: map);
+      if (Tracer.current != null) {
+        Tracer.current
+            .trace("Received response for ${method}/${params}}", map: map);
       }
       if (!_isServiceMap(map)) {
         var exception = new MalformedResponseRpcException(
@@ -878,23 +882,23 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
         return new Future.error(exception);
       }
       return new Future<Map>.value(map);
-    }).catchError((e, st) {
+    }).catchError((e) {
       // Errors pass through.
-      return new Future<Map>.error(e, st);
+      return new Future<Map>.error(e);
     });
   }
 
   Future<ServiceObject> invokeRpc(String method, Map params) {
     return invokeRpcNoUpgrade(method, params)
         .then<ServiceObject>((Map response) {
-      var obj = ServiceObject._fromMap(this, response);
+      var obj = new ServiceObject._fromMap(this, response);
       if ((obj != null) && obj.canCache) {
-        String objId = obj.id!;
+        String objId = obj.id;
         _cache.putIfAbsent(objId, () => obj);
       }
       return obj;
-    }).catchError((e, st) {
-      return new Future<ServiceObject>.error(e, st);
+    }).catchError((e) {
+      return new Future<ServiceObject>.error(e);
     });
   }
 
@@ -908,7 +912,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
   void _updateService(ServiceEvent event) {
     switch (event.kind) {
       case ServiceEvent.kServiceRegistered:
-        services.add(new Service(event.alias!, event.method!, event.service!));
+        services.add(new Service(event.alias, event.method, event.service));
         break;
       case ServiceEvent.kServiceUnregistered:
         services.removeWhere((s) => s.method == event.method);
@@ -951,7 +955,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
     };
     // Ignore network errors on stream listen.
     return invokeRpc('streamListen', params)
-        .catchError((e, st) => ignoreNetworkErrors(e, st));
+        .catchError((e) => ignoreNetworkErrors(e));
   }
 
   Future<ServiceObject> _streamCancel(String streamId) {
@@ -960,7 +964,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
     };
     // Ignore network errors on stream cancel.
     return invokeRpc('streamCancel', params)
-        .catchError((e, st) => ignoreNetworkErrors(e, st));
+        .catchError((e) => ignoreNetworkErrors(e));
   }
 
   // A map from stream id to event stream state.
@@ -989,7 +993,7 @@ abstract class VM extends ServiceObjectOwner implements M.VM {
 
   /// Helper function for listening to an event stream.
   Future<StreamSubscription> listenEventStream(
-      String streamId, void Function(ServiceEvent) function) async {
+      String streamId, Function function) async {
     var stream = await getEventStream(streamId);
     return stream.listen(function);
   }
@@ -1095,9 +1099,9 @@ class TagProfileSnapshot {
 class TagProfile {
   final List<String> names = <String>[];
   final List<TagProfileSnapshot> snapshots = <TagProfileSnapshot>[];
-  double get updatedAtSeconds => _seconds!;
-  double? _seconds;
-  TagProfileSnapshot? _maxSnapshot;
+  double get updatedAtSeconds => _seconds;
+  double _seconds;
+  TagProfileSnapshot _maxSnapshot;
   int _historySize;
   int _countersLength = 0;
 
@@ -1117,14 +1121,14 @@ class TagProfile {
       }
       // The counters monotonically grow, keep track of the maximum value.
       _maxSnapshot = new TagProfileSnapshot(0.0, _countersLength);
-      _maxSnapshot!.set(counters);
+      _maxSnapshot.set(counters);
       return;
     }
     var snapshot = new TagProfileSnapshot(seconds, _countersLength);
     // We snapshot the delta from the current counters to the maximum counter
     // values.
-    snapshot.delta(counters, _maxSnapshot!.counters);
-    _maxSnapshot!.max(counters);
+    snapshot.delta(counters, _maxSnapshot.counters);
+    _maxSnapshot.max(counters);
     snapshots.add(snapshot);
     // Only keep _historySize snapshots.
     if (snapshots.length > _historySize) {
@@ -1281,7 +1285,7 @@ class HeapSpace implements M.HeapSpace {
 }
 
 class IsolateGroup extends ServiceObjectOwner implements M.IsolateGroup {
-  IsolateGroup._empty(ServiceObjectOwner? owner)
+  IsolateGroup._empty(ServiceObjectOwner owner)
       : assert(owner is VM),
         super._empty(owner);
 
@@ -1305,6 +1309,9 @@ class IsolateGroup extends ServiceObjectOwner implements M.IsolateGroup {
 
   @override
   ServiceObject getFromMap(Map map) {
+    if (map == null) {
+      return null;
+    }
     final mapType = _stripRef(map['type']);
     if (mapType == 'IsolateGroup') {
       // There are sometimes isolate group refs in ServiceEvents.
@@ -1317,7 +1324,7 @@ class IsolateGroup extends ServiceObjectOwner implements M.IsolateGroup {
       return obj;
     }
     // Build the object from the map directly.
-    obj = ServiceObject._fromMap(this, map);
+    obj = new ServiceObject._fromMap(this, map);
     if ((obj != null) && obj.canCache) {
       _cache[mapId] = obj;
     }
@@ -1346,7 +1353,7 @@ class IsolateGroup extends ServiceObjectOwner implements M.IsolateGroup {
   final List<Isolate> isolates = <Isolate>[];
 
   @override
-  int? number;
+  int number;
 
   final Map<String, ServiceObject> _cache = Map<String, ServiceObject>();
 }
@@ -1356,28 +1363,28 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   static const kLoggingStream = 'Logging';
   static const kExtensionStream = 'Extension';
 
-  VM get vm => owner as VM;
+  VM get vm => owner;
   Isolate get isolate => this;
-  int? number;
-  int? originNumber;
-  DateTime? startTime;
-  Duration? get upTime {
+  int number;
+  int originNumber;
+  DateTime startTime;
+  Duration get upTime {
     if (startTime == null) {
       return null;
     }
-    return (new DateTime.now().difference(startTime!));
+    return (new DateTime.now().difference(startTime));
   }
 
   Map counters = {};
 
   void _updateRunState() {
-    topFrame = M.topFrame(pauseEvent) as Frame?;
+    topFrame = M.topFrame(pauseEvent);
     paused = (pauseEvent != null && !(pauseEvent is M.ResumeEvent));
     running = (!paused && topFrame != null);
     idle = (!paused && topFrame == null);
   }
 
-  M.DebugEvent? pauseEvent = null;
+  M.DebugEvent pauseEvent = null;
   bool paused = false;
   bool running = false;
   bool idle = false;
@@ -1402,7 +1409,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   Map<String, ServiceObject> _cache = new Map<String, ServiceObject>();
   final TagProfile tagProfile = new TagProfile(20);
 
-  Isolate._empty(ServiceObjectOwner? owner) : super._empty(owner) {
+  Isolate._empty(ServiceObjectOwner owner) : super._empty(owner) {
     assert(owner is VM);
   }
 
@@ -1423,7 +1430,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   static const kProfileReport = '_Profile';
 
   Future<ServiceObject> getSourceReport(List<String> report_kinds,
-      [Script? script, int? startPos, int? endPos]) {
+      [Script script, int startPos, int endPos]) {
     var params = <String, dynamic>{'reports': report_kinds};
     if (script != null) {
       params['scriptId'] = script.id;
@@ -1438,7 +1445,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   }
 
   Future<ServiceMap> reloadSources(
-      {String? rootLibUri, String? packagesUri, bool? pause}) {
+      {String rootLibUri, String packagesUri, bool pause}) {
     Map<String, dynamic> params = <String, dynamic>{};
     if (rootLibUri != null) {
       params['rootLibUri'] = rootLibUri;
@@ -1469,7 +1476,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   /// Object class object.
   Future<Class> getClassHierarchy() async {
     var classRefs = await invokeRpc('getClassList', {});
-    var classes = await _loadClasses(classRefs as ServiceMap);
+    var classes = await _loadClasses(classRefs);
     return _buildClassHierarchy(classes);
   }
 
@@ -1504,7 +1511,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
       }
       if ((cls.vmName == 'Object') &&
           (cls.isPatch == false) &&
-          (cls.library!.uri == 'dart:core')) {
+          (cls.library.uri == 'dart:core')) {
         objectClass = cls;
       }
     }
@@ -1513,6 +1520,9 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   }
 
   ServiceObject getFromMap(Map map) {
+    if (map == null) {
+      return null;
+    }
     var mapType = _stripRef(map['type']);
     if (mapType == 'Isolate') {
       // There are sometimes isolate refs in ServiceEvents.
@@ -1525,7 +1535,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
       return obj;
     }
     // Build the object from the map directly.
-    obj = ServiceObject._fromMap(this, map);
+    obj = new ServiceObject._fromMap(this, map);
     if ((obj != null) && obj.canCache) {
       _cache[mapId] = obj;
     }
@@ -1564,29 +1574,31 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   Future<List<Script>> getScripts() async {
     final response = await invokeRpc('getScripts', {}) as ServiceMap;
     assert(response.type == 'ScriptList');
-    return response['scripts'].cast<Script>() as List<Script>;
+    return response['scripts'].cast<Script>();
   }
 
   Future<Map> _fetchDirect({int count: kDefaultFieldLimit}) async {
     return invokeRpcNoUpgrade('getIsolate', {});
   }
 
-  Class? objectClass;
+  Class objectClass;
   final rootClasses = <Class>[];
 
-  late Library rootLibrary;
+  Library rootLibrary;
   List<Library> libraries = <Library>[];
-  Frame? topFrame;
+  Frame topFrame;
 
-  String? name;
-  String? vmName;
-  ServiceFunction? entry;
+  String name;
+  String vmName;
+  ServiceFunction entry;
 
   final HeapSpace newSpace = new HeapSpace();
   final HeapSpace oldSpace = new HeapSpace();
 
-  DartError? error;
-  SnapshotReader? _snapshotFetch;
+  String fileAndLine;
+
+  DartError error;
+  SnapshotReader _snapshotFetch;
 
   List<Thread> get threads => _threads;
   final List<Thread> _threads = <Thread>[];
@@ -1595,10 +1607,10 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   int _zoneHighWatermark = 0;
 
   int get numZoneHandles => _numZoneHandles;
-  int _numZoneHandles = 0;
+  int _numZoneHandles;
 
   int get numScopedHandles => _numScopedHandles;
-  int _numScopedHandles = 0;
+  int _numScopedHandles;
 
   void _loadHeapSnapshot(ServiceEvent event) {
     if (_snapshotFetch == null) {
@@ -1609,9 +1621,9 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
     }
 
     // Occasionally these actually arrive out of order.
-    _snapshotFetch!.add(event.data!);
-    if (event.lastChunk!) {
-      _snapshotFetch!.close();
+    _snapshotFetch.add(event.data);
+    if (event.lastChunk) {
+      _snapshotFetch.close();
       _snapshotFetch = null;
     }
   }
@@ -1622,7 +1634,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
       // isolate.vm.streamListen('HeapSnapshot');
       isolate.invokeRpcNoUpgrade('requestHeapSnapshot', {});
     }
-    return _snapshotFetch!;
+    return _snapshotFetch;
   }
 
   void updateHeapsFromMap(Map map) {
@@ -1656,7 +1668,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
       assert(names.length == counts.length);
       var sum = 0;
       for (var i = 0; i < counts.length; i++) {
-        sum += (counts[i] as int);
+        sum += counts[i];
       }
       var _counters = {};
       if (sum == 0) {
@@ -1683,8 +1695,8 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
     var newPauseEvent = map['pauseEvent'];
     assert((pauseEvent == null) ||
         (newPauseEvent == null) ||
-        !newPauseEvent.timestamp.isBefore(pauseEvent!.timestamp));
-    pauseEvent = createEventFromServiceEvent(newPauseEvent) as M.DebugEvent;
+        !newPauseEvent.timestamp.isBefore(pauseEvent.timestamp));
+    pauseEvent = createEventFromServiceEvent(newPauseEvent);
     _updateRunState();
     error = map['error'];
 
@@ -1707,7 +1719,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
 
     int currentZoneHighWatermark = 0;
     for (var i = 0; i < threads.length; i++) {
-      currentZoneHighWatermark += threads[i].zoneHighWatermark!;
+      currentZoneHighWatermark += threads[i].zoneHighWatermark;
     }
 
     if (currentZoneHighWatermark > _zoneHighWatermark) {
@@ -1727,7 +1739,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   }
 
   Map<int, Breakpoint> breakpoints = <int, Breakpoint>{};
-  String? exceptionsPauseInfo;
+  String exceptionsPauseInfo;
 
   void _updateBreakpoints(List newBpts) {
     // Build a set of new breakpoints.
@@ -1748,7 +1760,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   }
 
   void _addBreakpoint(Breakpoint bpt) {
-    breakpoints[bpt.number!] = bpt;
+    breakpoints[bpt.number] = bpt;
   }
 
   void _removeBreakpoint(Breakpoint bpt) {
@@ -1768,7 +1780,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
         _handleIsolateReloadEvent(event);
         break;
       case ServiceEvent.kBreakpointAdded:
-        _addBreakpoint(event.breakpoint!);
+        _addBreakpoint(event.breakpoint);
         break;
 
       case ServiceEvent.kIsolateUpdate:
@@ -1778,7 +1790,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
         break;
 
       case ServiceEvent.kBreakpointRemoved:
-        _removeBreakpoint(event.breakpoint!);
+        _removeBreakpoint(event.breakpoint);
         break;
 
       case ServiceEvent.kPauseStart:
@@ -1790,8 +1802,8 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
       case ServiceEvent.kNone:
       case ServiceEvent.kResume:
         assert((pauseEvent == null) ||
-            !event.timestamp!.isBefore(pauseEvent!.timestamp));
-        pauseEvent = createEventFromServiceEvent(event) as M.DebugEvent;
+            !event.timestamp.isBefore(pauseEvent.timestamp));
+        pauseEvent = createEventFromServiceEvent(event);
         _updateRunState();
         break;
 
@@ -1810,7 +1822,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
     }
   }
 
-  Future<Breakpoint> addBreakpoint(Script script, int line, [int? col]) {
+  Future<Breakpoint> addBreakpoint(Script script, int line, [int col]) {
     Map params = {
       'scriptId': script.id,
       'line': line,
@@ -1822,8 +1834,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
         .then((result) => result as Breakpoint);
   }
 
-  Future<Breakpoint> addBreakpointByScriptUri(String uri, int line,
-      [int? col]) {
+  Future<Breakpoint> addBreakpointByScriptUri(String uri, int line, [int col]) {
     Map params = {
       'scriptUri': uri,
       'line': line.toString(),
@@ -1908,7 +1919,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   }
 
   Future<ServiceObject> eval(ServiceObject target, String expression,
-      {Map<String, ServiceObject>? scope, bool disableBreakpoints: false}) {
+      {Map<String, ServiceObject> scope, bool disableBreakpoints: false}) {
     Map params = {
       'targetId': target.id,
       'expression': expression,
@@ -1917,7 +1928,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
     if (scope != null) {
       Map<String, String> scopeWithIds = new Map();
       scope.forEach((String name, ServiceObject object) {
-        scopeWithIds[name] = object.id!;
+        scopeWithIds[name] = object.id;
       });
       params["scope"] = scopeWithIds;
     }
@@ -1925,7 +1936,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
   }
 
   Future<ServiceObject> evalFrame(int frameIndex, String expression,
-      {Map<String, ServiceObject>? scope,
+      {Map<String, ServiceObject> scope,
       bool disableBreakpoints: false}) async {
     Map params = {
       'frameIndex': frameIndex,
@@ -1935,7 +1946,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
     if (scope != null) {
       Map<String, String> scopeWithIds = new Map();
       scope.forEach((String name, ServiceObject object) {
-        scopeWithIds[name] = object.id!;
+        scopeWithIds[name] = object.id;
       });
       params["scope"] = scopeWithIds;
     }
@@ -1951,7 +1962,7 @@ class Isolate extends ServiceObjectOwner implements M.Isolate {
           'exception': null,
           'stacktrace': null,
         };
-        return ServiceObject._fromMap(null, map);
+        return new ServiceObject._fromMap(null, map);
       } else
         rethrow;
     }
@@ -2045,7 +2056,7 @@ class NamedField implements M.NamedField {
 class ObjectStore extends ServiceObject implements M.ObjectStore {
   List<NamedField> fields = <NamedField>[];
 
-  ObjectStore._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ObjectStore._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     // Extract full properties.
@@ -2071,7 +2082,7 @@ class ServiceMap extends ServiceObject
 
   bool get immutable => false;
 
-  ServiceMap._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ServiceMap._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _loaded = !mapIsRef;
@@ -2081,7 +2092,7 @@ class ServiceMap extends ServiceObject
     // upgrading an already upgraded submap.  Is clearing really the
     // right thing to do here?
     _map.clear();
-    map.forEach((k, v) => _map[k] = v);
+    _map.addAll(map);
 
     name = _map['name'];
     vmName = (_map.containsKey('_vmName') ? _map['_vmName'] : name);
@@ -2093,20 +2104,19 @@ class ServiceMap extends ServiceObject
   int get clazz => _map['class'];
 
   // Forward Map interface calls.
-  void addAll(Map<String, dynamic> other) => _map.addAll(other);
+  void addAll(Map other) => _map.addAll(other);
   void clear() => _map.clear();
-  bool containsValue(dynamic v) => _map.containsValue(v);
-  bool containsKey(Object? k) => _map.containsKey(k);
-  void forEach(void f(String key, dynamic value)) => _map.forEach(f);
-  dynamic putIfAbsent(key, dynamic ifAbsent()) =>
-      _map.putIfAbsent(key, ifAbsent);
-  dynamic remove(Object? key) => _map.remove(key);
-  dynamic operator [](Object? k) => _map[k];
-  operator []=(String k, dynamic v) => _map[k] = v;
+  bool containsValue(v) => _map.containsValue(v);
+  bool containsKey(k) => _map.containsKey(k);
+  void forEach(Function f) => _map.forEach(f);
+  putIfAbsent(key, Function ifAbsent) => _map.putIfAbsent(key, ifAbsent);
+  void remove(key) => _map.remove(key);
+  operator [](k) => _map[k];
+  operator []=(k, v) => _map[k] = v;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
   Iterable<String> get keys => _map.keys;
-  Iterable<dynamic> get values => _map.values;
+  Iterable get values => _map.values;
   int get length => _map.length;
 
   // Suppress compile-time error about missing Map methods.
@@ -2133,12 +2143,12 @@ M.ErrorKind stringToErrorKind(String value) {
 
 /// A [DartError] is peered to a Dart Error object.
 class DartError extends HeapObject implements M.Error {
-  DartError._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  DartError._empty(ServiceObject owner) : super._empty(owner);
 
-  M.ErrorKind? kind;
-  String? message;
-  Instance? exception;
-  Instance? stacktrace;
+  M.ErrorKind kind;
+  String message;
+  Instance exception;
+  Instance stacktrace;
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, owner);
@@ -2202,46 +2212,46 @@ class ServiceEvent extends ServiceObject {
   static const kDartDevelopmentServiceConnected =
       'DartDevelopmentServiceConnected';
 
-  ServiceEvent._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ServiceEvent._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   ServiceEvent.connectionClosed(this.reason) : super._empty(null) {
     kind = kConnectionClosed;
   }
 
-  String? kind;
-  DateTime? timestamp;
-  String? flag;
-  String? newValue;
-  List<Breakpoint>? pauseBreakpoints;
-  Breakpoint? breakpoint;
-  Frame? topFrame;
-  DartError? error;
-  String? extensionRPC;
-  Instance? exception;
-  DartError? reloadError;
-  bool? atAsyncSuspension;
-  Instance? inspectee;
-  Uint8List? data;
-  int? count;
-  String? reason;
-  String? exceptions;
-  String? bytesAsString;
-  Map? logRecord;
-  String? extensionKind;
-  Map? extensionData;
-  List? timelineEvents;
-  List<String>? updatedStreams;
-  String? spawnToken;
-  String? spawnError;
-  String? editor;
-  ServiceObject? object;
-  String? method;
-  String? service;
-  String? alias;
-  String? message;
-  Uri? uri;
+  String kind;
+  DateTime timestamp;
+  String flag;
+  String newValue;
+  List<M.Breakpoint> pauseBreakpoints;
+  Breakpoint breakpoint;
+  Frame topFrame;
+  DartError error;
+  String extensionRPC;
+  Instance exception;
+  DartError reloadError;
+  bool atAsyncSuspension;
+  Instance inspectee;
+  Uint8List data;
+  int count;
+  String reason;
+  String exceptions;
+  String bytesAsString;
+  Map logRecord;
+  String extensionKind;
+  Map extensionData;
+  List timelineEvents;
+  List<String> updatedStreams;
+  String spawnToken;
+  String spawnError;
+  String editor;
+  ServiceObject object;
+  String method;
+  String service;
+  String alias;
+  String message;
+  Uri uri;
 
-  bool? lastChunk;
+  bool lastChunk;
 
   bool get isPauseEvent {
     return (kind == kPauseStart ||
@@ -2267,8 +2277,8 @@ class ServiceEvent extends ServiceObject {
     }
     if (map['pauseBreakpoints'] != null) {
       pauseBreakpoints = new List<Breakpoint>.from(map['pauseBreakpoints']);
-      if (pauseBreakpoints!.length > 0) {
-        breakpoint = pauseBreakpoints![0];
+      if (pauseBreakpoints.length > 0) {
+        breakpoint = pauseBreakpoints[0];
       }
     } else {
       pauseBreakpoints = const [];
@@ -2305,9 +2315,9 @@ class ServiceEvent extends ServiceObject {
     }
     if (map['logRecord'] != null) {
       logRecord = map['logRecord'];
-      logRecord!['time'] =
-          new DateTime.fromMillisecondsSinceEpoch(logRecord!['time']);
-      logRecord!['level'] = _findLogLevel(logRecord!['level']);
+      logRecord['time'] =
+          new DateTime.fromMillisecondsSinceEpoch(logRecord['time']);
+      logRecord['level'] = _findLogLevel(logRecord['level']);
     }
     if (map['extensionKind'] != null) {
       extensionKind = map['extensionKind'];
@@ -2355,42 +2365,42 @@ class ServiceEvent extends ServiceObject {
   }
 
   String toString() {
-    var ownerName = owner!.id != null ? owner!.id.toString() : owner!.name;
+    var ownerName = owner.id != null ? owner.id.toString() : owner.name;
     if (data == null) {
       return "ServiceEvent(owner='${ownerName}', kind='${kind}', "
           "time=${timestamp})";
     } else {
       return "ServiceEvent(owner='${ownerName}', kind='${kind}', "
-          "data.lengthInBytes=${data!.lengthInBytes}, time=${timestamp})";
+          "data.lengthInBytes=${data.lengthInBytes}, time=${timestamp})";
     }
   }
 }
 
 class Breakpoint extends ServiceObject implements M.Breakpoint {
-  Breakpoint._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Breakpoint._empty(ServiceObjectOwner owner) : super._empty(owner);
 
-  final M.ClassRef? clazz = null;
-  final int? size = null;
+  final M.ClassRef clazz = null;
+  final int size = null;
 
   // TODO(turnidge): Add state to track if a breakpoint has been
   // removed from the program.  Remove from the cache when deleted.
   bool get immutable => false;
 
   // A unique integer identifier for this breakpoint.
-  int? number;
+  int number;
 
   // Either SourceLocation or UnresolvedSourceLocation.
-  Location? location;
+  Location location;
 
   // The breakpoint is in a file which is not yet loaded.
-  bool? latent;
+  bool latent;
 
   // The breakpoint has been assigned to a final source location.
-  bool? resolved;
+  bool resolved;
 
   // The breakpoint was synthetically created as part of an
   // 'OverAsyncContinuation' resume request.
-  bool? isSyntheticAsyncContinuation;
+  bool isSyntheticAsyncContinuation;
 
   void _update(Map map, bool mapIsRef) {
     _loaded = true;
@@ -2413,23 +2423,23 @@ class Breakpoint extends ServiceObject implements M.Breakpoint {
       }
     }
     location = newLocation;
-    var newScript = location!.script;
+    var newScript = location.script;
     if (newScript != null && newScript.loaded) {
       newScript._addBreakpoint(this);
     }
 
     isSyntheticAsyncContinuation = map['isSyntheticAsyncContinuation'] != null;
 
-    assert(resolved! || location is UnresolvedSourceLocation);
+    assert(resolved || location is UnresolvedSourceLocation);
   }
 
   void remove() {
-    location!.script._removeBreakpoint(this);
+    location.script._removeBreakpoint(this);
   }
 
   String toString() {
     if (number != null) {
-      if (isSyntheticAsyncContinuation!) {
+      if (isSyntheticAsyncContinuation) {
         return 'Synthetic Async Continuation Breakpoint ${number}';
       } else {
         return 'Breakpoint ${number} at ${location}';
@@ -2455,33 +2465,33 @@ class LibraryDependency implements M.LibraryDependency {
 }
 
 class Library extends HeapObject implements M.Library {
-  String? uri;
+  String uri;
   final List<LibraryDependency> dependencies = <LibraryDependency>[];
   final List<Script> scripts = <Script>[];
   final List<Class> classes = <Class>[];
   final List<Field> variables = <Field>[];
   final List<ServiceFunction> functions = <ServiceFunction>[];
-  bool? _debuggable;
-  bool get debuggable => _debuggable!;
+  bool _debuggable;
+  bool get debuggable => _debuggable;
   bool get immutable => false;
 
   bool isDart(String libraryName) {
     return uri == 'dart:$libraryName';
   }
 
-  Library._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Library._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
     super._update(map, mapIsRef);
 
     uri = map['uri'];
-    var shortUri = uri!;
-    if (shortUri.startsWith('file://') || shortUri.startsWith('http://')) {
-      shortUri = shortUri.substring(shortUri.lastIndexOf('/') + 1);
+    var shortUri = uri;
+    if (uri.startsWith('file://') || uri.startsWith('http://')) {
+      shortUri = uri.substring(uri.lastIndexOf('/') + 1);
     }
-    name = map['name'] as String;
-    if (name!.isEmpty) {
+    name = map['name'];
+    if (name.isEmpty) {
       // When there is no name for a library, use the shortUri.
       name = shortUri;
     }
@@ -2510,12 +2520,12 @@ class Library extends HeapObject implements M.Library {
   }
 
   Future<ServiceObject> evaluate(String expression,
-      {Map<String, ServiceObject>? scope, bool disableBreakpoints: false}) {
-    return isolate!.eval(this, expression,
+      {Map<String, ServiceObject> scope, bool disableBreakpoints: false}) {
+    return isolate.eval(this, expression,
         scope: scope, disableBreakpoints: disableBreakpoints);
   }
 
-  Script? get rootScript {
+  Script get rootScript {
     for (Script script in scripts) {
       if (script.uri == uri) return script;
     }
@@ -2552,17 +2562,17 @@ class Allocations implements M.Allocations {
 }
 
 class Class extends HeapObject implements M.Class {
-  Library? library;
+  Library library;
 
-  bool? isAbstract;
-  bool? isConst;
-  bool? isFinalized;
-  bool? isPatch;
-  bool? isImplemented;
+  bool isAbstract;
+  bool isConst;
+  bool isFinalized;
+  bool isPatch;
+  bool isImplemented;
 
-  SourceLocation? location;
+  SourceLocation location;
 
-  DartError? error;
+  DartError error;
 
   final Allocations newSpace = new Allocations();
   final Allocations oldSpace = new Allocations();
@@ -2573,16 +2583,16 @@ class Class extends HeapObject implements M.Class {
   final List<Field> fields = <Field>[];
   final List<ServiceFunction> functions = <ServiceFunction>[];
 
-  Class? superclass;
+  Class superclass;
   final List<Instance> interfaces = <Instance>[];
   final List<Class> subclasses = <Class>[];
 
-  Instance? superType;
-  Instance? mixin;
+  Instance superType;
+  Instance mixin;
 
   bool get immutable => false;
 
-  Class._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Class._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -2594,7 +2604,7 @@ class Class extends HeapObject implements M.Class {
       name = 'top-level-class'; // Better than ''
     }
     var idPrefix = "classes/";
-    assert(id!.startsWith(idPrefix));
+    assert(id.startsWith(idPrefix));
 
     if (mapIsRef) {
       return;
@@ -2638,8 +2648,8 @@ class Class extends HeapObject implements M.Class {
 
     superclass = map['super'];
     // Work-around Object not tracking its subclasses in the VM.
-    if (superclass != null && superclass!.name == "Object") {
-      superclass!._addSubclass(this);
+    if (superclass != null && superclass.name == "Object") {
+      superclass._addSubclass(this);
     }
     superType = map['superType'];
     mixin = map['mixin'];
@@ -2659,13 +2669,13 @@ class Class extends HeapObject implements M.Class {
   }
 
   Future<ServiceObject> evaluate(String expression,
-      {Map<String, ServiceObject>? scope, disableBreakpoints: false}) {
-    return isolate!.eval(this, expression,
+      {Map<String, ServiceObject> scope, disableBreakpoints: false}) {
+    return isolate.eval(this, expression,
         scope: scope, disableBreakpoints: disableBreakpoints);
   }
 
   Future<ServiceObject> setTraceAllocations(bool enable) {
-    return isolate!.invokeRpc('_setTraceClassAllocation', {
+    return isolate.invokeRpc('_setTraceClassAllocation', {
       'enable': enable,
       'classId': id,
     });
@@ -2675,7 +2685,7 @@ class Class extends HeapObject implements M.Class {
     var params = {
       'classId': id,
     };
-    return isolate!.invokeRpc('_getAllocationSamples', params);
+    return isolate.invokeRpc('_getAllocationSamples', params);
   }
 
   String toString() => 'Class($vmName)';
@@ -2758,8 +2768,8 @@ M.InstanceKind stringToInstanceKind(String s) {
 class Guarded<T extends ServiceObject> implements M.Guarded<T> {
   bool get isValue => asValue != null;
   bool get isSentinel => asSentinel != null;
-  final Sentinel? asSentinel;
-  final T? asValue;
+  final Sentinel asSentinel;
+  final T asValue;
 
   factory Guarded(ServiceObject obj) {
     if (obj is Sentinel) {
@@ -2794,41 +2804,41 @@ class MapAssociation implements M.MapAssociation {
 }
 
 class Instance extends HeapObject implements M.Instance {
-  M.InstanceKind? kind;
-  String? valueAsString; // If primitive.
-  bool? valueAsStringIsTruncated;
-  ServiceFunction? closureFunction; // If a closure.
-  Context? closureContext; // If a closure.
-  int? length; // If a List, Map or TypedData.
-  int? count;
-  int? offset;
-  Instance? pattern; // If a RegExp.
+  M.InstanceKind kind;
+  String valueAsString; // If primitive.
+  bool valueAsStringIsTruncated;
+  ServiceFunction closureFunction; // If a closure.
+  Context closureContext; // If a closure.
+  int length; // If a List, Map or TypedData.
+  int count;
+  int offset;
+  Instance pattern; // If a RegExp.
 
-  String? name;
-  Class? typeClass;
-  Class? parameterizedClass;
-  TypeArguments? typeArguments;
-  int? parameterIndex;
-  Instance? targetType;
-  Instance? bound;
+  String name;
+  Class typeClass;
+  Class parameterizedClass;
+  TypeArguments typeArguments;
+  int parameterIndex;
+  Instance targetType;
+  Instance bound;
 
-  Iterable<BoundField>? fields;
+  Iterable<BoundField> fields;
   var nativeFields;
-  Iterable<Guarded<HeapObject>>? elements; // If a List.
-  Iterable<MapAssociation>? associations; // If a Map.
-  List<dynamic>? typedElements; // If a TypedData.
-  HeapObject? referent; // If a MirrorReference.
-  Instance? key; // If a WeakProperty.
-  Instance? value; // If a WeakProperty.
-  Breakpoint? activationBreakpoint; // If a Closure.
-  ServiceFunction? oneByteFunction; // If a RegExp.
-  ServiceFunction? twoByteFunction; // If a RegExp.
-  ServiceFunction? externalOneByteFunction; // If a RegExp.
-  ServiceFunction? externalTwoByteFunction; // If a RegExp.
-  Instance? oneByteBytecode; // If a RegExp.
-  Instance? twoByteBytecode; // If a RegExp.
-  bool? isCaseSensitive; // If a RegExp.
-  bool? isMultiLine; // If a RegExp.
+  Iterable<Guarded<HeapObject>> elements; // If a List.
+  Iterable<MapAssociation> associations; // If a Map.
+  List<dynamic> typedElements; // If a TypedData.
+  HeapObject referent; // If a MirrorReference.
+  Instance key; // If a WeakProperty.
+  Instance value; // If a WeakProperty.
+  Breakpoint activationBreakpoint; // If a Closure.
+  ServiceFunction oneByteFunction; // If a RegExp.
+  ServiceFunction twoByteFunction; // If a RegExp.
+  ServiceFunction externalOneByteFunction; // If a RegExp.
+  ServiceFunction externalTwoByteFunction; // If a RegExp.
+  Instance oneByteBytecode; // If a RegExp.
+  Instance twoByteBytecode; // If a RegExp.
+  bool isCaseSensitive; // If a RegExp.
+  bool isMultiLine; // If a RegExp.
 
   bool get isAbstractType => M.isAbstractType(kind);
   bool get isNull => kind == M.InstanceKind.vNull;
@@ -2838,8 +2848,14 @@ class Instance extends HeapObject implements M.Instance {
   bool get isInt => kind == M.InstanceKind.int;
   bool get isList => kind == M.InstanceKind.list;
   bool get isMap => kind == M.InstanceKind.map;
-  bool get isTypedData => M.isTypedData(kind);
-  bool get isSimdValue => M.isSimdValue(kind);
+  bool get isTypedData {
+    return M.isTypedData(kind);
+  }
+
+  bool get isSimdValue {
+    return M.isSimdValue(kind);
+  }
+
   bool get isRegExp => kind == M.InstanceKind.regExp;
   bool get isMirrorReference => kind == M.InstanceKind.mirrorReference;
   bool get isWeakProperty => kind == M.InstanceKind.weakProperty;
@@ -2849,29 +2865,27 @@ class Instance extends HeapObject implements M.Instance {
     if (clazz == null) {
       return false;
     }
-    if (clazz!.library == null) {
+    if (clazz.library == null) {
       return false;
     }
-    return (clazz!.name == 'StackOverflowError') &&
-        clazz!.library!.isDart('core');
+    return (clazz.name == 'StackOverflowError') && clazz.library.isDart('core');
   }
 
   bool get isOutOfMemoryError {
     if (clazz == null) {
       return false;
     }
-    if (clazz!.library == null) {
+    if (clazz.library == null) {
       return false;
     }
-    return (clazz!.name == 'OutOfMemoryError') &&
-        clazz!.library!.isDart('core');
+    return (clazz.name == 'OutOfMemoryError') && clazz.library.isDart('core');
   }
 
   // TODO(turnidge): Is this properly backwards compatible when new
   // instance kinds are added?
   bool get isPlainInstance => kind == 'PlainInstance';
 
-  Instance._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Instance._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     // Extract full properties.1
@@ -3015,17 +3029,17 @@ class Instance extends HeapObject implements M.Instance {
 
   String get shortName {
     if (isClosure) {
-      return closureFunction!.qualifiedName!;
+      return closureFunction.qualifiedName;
     }
     if (valueAsString != null) {
-      return valueAsString!;
+      return valueAsString;
     }
-    return 'a ${clazz!.name}';
+    return 'a ${clazz.name}';
   }
 
   Future<ServiceObject> evaluate(String expression,
-      {Map<String, ServiceObject>? scope, bool disableBreakpoints: false}) {
-    return isolate!.eval(this, expression,
+      {Map<String, ServiceObject> scope, bool disableBreakpoints: false}) {
+    return isolate.eval(this, expression,
         scope: scope, disableBreakpoints: disableBreakpoints);
   }
 
@@ -3033,11 +3047,11 @@ class Instance extends HeapObject implements M.Instance {
 }
 
 class Context extends HeapObject implements M.Context {
-  Context? parentContext;
-  int? length;
-  Iterable<ContextElement>? variables;
+  Context parentContext;
+  int length;
+  Iterable<ContextElement> variables;
 
-  Context._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Context._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     // Extract full properties.
@@ -3126,31 +3140,31 @@ M.FunctionKind stringToFunctionKind(String value) {
 
 class ServiceFunction extends HeapObject implements M.ServiceFunction {
   // owner is a Library, Class, or ServiceFunction.
-  M.ObjectRef? dartOwner;
-  Library? library;
-  bool? isStatic;
-  bool? isConst;
-  SourceLocation? location;
-  Code? code;
-  Code? unoptimizedCode;
-  Code? bytecode;
-  bool? isOptimizable;
-  bool? isInlinable;
-  bool? hasIntrinsic;
-  bool? isRecognized;
-  bool? isNative;
-  M.FunctionKind? kind;
-  int? deoptimizations;
-  String? qualifiedName;
-  int? usageCounter;
-  bool? isDart;
-  ProfileFunction? profile;
-  Instance? icDataArray;
-  Field? field;
+  M.ObjectRef dartOwner;
+  Library library;
+  bool isStatic;
+  bool isConst;
+  SourceLocation location;
+  Code code;
+  Code unoptimizedCode;
+  Code bytecode;
+  bool isOptimizable;
+  bool isInlinable;
+  bool hasIntrinsic;
+  bool isRecognized;
+  bool isNative;
+  M.FunctionKind kind;
+  int deoptimizations;
+  String qualifiedName;
+  int usageCounter;
+  bool isDart;
+  ProfileFunction profile;
+  Instance icDataArray;
+  Field field;
 
   bool get immutable => false;
 
-  ServiceFunction._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ServiceFunction._empty(ServiceObject owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, owner);
@@ -3164,15 +3178,15 @@ class ServiceFunction extends HeapObject implements M.ServiceFunction {
     isDart = M.isDartFunction(kind);
 
     if (dartOwner is ServiceFunction) {
-      ServiceFunction ownerFunction = dartOwner as ServiceFunction;
+      ServiceFunction ownerFunction = dartOwner;
       library = ownerFunction.library;
       qualifiedName = "${ownerFunction.qualifiedName}.${name}";
     } else if (dartOwner is Class) {
-      Class ownerClass = dartOwner as Class;
+      Class ownerClass = dartOwner;
       library = ownerClass.library;
       qualifiedName = "${ownerClass.name}.${name}";
     } else {
-      library = dartOwner as Library;
+      library = dartOwner;
       qualifiedName = name;
     }
 
@@ -3202,7 +3216,7 @@ class ServiceFunction extends HeapObject implements M.ServiceFunction {
   ServiceFunction get homeMethod {
     var m = this;
     while (m.dartOwner is ServiceFunction) {
-      m = m.dartOwner as ServiceFunction;
+      m = m.dartOwner;
     }
     return m;
   }
@@ -3233,10 +3247,10 @@ M.SentinelKind stringToSentinelKind(String s) {
 }
 
 class Sentinel extends ServiceObject implements M.Sentinel {
-  late M.SentinelKind kind;
-  late String valueAsString;
+  M.SentinelKind kind;
+  String valueAsString;
 
-  Sentinel._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Sentinel._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     // Extract full properties.
@@ -3252,16 +3266,16 @@ class Sentinel extends ServiceObject implements M.Sentinel {
 }
 
 class Thread extends ServiceObject implements M.Thread {
-  M.ThreadKind? get kind => _kind;
-  M.ThreadKind? _kind;
-  String? get kindString => _kindString;
-  String? _kindString;
-  int? get zoneHighWatermark => _zoneHighWatermark;
-  int? _zoneHighWatermark;
-  int? get zoneCapacity => _zoneCapacity;
-  int? _zoneCapacity;
+  M.ThreadKind get kind => _kind;
+  M.ThreadKind _kind;
+  String get kindString => _kindString;
+  String _kindString;
+  int get zoneHighWatermark => _zoneHighWatermark;
+  int _zoneHighWatermark;
+  int get zoneCapacity => _zoneCapacity;
+  int _zoneCapacity;
 
-  Thread._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Thread._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     String rawKind = map['kind'];
@@ -3307,23 +3321,23 @@ class Zone implements M.Zone {
 
 class Field extends HeapObject implements M.Field {
   // Library or Class.
-  HeapObject? dartOwner;
-  Library? library;
-  Instance? declaredType;
-  bool? isStatic;
-  bool? isFinal;
-  bool? isConst;
-  ServiceObject? staticValue;
-  String? name;
-  String? vmName;
+  HeapObject dartOwner;
+  Library library;
+  Instance declaredType;
+  bool isStatic;
+  bool isFinal;
+  bool isConst;
+  ServiceObject staticValue;
+  String name;
+  String vmName;
 
-  bool? guardNullable;
-  M.GuardClassKind? guardClassKind;
-  Class? guardClass;
-  String? guardLength;
-  SourceLocation? location;
+  bool guardNullable;
+  M.GuardClassKind guardClassKind;
+  Class guardClass;
+  String guardLength;
+  SourceLocation location;
 
-  Field._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Field._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     // Extract full properties.
@@ -3339,10 +3353,10 @@ class Field extends HeapObject implements M.Field {
     isConst = map['const'];
 
     if (dartOwner is Class) {
-      Class ownerClass = dartOwner as Class;
+      Class ownerClass = dartOwner;
       library = ownerClass.library;
     } else {
-      library = dartOwner as Library;
+      library = dartOwner;
     }
 
     if (mapIsRef) {
@@ -3371,14 +3385,14 @@ class Field extends HeapObject implements M.Field {
     _loaded = true;
   }
 
-  String toString() => 'Field(${dartOwner!.name}.$name)';
+  String toString() => 'Field(${dartOwner.name}.$name)';
 }
 
 class ScriptLine {
   final Script script;
   final int line;
   final String text;
-  Set<Breakpoint>? breakpoints;
+  Set<Breakpoint> breakpoints;
 
   ScriptLine(this.script, this.line, this.text);
 
@@ -3386,12 +3400,12 @@ class ScriptLine {
     return text.isEmpty || text.trim().isEmpty;
   }
 
-  bool? _isTrivial = null;
+  bool _isTrivial = null;
   bool get isTrivial {
     if (_isTrivial == null) {
       _isTrivial = _isTrivialLine(text);
     }
-    return _isTrivial!;
+    return _isTrivial;
   }
 
   static bool _isTrivialToken(String token) {
@@ -3433,13 +3447,13 @@ class ScriptLine {
     if (breakpoints == null) {
       breakpoints = new Set<Breakpoint>();
     }
-    breakpoints!.add(bpt);
+    breakpoints.add(bpt);
   }
 
   void removeBreakpoint(Breakpoint bpt) {
-    assert(breakpoints != null && breakpoints!.contains(bpt));
-    breakpoints!.remove(bpt);
-    if (breakpoints!.isEmpty) {
+    assert(breakpoints != null && breakpoints.contains(bpt));
+    breakpoints.remove(bpt);
+    if (breakpoints.isEmpty) {
       breakpoints = null;
     }
   }
@@ -3454,8 +3468,8 @@ class CallSite {
 
   CallSite(this.name, this.script, this.tokenPos, this.entries);
 
-  int get line => script.tokenToLine(tokenPos)!;
-  int get column => script.tokenToCol(tokenPos)!;
+  int get line => script.tokenToLine(tokenPos);
+  int get column => script.tokenToCol(tokenPos);
 
   int get aggregateCount {
     var count = 0;
@@ -3512,46 +3526,46 @@ class LocalVarLocation {
 
 class Script extends HeapObject implements M.Script {
   final lines = <ScriptLine>[];
-  late String uri;
-  late String kind;
-  DateTime? loadTime;
-  int? firstTokenPos;
-  int? lastTokenPos;
-  int? lineOffset;
-  int? columnOffset;
-  Library? library;
+  String uri;
+  String kind;
+  DateTime loadTime;
+  int firstTokenPos;
+  int lastTokenPos;
+  int lineOffset;
+  int columnOffset;
+  Library library;
 
-  String? source;
+  String source;
 
   bool get immutable => true;
 
-  String? _shortUri;
+  String _shortUri;
 
-  Script._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Script._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   /// Retrieves line number [line] if it exists.
-  ScriptLine? getLine(int line) {
+  ScriptLine getLine(int line) {
     assert(_loaded);
     assert(line >= 1);
-    var index = (line - lineOffset! - 1);
+    var index = (line - lineOffset - 1);
     if (lines.length < index) {
       return null;
     }
-    return lines[line - lineOffset! - 1];
+    return lines[line - lineOffset - 1];
   }
 
   /// This function maps a token position to a line number.
   /// The VM considers the first line to be line 1.
-  int? tokenToLine(int? tokenPos) => _tokenToLine[tokenPos];
+  int tokenToLine(int tokenPos) => _tokenToLine[tokenPos];
   Map _tokenToLine = {};
 
   /// This function maps a token position to a column number.
   /// The VM considers the first column to be column 1.
-  int? tokenToCol(int? tokenPos) => _tokenToCol[tokenPos];
+  int tokenToCol(int tokenPos) => _tokenToCol[tokenPos];
   Map _tokenToCol = {};
 
-  int? guessTokenLength(int line, int column) {
-    String source = getLine(line)!.text;
+  int guessTokenLength(int line, int column) {
+    String source = getLine(line).text;
 
     var pos = column;
     if (pos >= source.length) {
@@ -3660,9 +3674,9 @@ class Script extends HeapObject implements M.Script {
         } else {
           // Keep track of max and min token positions.
           firstTokenPos =
-              (firstTokenPos! <= tokenOffset) ? firstTokenPos : tokenOffset;
+              (firstTokenPos <= tokenOffset) ? firstTokenPos : tokenOffset;
           lastTokenPos =
-              (lastTokenPos! >= tokenOffset) ? lastTokenPos : tokenOffset;
+              (lastTokenPos >= tokenOffset) ? lastTokenPos : tokenOffset;
         }
         _tokenToLine[tokenOffset] = lineNumber;
         _tokenToCol[tokenOffset] = colNumber;
@@ -3681,10 +3695,10 @@ class Script extends HeapObject implements M.Script {
     lines.clear();
     Logger.root.info('Adding ${sourceLines.length} source lines for ${uri}');
     for (var i = 0; i < sourceLines.length; i++) {
-      lines.add(new ScriptLine(this, i + lineOffset! + 1, sourceLines[i]));
+      lines.add(new ScriptLine(this, i + lineOffset + 1, sourceLines[i]));
     }
-    for (var bpt in isolate!.breakpoints.values) {
-      if (bpt.location!.script == this) {
+    for (var bpt in isolate.breakpoints.values) {
+      if (bpt.location.script == this) {
         _addBreakpoint(bpt);
       }
     }
@@ -3692,19 +3706,19 @@ class Script extends HeapObject implements M.Script {
 
   // Note, this may return source beyond the token length if [guessTokenLength]
   // fails.
-  String? getToken(int tokenPos) {
-    final int? line = tokenToLine(tokenPos);
-    int? column = tokenToCol(tokenPos);
+  String getToken(int tokenPos) {
+    final int line = tokenToLine(tokenPos);
+    int column = tokenToCol(tokenPos);
     if ((line == null) || (column == null)) {
       return null;
     }
     // Line and column numbers start at 1 in the VM.
     column -= 1;
-    String? sourceLine = getLine(line)?.text;
+    String sourceLine = getLine(line)?.text;
     if (sourceLine == null) {
       return null;
     }
-    final int? length = guessTokenLength(line, column);
+    final int length = guessTokenLength(line, column);
     if (length == null) {
       return sourceLine.substring(column);
     } else {
@@ -3714,21 +3728,21 @@ class Script extends HeapObject implements M.Script {
 
   void _addBreakpoint(Breakpoint bpt) {
     var line;
-    if (bpt.location!.tokenPos != null) {
-      line = tokenToLine(bpt.location!.tokenPos);
+    if (bpt.location.tokenPos != null) {
+      line = tokenToLine(bpt.location.tokenPos);
     } else {
-      UnresolvedSourceLocation loc = bpt.location as UnresolvedSourceLocation;
+      UnresolvedSourceLocation loc = bpt.location;
       line = loc.line;
     }
-    getLine(line!)?.addBreakpoint(bpt);
+    getLine(line)?.addBreakpoint(bpt);
   }
 
   void _removeBreakpoint(Breakpoint bpt) {
     var line;
-    if (bpt.location!.tokenPos != null) {
-      line = tokenToLine(bpt.location!.tokenPos);
+    if (bpt.location.tokenPos != null) {
+      line = tokenToLine(bpt.location.tokenPos);
     } else {
-      UnresolvedSourceLocation loc = bpt.location as UnresolvedSourceLocation;
+      UnresolvedSourceLocation loc = bpt.location;
       line = loc.line;
     }
     if (line != null) {
@@ -3779,16 +3793,14 @@ class Script extends HeapObject implements M.Script {
       return r;
     }
     // Current scan position.
-    int? maybeLine = tokenToLine(tokenPos);
-    if (maybeLine == null) {
+    var line = tokenToLine(tokenPos);
+    if (line == null) {
       return r;
     }
-    int line = maybeLine;
-    int? maybeColumn = tokenToCol(tokenPos);
-    if (maybeColumn == null) {
+    var column = tokenToCol(tokenPos);
+    if (column == null) {
       return r;
     }
-    int column = maybeColumn;
 
     // Move back by name length.
     // TODO(johnmccutchan): Fix LocalVarDescriptor to set column before the
@@ -3799,44 +3811,44 @@ class Script extends HeapObject implements M.Script {
 
     if (line == lastLine) {
       // Only one line.
-      if (!getLine(line)!.isTrivial) {
+      if (!getLine(line).isTrivial) {
         // TODO(johnmccutchan): end token pos -> column can lie for snapshotted
         // code. e.g.:
         // io_sink.dart source line 23 ends at column 39
         // io_sink.dart snapshotted source line 23 ends at column 35.
-        lastColumn = math.min(getLine(line)!.text.length, lastColumn);
-        lineContents = getLine(line)!.text.substring(column, lastColumn - 1);
+        lastColumn = math.min(getLine(line).text.length, lastColumn);
+        lineContents = getLine(line).text.substring(column, lastColumn - 1);
         return scanLineForLocalVariableLocations(
             pattern, name, lineContents, line, column);
       }
     }
 
     // Scan first line.
-    if (!getLine(line)!.isTrivial) {
-      lineContents = getLine(line)!.text.substring(column);
+    if (!getLine(line).isTrivial) {
+      lineContents = getLine(line).text.substring(column);
       r.addAll(scanLineForLocalVariableLocations(
           pattern, name, lineContents, line++, column));
     }
 
     // Scan middle lines.
     while (line < (lastLine - 1)) {
-      if (getLine(line)!.isTrivial) {
+      if (getLine(line).isTrivial) {
         line++;
         continue;
       }
-      lineContents = getLine(line)!.text;
+      lineContents = getLine(line).text;
       r.addAll(scanLineForLocalVariableLocations(
           pattern, name, lineContents, line++, 0));
     }
 
     // Scan last line.
-    if (!getLine(line)!.isTrivial) {
+    if (!getLine(line).isTrivial) {
       // TODO(johnmccutchan): end token pos -> column can lie for snapshotted
       // code. e.g.:
       // io_sink.dart source line 23 ends at column 39
       // io_sink.dart snapshotted source line 23 ends at column 35.
-      lastColumn = math.min(getLine(line)!.text.length, lastColumn);
-      lineContents = getLine(line)!.text.substring(0, lastColumn - 1);
+      lastColumn = math.min(getLine(line).text.length, lastColumn);
+      lineContents = getLine(line).text.substring(0, lastColumn - 1);
       r.addAll(scanLineForLocalVariableLocations(
           pattern, name, lineContents, line, 0));
     }
@@ -3850,8 +3862,8 @@ class PcDescriptor {
   final int tokenPos;
   final int tryIndex;
   final String kind;
-  Script? script;
-  String? formattedLine;
+  Script script;
+  String formattedLine;
   PcDescriptor(
       this.pcOffset, this.deoptId, this.tokenPos, this.tryIndex, this.kind);
 
@@ -3880,17 +3892,17 @@ class PcDescriptor {
     }
     this.script = script;
     var scriptLine = script.getLine(line);
-    formattedLine = scriptLine!.text;
+    formattedLine = scriptLine.text;
   }
 }
 
 class PcDescriptors extends ServiceObject implements M.PcDescriptorsRef {
-  Class? clazz;
-  int? size;
+  Class clazz;
+  int size;
   bool get immutable => true;
   final List<PcDescriptor> descriptors = <PcDescriptor>[];
 
-  PcDescriptors._empty(ServiceObjectOwner? owner) : super._empty(owner) {}
+  PcDescriptors._empty(ServiceObjectOwner owner) : super._empty(owner) {}
 
   void _update(Map m, bool mapIsRef) {
     if (mapIsRef) {
@@ -3927,11 +3939,11 @@ class LocalVarDescriptor implements M.LocalVarDescriptorsRef {
 }
 
 class LocalVarDescriptors extends ServiceObject {
-  Class? clazz;
-  int? size;
+  Class clazz;
+  int size;
   bool get immutable => true;
   final List<LocalVarDescriptor> descriptors = <LocalVarDescriptor>[];
-  LocalVarDescriptors._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  LocalVarDescriptors._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map m, bool mapIsRef) {
     if (mapIsRef) {
@@ -3959,10 +3971,10 @@ class LocalVarDescriptors extends ServiceObject {
 class ObjectPool extends HeapObject implements M.ObjectPool {
   bool get immutable => false;
 
-  int? length;
-  List<ObjectPoolEntry>? entries;
+  int length;
+  List<ObjectPoolEntry> entries;
 
-  ObjectPool._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ObjectPool._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -3981,8 +3993,8 @@ class ObjectPool extends HeapObject implements M.ObjectPool {
 class ObjectPoolEntry implements M.ObjectPoolEntry {
   final int offset;
   final M.ObjectPoolEntryKind kind;
-  final M.ObjectRef? asObject;
-  final int? asInteger;
+  final M.ObjectRef asObject;
+  final int asInteger;
 
   factory ObjectPoolEntry(map) {
     M.ObjectPoolEntryKind kind = stringToObjectPoolEntryKind(map['kind']);
@@ -4020,14 +4032,14 @@ M.ObjectPoolEntryKind stringToObjectPoolEntryKind(String kind) {
 }
 
 class ICData extends HeapObject implements M.ICData {
-  HeapObject? dartOwner;
-  String? selector;
-  Instance? argumentsDescriptor;
-  Instance? entries;
+  HeapObject dartOwner;
+  String selector;
+  Instance argumentsDescriptor;
+  Instance entries;
 
   bool get immutable => false;
 
-  ICData._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ICData._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -4044,12 +4056,12 @@ class ICData extends HeapObject implements M.ICData {
 }
 
 class UnlinkedCall extends HeapObject implements M.UnlinkedCall {
-  String? selector;
-  Instance? argumentsDescriptor;
+  String selector;
+  Instance argumentsDescriptor;
 
   bool get immutable => false;
 
-  UnlinkedCall._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  UnlinkedCall._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -4064,13 +4076,13 @@ class UnlinkedCall extends HeapObject implements M.UnlinkedCall {
 }
 
 class SingleTargetCache extends HeapObject implements M.SingleTargetCache {
-  Code? target;
-  int? lowerLimit;
-  int? upperLimit;
+  Code target;
+  int lowerLimit;
+  int upperLimit;
 
   bool get immutable => false;
 
-  SingleTargetCache._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  SingleTargetCache._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -4086,11 +4098,11 @@ class SingleTargetCache extends HeapObject implements M.SingleTargetCache {
 }
 
 class SubtypeTestCache extends HeapObject implements M.SubtypeTestCache {
-  Instance? cache;
+  Instance cache;
 
   bool get immutable => false;
 
-  SubtypeTestCache._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  SubtypeTestCache._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -4104,11 +4116,11 @@ class SubtypeTestCache extends HeapObject implements M.SubtypeTestCache {
 }
 
 class TypeArguments extends HeapObject implements M.TypeArguments {
-  HeapObject? dartOwner;
-  String? name;
-  Iterable<Instance>? types;
+  HeapObject dartOwner;
+  String name;
+  Iterable<Instance> types;
 
-  TypeArguments._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  TypeArguments._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -4124,11 +4136,11 @@ class TypeArguments extends HeapObject implements M.TypeArguments {
 }
 
 class InstanceSet extends HeapObject implements M.InstanceSet {
-  HeapObject? dartOwner;
-  int? count;
-  Iterable<HeapObject>? instances;
+  HeapObject dartOwner;
+  int count;
+  Iterable<HeapObject> instances;
 
-  InstanceSet._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  InstanceSet._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -4143,14 +4155,14 @@ class InstanceSet extends HeapObject implements M.InstanceSet {
 }
 
 class MegamorphicCache extends HeapObject implements M.MegamorphicCache {
-  int? mask;
-  Instance? buckets;
-  String? selector;
-  Instance? argumentsDescriptor;
+  int mask;
+  Instance buckets;
+  String selector;
+  Instance argumentsDescriptor;
 
   bool get immutable => false;
 
-  MegamorphicCache._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  MegamorphicCache._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     _upgradeCollection(map, isolate);
@@ -4173,7 +4185,7 @@ class CodeInstruction {
   final String machine;
   final String human;
   final ServiceObject object;
-  CodeInstruction? jumpTarget;
+  CodeInstruction jumpTarget;
   List<PcDescriptor> descriptors = <PcDescriptor>[];
 
   CodeInstruction(
@@ -4206,7 +4218,7 @@ class CodeInstruction {
   }
 
   void _resolveJumpTarget(
-      List<CodeInstruction?> instructionsByAddressOffset, int startAddress) {
+      List<CodeInstruction> instructionsByAddressOffset, int startAddress) {
     if (!_isJumpInstruction()) {
       return;
     }
@@ -4223,7 +4235,7 @@ class CodeInstruction {
       Logger.root.warning('Bad address resolving jump target $relativeAddress');
       return;
     }
-    jumpTarget = instructionsByAddressOffset[relativeAddress]!;
+    jumpTarget = instructionsByAddressOffset[relativeAddress];
   }
 }
 
@@ -4253,26 +4265,26 @@ class CodeInlineInterval {
 }
 
 class Code extends HeapObject implements M.Code {
-  M.CodeKind? kind;
-  ObjectPool? objectPool;
-  ServiceFunction? function;
-  Script? script;
-  bool? isOptimized;
-  bool? hasIntrinsic;
-  bool? isNative;
+  M.CodeKind kind;
+  ObjectPool objectPool;
+  ServiceFunction function;
+  Script script;
+  bool isOptimized;
+  bool hasIntrinsic;
+  bool isNative;
 
   int startAddress = 0;
   int endAddress = 0;
   final instructions = <CodeInstruction>[];
-  List<CodeInstruction?>? instructionsByAddressOffset;
+  List<CodeInstruction> instructionsByAddressOffset;
 
-  ProfileCode? profile;
+  ProfileCode profile;
   final List<CodeInlineInterval> inlineIntervals = <CodeInlineInterval>[];
   final List<ServiceFunction> inlinedFunctions = <ServiceFunction>[];
 
   bool get immutable => true;
 
-  Code._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Code._empty(ServiceObjectOwner owner) : super._empty(owner);
 
   void _updateDescriptors(Script script) {
     this.script = script;
@@ -4283,7 +4295,7 @@ class Code extends HeapObject implements M.Code {
     }
   }
 
-  Future loadScript() async {
+  Future loadScript() {
     if (script != null) {
       // Already done.
       return null;
@@ -4294,10 +4306,10 @@ class Code extends HeapObject implements M.Code {
     if (function == null) {
       return null;
     }
-    if ((function!.location == null) || (function!.location!.script == null)) {
+    if ((function.location == null) || (function.location.script == null)) {
       // Attempt to load the function.
-      return function!.load().then((func) {
-        var script = function!.location!.script;
+      return function.load().then((func) {
+        var script = function.location.script;
         if (script == null) {
           // Function doesn't have an associated script.
           return null;
@@ -4308,7 +4320,7 @@ class Code extends HeapObject implements M.Code {
     }
     {
       // Load the script and then update descriptors.
-      var script = function!.location!.script;
+      var script = function.location.script;
       return script.load().then((_) => _updateDescriptors(script));
     }
   }
@@ -4337,8 +4349,8 @@ class Code extends HeapObject implements M.Code {
     _loaded = true;
     startAddress = int.parse(m['_startAddress'], radix: 16);
     endAddress = int.parse(m['_endAddress'], radix: 16);
-    function = isolate!.getFromMap(m['function']) as ServiceFunction;
-    objectPool = isolate!.getFromMap(m['_objectPool']) as ObjectPool;
+    function = isolate.getFromMap(m['function']);
+    objectPool = isolate.getFromMap(m['_objectPool']);
     var disassembly = m['_disassembly'];
     if (disassembly != null) {
       _processDisassembly(disassembly);
@@ -4356,8 +4368,7 @@ class Code extends HeapObject implements M.Code {
       // Iterate and upgrade each ServiceFunction.
       for (var i = 0; i < inlinedFunctionsTable.length; i++) {
         // Upgrade each function and set it back in the list.
-        var func =
-            isolate!.getFromMap(inlinedFunctionsTable[i]) as ServiceFunction;
+        var func = isolate.getFromMap(inlinedFunctionsTable[i]);
         inlinedFunctionsTable[i] = func;
         if (!inlinedFunctions.contains(func)) {
           inlinedFunctions.add(func);
@@ -4375,7 +4386,7 @@ class Code extends HeapObject implements M.Code {
     super._update(m, mapIsRef);
   }
 
-  CodeInlineInterval? findInterval(int pc) {
+  CodeInlineInterval findInterval(int pc) {
     for (var i = 0; i < inlineIntervals.length; i++) {
       var interval = inlineIntervals[i];
       if (interval.contains(pc)) {
@@ -4409,8 +4420,7 @@ class Code extends HeapObject implements M.Code {
   void _processDisassembly(List disassembly) {
     assert(disassembly != null);
     instructions.clear();
-    instructionsByAddressOffset =
-        new List<CodeInstruction?>.filled(endAddress - startAddress, null);
+    instructionsByAddressOffset = new List(endAddress - startAddress);
 
     assert((disassembly.length % 4) == 0);
     for (var i = 0; i < disassembly.length; i += 4) {
@@ -4419,7 +4429,7 @@ class Code extends HeapObject implements M.Code {
       var human = disassembly[i + 2];
       var object = disassembly[i + 3];
       if (object != null) {
-        object = ServiceObject._fromMap(owner, object);
+        object = new ServiceObject._fromMap(owner, object);
       }
       var pcOffset = 0;
       if (disassembly[i] != null) {
@@ -4432,12 +4442,11 @@ class Code extends HeapObject implements M.Code {
       instructions.add(instruction);
       if (disassembly[i] != null) {
         // Not a code comment.
-        instructionsByAddressOffset![pcOffset] = instruction;
+        instructionsByAddressOffset[pcOffset] = instruction;
       }
     }
     for (var instruction in instructions) {
-      instruction._resolveJumpTarget(
-          instructionsByAddressOffset!, startAddress);
+      instruction._resolveJumpTarget(instructionsByAddressOffset, startAddress);
     }
   }
 
@@ -4450,7 +4459,7 @@ class Code extends HeapObject implements M.Code {
       var tryIndex = descriptor['tryIndex'];
       var kind = descriptor['kind'].trim();
 
-      var instruction = instructionsByAddressOffset![address - startAddress];
+      var instruction = instructionsByAddressOffset[address - startAddress];
       if (instruction != null) {
         instruction.descriptors
             .add(new PcDescriptor(pcOffset, deoptId, tokenPos, tryIndex, kind));
@@ -4513,16 +4522,16 @@ class SocketStats {
 /// OS pipes. Each socket is owned by another ServceObject, for example,
 /// a process or an HTTP server.
 class Socket extends ServiceObject {
-  Socket._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Socket._empty(ServiceObjectOwner owner) : super._empty(owner);
 
-  ServiceObject? socketOwner;
+  ServiceObject socketOwner;
 
   bool get isPipe => (kind == SocketKind.Pipe);
 
-  SocketStats? latest;
-  SocketStats? previous;
+  SocketStats latest;
+  SocketStats previous;
 
-  SocketKind? kind;
+  SocketKind kind;
 
   String protocol = '';
 
@@ -4533,12 +4542,12 @@ class Socket extends ServiceObject {
   /// Listening for connections.
   bool listening = false;
 
-  int? fd;
+  int fd;
 
-  String? localAddress;
-  int? localPort;
-  String? remoteAddress;
-  int? remotePort;
+  String localAddress;
+  int localPort;
+  String remoteAddress;
+  int remotePort;
 
   // Updates internal state from [map]. [map] can be a reference.
   void _update(Map map, bool mapIsRef) {
@@ -4573,20 +4582,20 @@ class Socket extends ServiceObject {
 }
 
 class ServiceMetric extends ServiceObject implements M.Metric {
-  ServiceMetric._empty(ServiceObjectOwner? owner) : super._empty(owner) {}
+  ServiceMetric._empty(ServiceObjectOwner owner) : super._empty(owner) {}
 
   bool get immutable => false;
 
   Future<Map> _fetchDirect({int count: kDefaultFieldLimit}) {
     assert(owner is Isolate);
-    return isolate!.invokeRpcNoUpgrade('_getIsolateMetric', {'metricId': id});
+    return isolate.invokeRpcNoUpgrade('_getIsolateMetric', {'metricId': id});
   }
 
-  String? description;
+  String description;
   double value = 0.0;
   // Only a gauge has a non-null min and max.
-  double? min;
-  double? max;
+  double min;
+  double max;
 
   bool get isGauge => (min != null) && (max != null);
 
@@ -4612,14 +4621,14 @@ Future<Null> printFrames(List frames) async {
 
 class Frame extends ServiceObject implements M.Frame {
   M.FrameKind kind = M.FrameKind.regular;
-  int? index;
-  ServiceFunction? function;
-  SourceLocation? location;
-  Code? code;
+  int index;
+  ServiceFunction function;
+  SourceLocation location;
+  Code code;
   List<ServiceMap> variables = <ServiceMap>[];
-  String? marker;
+  String marker;
 
-  Frame._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  Frame._empty(ServiceObject owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     assert(!mapIsRef);
@@ -4658,7 +4667,7 @@ class Frame extends ServiceObject implements M.Frame {
 
   String toString() {
     if (function != null) {
-      return "Frame([$kind] ${function!.qualifiedName} $location)";
+      return "Frame([$kind] ${function.qualifiedName} $location)";
     } else if (location != null) {
       return "Frame([$kind] $location)";
     } else {
@@ -4668,10 +4677,10 @@ class Frame extends ServiceObject implements M.Frame {
 
   Future<String> toUserString() async {
     if (function != null) {
-      return "Frame([$kind] ${function!.qualifiedName} "
-          "${await location!.toUserString()})";
+      return "Frame([$kind] ${function.qualifiedName} "
+          "${await location.toUserString()})";
     } else if (location != null) {
-      return "Frame([$kind] ${await location!.toUserString()}";
+      return "Frame([$kind] ${await location.toUserString()}";
     } else {
       return "Frame([$kind])";
     }
@@ -4679,13 +4688,13 @@ class Frame extends ServiceObject implements M.Frame {
 }
 
 class ServiceMessage extends ServiceObject {
-  int? index;
-  String? messageObjectId;
-  int? size;
-  ServiceFunction? handler;
-  SourceLocation? location;
+  int index;
+  String messageObjectId;
+  int size;
+  ServiceFunction handler;
+  SourceLocation location;
 
-  ServiceMessage._empty(ServiceObjectOwner? owner) : super._empty(owner);
+  ServiceMessage._empty(ServiceObject owner) : super._empty(owner);
 
   void _update(Map map, bool mapIsRef) {
     assert(!mapIsRef);
@@ -4726,14 +4735,14 @@ Set<int> getPossibleBreakpointLines(ServiceMap report, Script script) {
         var possibleBpts = range['possibleBreakpoints'];
         if (possibleBpts != null) {
           for (var tokenPos in possibleBpts) {
-            result.add(script.tokenToLine(tokenPos!)!);
+            result.add(script.tokenToLine(tokenPos));
           }
         }
       } else {
-        int startLine = script.tokenToLine(range['startPos'])!;
-        int endLine = script.tokenToLine(range['endPos'])!;
+        int startLine = script.tokenToLine(range['startPos']);
+        int endLine = script.tokenToLine(range['endPos']);
         for (int line = startLine; line <= endLine; line++) {
-          if (!script.getLine(line)!.isTrivial) {
+          if (!script.getLine(line).isTrivial) {
             result.add(line);
           }
         }
@@ -4755,7 +4764,7 @@ String _stripRef(String type) => (_hasRef(type) ? type.substring(1) : type);
 /// Recursively upgrades all [ServiceObject]s inside [collection] which must
 /// be an [Map] or an [List]. Upgraded elements will be
 /// associated with [vm] and [isolate].
-void _upgradeCollection(collection, ServiceObjectOwner? owner) {
+void _upgradeCollection(collection, ServiceObjectOwner owner) {
   if (collection is ServiceMap) {
     return; // Already upgraded.
   }
@@ -4767,10 +4776,10 @@ void _upgradeCollection(collection, ServiceObjectOwner? owner) {
   }
 }
 
-void _upgradeMap(Map map, ServiceObjectOwner? owner) {
+void _upgradeMap(Map map, ServiceObjectOwner owner) {
   map.forEach((k, v) {
     if ((v is Map) && _isServiceMap(v)) {
-      map[k] = owner!.getFromMap(v);
+      map[k] = owner.getFromMap(v);
     } else if (v is List) {
       _upgradeList(v, owner);
     } else if (v is Map) {
@@ -4779,7 +4788,7 @@ void _upgradeMap(Map map, ServiceObjectOwner? owner) {
   });
 }
 
-void _upgradeList(List list, ServiceObjectOwner? owner) {
+void _upgradeList(List list, ServiceObjectOwner owner) {
   if (list is Uint8List) {
     // Nothing to upgrade; avoid slowly visiting every byte
     // of large binary responses.
@@ -4789,7 +4798,7 @@ void _upgradeList(List list, ServiceObjectOwner? owner) {
   for (var i = 0; i < list.length; i++) {
     var v = list[i];
     if ((v is Map) && _isServiceMap(v)) {
-      list[i] = owner!.getFromMap(v);
+      list[i] = owner.getFromMap(v);
     } else if (v is List) {
       _upgradeList(v, owner);
     } else if (v is Map) {
diff --git a/runtime/observatory/lib/tracer.dart b/runtime/observatory/lib/tracer.dart
index 3f9a07fe0d3..3a18cfd7294 100644
--- a/runtime/observatory/lib/tracer.dart
+++ b/runtime/observatory/lib/tracer.dart
@@ -27,7 +27,7 @@ _deepCopy(src) {
 }
 
 class TraceEvent {
-  TraceEvent.msg(this._time, this.message, Map? originalMap) {
+  TraceEvent.msg(this._time, this.message, Map originalMap) {
     map = _deepCopy(originalMap);
   }
 
@@ -39,14 +39,14 @@ class TraceEvent {
 
   int _time;
   String message;
-  Map? map;
+  Map map;
 }
 
 class Tracer {
   // The current global tracer.
-  static Tracer? get current => _current;
+  static Tracer get current => _current;
 
-  static Tracer? _current;
+  static Tracer _current;
 
   static void start() {
     if (_current == null) {
@@ -56,23 +56,23 @@ class Tracer {
 
   static void stop() {
     if (_current != null) {
-      _current!.cancel();
+      _current.cancel();
       _current = null;
     }
   }
 
   // The tracer subscribes to all logging events.
-  StreamSubscription? loggerSub = null;
+  StreamSubscription loggerSub = null;
 
   // The start time for the current request.
-  Stopwatch? _time;
+  Stopwatch _time;
 
   // A list of all tracing events for thre current request.
   List<TraceEvent> events = <TraceEvent>[];
 
   Tracer() {
     _time = new Stopwatch();
-    _time!.start();
+    _time.start();
     loggerSub = Logger.root.onRecord.listen((LogRecord rec) {
       // Echo all log messages to the trace.
       trace('${rec.level.name}: ${rec.message}');
@@ -81,16 +81,16 @@ class Tracer {
   }
 
   void cancel() {
-    loggerSub!.cancel();
+    loggerSub.cancel();
   }
 
   void reset() {
-    _time!.reset();
+    _time.reset();
     events.clear();
   }
 
-  TraceEvent trace(String message, {Map? map}) {
-    var event = new TraceEvent.msg(_time!.elapsedMicroseconds, message, map);
+  TraceEvent trace(String message, {Map map: null}) {
+    var event = new TraceEvent.msg(_time.elapsedMicroseconds, message, map);
     events.add(event);
     return event;
   }
diff --git a/runtime/observatory/lib/utils.dart b/runtime/observatory/lib/utils.dart
index 940a6e16f11..821e88f7882 100644
--- a/runtime/observatory/lib/utils.dart
+++ b/runtime/observatory/lib/utils.dart
@@ -29,7 +29,7 @@ class Utils {
   static String zeroPad(int value, int pad) {
     String prefix = "";
     while (pad > 1) {
-      int pow10 = pow(10, pad - 1) as int;
+      int pow10 = pow(10, pad - 1);
       if (value < pow10) {
         prefix = prefix + "0";
       }
@@ -289,7 +289,7 @@ class Utils {
 
 /// A [Task] that can be scheduled on the Dart event queue.
 class Task {
-  Timer? _timer;
+  Timer _timer;
   final Function callback;
 
   Task(this.callback);
diff --git a/runtime/observatory/pubspec.yaml b/runtime/observatory/pubspec.yaml
index b273980cfbe..8c4c3783a1e 100644
--- a/runtime/observatory/pubspec.yaml
+++ b/runtime/observatory/pubspec.yaml
@@ -1,3 +1,3 @@
 name: observatory
 environment:
-  sdk: '>=2.9.0 <3.0.0'
+  sdk: '>=2.2.2 <3.0.0'
\ No newline at end of file
diff --git a/runtime/observatory/tests/observatory_ui/app_test.dart b/runtime/observatory/tests/observatory_ui/app_test.dart
index 9622ec4ad0b..0fab60c974b 100644
--- a/runtime/observatory/tests/observatory_ui/app_test.dart
+++ b/runtime/observatory/tests/observatory_ui/app_test.dart
@@ -14,7 +14,7 @@ main() async {
       print('${rec.level.name}: ${rec.time}: ${rec.message}');
     });
     Logger.root.info('Starting Observatory');
-    document.body!.children
+    document.body.children
         .insert(0, new ObservatoryApplicationElement.created().element);
   });
 }
diff --git a/runtime/observatory/tests/service/add_breakpoint_rpc_kernel_test.dart b/runtime/observatory/tests/service/add_breakpoint_rpc_kernel_test.dart
index 96ae4b5783a..34feaf0df0f 100644
--- a/runtime/observatory/tests/service/add_breakpoint_rpc_kernel_test.dart
+++ b/runtime/observatory/tests/service/add_breakpoint_rpc_kernel_test.dart
@@ -38,15 +38,15 @@ var tests = <IsolateTest>[
     var futureBpt1 = await isolate.addBreakpoint(script, LINE_A);
     expect(futureBpt1.number, equals(1));
     expect(futureBpt1.resolved, isFalse);
-    expect(await futureBpt1.location!.getLine(), equals(LINE_A));
-    expect(await futureBpt1.location!.getColumn(), equals(null));
+    expect(await futureBpt1.location.getLine(), equals(LINE_A));
+    expect(await futureBpt1.location.getColumn(), equals(null));
 
     // Future breakpoint with specific column.
     var futureBpt2 = await isolate.addBreakpoint(script, LINE_A, 3);
     expect(futureBpt2.number, equals(2));
     expect(futureBpt2.resolved, isFalse);
-    expect(await futureBpt2.location!.getLine(), equals(LINE_A));
-    expect(await futureBpt2.location!.getColumn(), equals(3));
+    expect(await futureBpt2.location.getLine(), equals(LINE_A));
+    expect(await futureBpt2.location.getColumn(), equals(3));
 
     int resolvedCount =
         await resumeAndCountResolvedBreakpointsUntilPause(isolate);
@@ -54,21 +54,21 @@ var tests = <IsolateTest>[
     // After resolution the breakpoints have assigned line & column.
     expect(resolvedCount, equals(2));
     expect(futureBpt1.resolved, isTrue);
-    expect(await futureBpt1.location!.getLine(), equals(LINE_A));
-    expect(await futureBpt1.location!.getColumn(), equals(12));
+    expect(await futureBpt1.location.getLine(), equals(LINE_A));
+    expect(await futureBpt1.location.getColumn(), equals(12));
     expect(futureBpt2.resolved, isTrue);
-    expect(await futureBpt2.location!.getLine(), equals(LINE_A));
-    expect(await futureBpt2.location!.getColumn(), equals(3));
+    expect(await futureBpt2.location.getLine(), equals(LINE_A));
+    expect(await futureBpt2.location.getColumn(), equals(3));
 
     // The first breakpoint hits before value is modified.
-    Instance result = await rootLib.evaluate('value') as Instance;
+    Instance result = await rootLib.evaluate('value');
     expect(result.valueAsString, equals('0'));
 
     isolate.resume();
     await hasStoppedAtBreakpoint(isolate);
 
     // The second breakpoint hits after value has been modified once.
-    result = await rootLib.evaluate('value') as Instance;
+    result = await rootLib.evaluate('value');
     expect(result.valueAsString, equals('1'));
 
     // Remove the breakpoints.
@@ -85,8 +85,8 @@ var tests = <IsolateTest>[
     for (int col = 1; col <= 50; col++) {
       var bpt = await isolate.addBreakpoint(script, LINE_A, col);
       expect(bpt.resolved, isTrue);
-      int resolvedLine = await bpt.location!.getLine() as int;
-      int resolvedCol = await bpt.location!.getColumn() as int;
+      int resolvedLine = await bpt.location.getLine();
+      int resolvedCol = await bpt.location.getColumn();
       print('$LINE_A:${col} -> ${resolvedLine}:${resolvedCol}');
       if (col <= 12) {
         expect(resolvedLine, equals(LINE_A));
diff --git a/runtime/observatory/tests/service/allocations_test.dart b/runtime/observatory/tests/service/allocations_test.dart
index 6a7d002f618..b2d9085fc07 100644
--- a/runtime/observatory/tests/service/allocations_test.dart
+++ b/runtime/observatory/tests/service/allocations_test.dart
@@ -13,7 +13,7 @@ class Foo {}
 // Prevent TFA from removing this static field to ensure the objects are kept
 // alive, so the allocation stats will report them via the service api.
 @pragma('vm:entry-point')
-List<Foo>? foos;
+List<Foo> foos;
 
 void script() {
   foos = [new Foo(), new Foo(), new Foo()];
diff --git a/runtime/observatory/tests/service/async_single_step_exception_test.dart b/runtime/observatory/tests/service/async_single_step_exception_test.dart
index d67f8c0c122..6393d325e6e 100644
--- a/runtime/observatory/tests/service/async_single_step_exception_test.dart
+++ b/runtime/observatory/tests/service/async_single_step_exception_test.dart
@@ -27,7 +27,7 @@ testMain() async {
   print('mmmmm'); // LINE_C.
   try {
     await helper(); // LINE_D.
-  } on dynamic catch (e) {
+  } catch (e) {
     // arrive here on error.
     print('error: $e'); // LINE_E.
   } finally {
diff --git a/runtime/observatory/tests/service/auth_token_test.dart b/runtime/observatory/tests/service/auth_token_test.dart
index 27a0b47ed00..f2ba2758b8b 100644
--- a/runtime/observatory/tests/service/auth_token_test.dart
+++ b/runtime/observatory/tests/service/auth_token_test.dart
@@ -17,13 +17,13 @@ Future<Null> testeeBefore() async {
   ServiceProtocolInfo info = await Service.controlWebServer(enable: true);
   Expect.isNotNull(info.serverUri);
   // Ensure that we have the auth token in the path segments.
-  Expect.isTrue(info.serverUri!.pathSegments.length > 1);
+  Expect.isTrue(info.serverUri.pathSegments.length > 1);
   // Sanity check the length of the auth token.
-  Expect.isTrue(info.serverUri!.pathSegments[0].length > 8);
+  Expect.isTrue(info.serverUri.pathSegments[0].length > 8);
 
   // Try connecting to the server without the auth token, it should throw
   // an exception.
-  var port = info.serverUri!.port;
+  var port = info.serverUri.port;
   var url = Uri.parse('http://localhost:$port');
   var httpClient = new io.HttpClient();
   try {
@@ -35,7 +35,7 @@ Future<Null> testeeBefore() async {
 
   // Try connecting to the server with the auth token, it should succeed.
   try {
-    var request = await httpClient.getUrl(info.serverUri!);
+    var request = await httpClient.getUrl(info.serverUri);
   } catch (e) {
     fail('could not connect');
   }
diff --git a/runtime/observatory/tests/service/bad_reload_test.dart b/runtime/observatory/tests/service/bad_reload_test.dart
index eb40b71f280..9c4219c8b70 100644
--- a/runtime/observatory/tests/service/bad_reload_test.dart
+++ b/runtime/observatory/tests/service/bad_reload_test.dart
@@ -32,9 +32,9 @@ Future<String> invokeTest(Isolate isolate) async {
   await isolate.reload();
   Library lib = isolate.rootLibrary;
   await lib.load();
-  Instance result = await lib.evaluate('test()') as Instance;
+  Instance result = await lib.evaluate('test()');
   expect(result.isString, isTrue);
-  return result.valueAsString as String;
+  return result.valueAsString;
 }
 
 var tests = <IsolateTest>[
diff --git a/runtime/observatory/tests/service/bad_web_socket_address_test.dart b/runtime/observatory/tests/service/bad_web_socket_address_test.dart
index 69ac6062669..8f83fd8c655 100644
--- a/runtime/observatory/tests/service/bad_web_socket_address_test.dart
+++ b/runtime/observatory/tests/service/bad_web_socket_address_test.dart
@@ -24,5 +24,5 @@ main() async {
     print('${rec.level.name}: ${rec.time}: ${rec.message}');
   });
 
-  test('bad web socket address', testBadWebSocket);
+  await test('bad web socket address', testBadWebSocket);
 }
diff --git a/runtime/observatory/tests/service/break_on_activation_test.dart b/runtime/observatory/tests/service/break_on_activation_test.dart
index 1b6191b68d2..fac268454ad 100644
--- a/runtime/observatory/tests/service/break_on_activation_test.dart
+++ b/runtime/observatory/tests/service/break_on_activation_test.dart
@@ -48,7 +48,7 @@ void testeeDoNamed() {
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library rootLib = await isolate.rootLibrary.load() as Library;
+    Library rootLib = await isolate.rootLibrary.load();
 
     var breaksHit = 0;
 
@@ -92,7 +92,7 @@ var tests = <IsolateTest>[
     await cancelFutureSubscription(subscriptionFuture);
   },
   (Isolate isolate) async {
-    Library rootLib = await isolate.rootLibrary.load() as Library;
+    Library rootLib = await isolate.rootLibrary.load();
 
     var breaksHit = 0;
 
@@ -136,7 +136,7 @@ var tests = <IsolateTest>[
     await cancelFutureSubscription(subscriptionFuture);
   },
   (Isolate isolate) async {
-    Library rootLib = await isolate.rootLibrary.load() as Library;
+    Library rootLib = await isolate.rootLibrary.load();
 
     var breaksHit = 0;
 
diff --git a/runtime/observatory/tests/service/break_on_async_function_test.dart b/runtime/observatory/tests/service/break_on_async_function_test.dart
index ba6bb1881da..dd52294e35d 100644
--- a/runtime/observatory/tests/service/break_on_async_function_test.dart
+++ b/runtime/observatory/tests/service/break_on_async_function_test.dart
@@ -26,7 +26,7 @@ var tests = <IsolateTest>[
 
 // Add breakpoint at the entry of async function
   (Isolate isolate) async {
-    Library rootLib = await isolate.rootLibrary.load() as Library;
+    Library rootLib = await isolate.rootLibrary.load();
     var function =
         rootLib.functions.singleWhere((f) => f.name == 'testFunction');
 
diff --git a/runtime/observatory/tests/service/break_on_default_constructor_test.dart b/runtime/observatory/tests/service/break_on_default_constructor_test.dart
index 83e09fafbaf..d968ca21cfa 100644
--- a/runtime/observatory/tests/service/break_on_default_constructor_test.dart
+++ b/runtime/observatory/tests/service/break_on_default_constructor_test.dart
@@ -51,7 +51,7 @@ var tests = <IsolateTest>[
     if (loc.valid) {
       if (loc.function != null) {
         try {
-          await debugger.isolate.addBreakpointAtEntry(loc.function!);
+          await debugger.isolate.addBreakpointAtEntry(loc.function);
         } on S.ServerRpcException catch (e) {
           if (e.code == S.ServerRpcException.kCannotAddBreakpoint) {
             // Expected
diff --git a/runtime/observatory/tests/service/break_on_function_test.dart b/runtime/observatory/tests/service/break_on_function_test.dart
index acdf910ff3f..89f60fb3202 100644
--- a/runtime/observatory/tests/service/break_on_function_test.dart
+++ b/runtime/observatory/tests/service/break_on_function_test.dart
@@ -32,7 +32,7 @@ var tests = <IsolateTest>[
 
 // Add breakpoint
   (Isolate isolate) async {
-    Library rootLib = await isolate.rootLibrary.load() as Library;
+    Library rootLib = await isolate.rootLibrary.load();
     var function =
         rootLib.functions.singleWhere((f) => f.name == 'testFunction');
 
diff --git a/runtime/observatory/tests/service/breakpoint_async_break_test.dart b/runtime/observatory/tests/service/breakpoint_async_break_test.dart
index 671577ec344..b59613eaa4d 100644
--- a/runtime/observatory/tests/service/breakpoint_async_break_test.dart
+++ b/runtime/observatory/tests/service/breakpoint_async_break_test.dart
@@ -33,8 +33,8 @@ var tests = <IsolateTest>[
     var futureBpt = await isolate.addBreakpoint(script, LINE);
     expect(futureBpt.number, 1);
     expect(futureBpt.resolved, isFalse);
-    expect(await futureBpt.location!.getLine(), LINE);
-    expect(await futureBpt.location!.getColumn(), null);
+    expect(await futureBpt.location.getLine(), LINE);
+    expect(await futureBpt.location.getColumn(), null);
 
     var stream = await isolate.vm.getEventStream(VM.kDebugStream);
     Completer completer = new Completer();
@@ -55,8 +55,8 @@ var tests = <IsolateTest>[
     // After resolution the breakpoints have assigned line & column.
     expect(resolvedCount, 1);
     expect(futureBpt.resolved, isTrue);
-    expect(await futureBpt.location!.getLine(), LINE);
-    expect(await futureBpt.location!.getColumn(), 7);
+    expect(await futureBpt.location.getLine(), LINE);
+    expect(await futureBpt.location.getColumn(), 7);
 
     // Remove the breakpoints.
     expect((await isolate.removeBreakpoint(futureBpt)).type, 'Success');
diff --git a/runtime/observatory/tests/service/breakpoint_non_debuggable_library_test.dart b/runtime/observatory/tests/service/breakpoint_non_debuggable_library_test.dart
index e4339b408b0..1c7af666465 100644
--- a/runtime/observatory/tests/service/breakpoint_non_debuggable_library_test.dart
+++ b/runtime/observatory/tests/service/breakpoint_non_debuggable_library_test.dart
@@ -54,7 +54,7 @@ var tests = <IsolateTest>[
       'isDebuggable': false,
     };
     Map<String, dynamic> result = await isolate.invokeRpcNoUpgrade(
-        'setLibraryDebuggable', setDebugParams) as Map<String, dynamic>;
+        'setLibraryDebuggable', setDebugParams);
     expect(result['type'], 'Success');
     await has_part.reload();
     expect(has_part.debuggable, false);
@@ -63,7 +63,7 @@ var tests = <IsolateTest>[
     // Breakpoints are not allowed to set on non-debuggable libraries.
     try {
       await isolate.addBreakpoint(script, LINE_B);
-    } on dynamic catch (e) {
+    } catch (e) {
       expect(e is ServerRpcException, true);
       expect(e.code == ServerRpcException.kCannotAddBreakpoint, true);
       print("Set Breakpoint to non-debuggable library is not allowed");
diff --git a/runtime/observatory/tests/service/breakpoint_two_args_checked_test.dart b/runtime/observatory/tests/service/breakpoint_two_args_checked_test.dart
index 9832fa41bda..47235458965 100644
--- a/runtime/observatory/tests/service/breakpoint_two_args_checked_test.dart
+++ b/runtime/observatory/tests/service/breakpoint_two_args_checked_test.dart
@@ -19,7 +19,7 @@ const int LINE_C = 28;
 class NotGeneric {}
 
 testeeMain() {
-  var x = new List<dynamic>.filled(1, null);
+  var x = new List(1);
   var y = 7;
   debugger();
   print("Statement");
@@ -33,23 +33,23 @@ var tests = <IsolateTest>[
 
 // Add breakpoints.
   (Isolate isolate) async {
-    Library rootLib = await isolate.rootLibrary.load() as Library;
+    Library rootLib = await isolate.rootLibrary.load();
     var script = rootLib.scripts[0];
 
     var bpt1 = await isolate.addBreakpoint(script, LINE_A);
     print(bpt1);
     expect(bpt1.resolved, isTrue);
-    expect(await bpt1.location!.getLine(), equals(LINE_A));
+    expect(await bpt1.location.getLine(), equals(LINE_A));
 
     var bpt2 = await isolate.addBreakpoint(script, LINE_B);
     print(bpt2);
     expect(bpt2.resolved, isTrue);
-    expect(await bpt2.location!.getLine(), equals(LINE_B));
+    expect(await bpt2.location.getLine(), equals(LINE_B));
 
     var bpt3 = await isolate.addBreakpoint(script, LINE_C);
     print(bpt3);
     expect(bpt3.resolved, isTrue);
-    expect(await bpt3.location!.getLine(), equals(LINE_C));
+    expect(await bpt3.location.getLine(), equals(LINE_C));
   },
 
   resumeIsolate,
diff --git a/runtime/observatory/tests/service/caching_test.dart b/runtime/observatory/tests/service/caching_test.dart
index 9f374f79734..0c89eca9073 100644
--- a/runtime/observatory/tests/service/caching_test.dart
+++ b/runtime/observatory/tests/service/caching_test.dart
@@ -14,9 +14,9 @@ import 'test_helper.dart';
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
-    Script script = await lib.scripts.single.load() as Script;
-    Script script2 = await isolate.getObject(script.id!) as Script;
+    Library lib = await isolate.rootLibrary.load();
+    Script script = await lib.scripts.single.load();
+    Script script2 = await isolate.getObject(script.id);
     expect(identical(script, script2), isTrue);
   },
 ];
diff --git a/runtime/observatory/tests/service/client_resume_approvals_approve_then_disconnect_test.dart b/runtime/observatory/tests/service/client_resume_approvals_approve_then_disconnect_test.dart
index 3b4fda0c390..439f170b9cf 100644
--- a/runtime/observatory/tests/service/client_resume_approvals_approve_then_disconnect_test.dart
+++ b/runtime/observatory/tests/service/client_resume_approvals_approve_then_disconnect_test.dart
@@ -16,13 +16,13 @@ void fooBar() {
   print(i);
 }
 
-late WebSocketVM client1;
-late WebSocketVM client2;
+WebSocketVM client1;
+WebSocketVM client2;
 
 final test = <IsolateTest>[
   hasPausedAtStart,
   (Isolate isolate) async {
-    client1 = await createClient(isolate.owner as WebSocketVM);
+    client1 = await createClient(isolate.owner);
     await setRequireApprovalForResume(
       client1,
       isolate,
@@ -30,7 +30,7 @@ final test = <IsolateTest>[
       pauseOnExit: true,
     );
     client2 = await createClient(
-      isolate.owner as WebSocketVM,
+      isolate.owner,
       clientName: otherClientName,
     );
     await setRequireApprovalForResume(
diff --git a/runtime/observatory/tests/service/client_resume_approvals_common.dart b/runtime/observatory/tests/service/client_resume_approvals_common.dart
index b6a80063481..a2221e57103 100644
--- a/runtime/observatory/tests/service/client_resume_approvals_common.dart
+++ b/runtime/observatory/tests/service/client_resume_approvals_common.dart
@@ -81,5 +81,5 @@ Future<void> waitForResume(Isolate isolate) async {
       }
     });
   });
-  await completer.future;
+  return completer.future;
 }
diff --git a/runtime/observatory/tests/service/client_resume_approvals_disconnect_test.dart b/runtime/observatory/tests/service/client_resume_approvals_disconnect_test.dart
index ce23eb98cdf..d82317af6bb 100644
--- a/runtime/observatory/tests/service/client_resume_approvals_disconnect_test.dart
+++ b/runtime/observatory/tests/service/client_resume_approvals_disconnect_test.dart
@@ -16,14 +16,14 @@ void fooBar() {
   print(i);
 }
 
-late WebSocketVM client1;
-late WebSocketVM client2;
+WebSocketVM client1;
+WebSocketVM client2;
 
 final test = <IsolateTest>[
   // Multiple clients, disconnect client awaiting approval.
   hasPausedAtStart,
   (Isolate isolate) async {
-    client1 = await createClient(isolate.owner as WebSocketVM);
+    client1 = await createClient(isolate.owner);
     await setRequireApprovalForResume(
       client1,
       isolate,
@@ -31,7 +31,7 @@ final test = <IsolateTest>[
       pauseOnExit: true,
     );
     client2 = await createClient(
-      isolate.owner as WebSocketVM,
+      isolate.owner,
       clientName: otherClientName,
     );
     await setRequireApprovalForResume(
diff --git a/runtime/observatory/tests/service/client_resume_approvals_identical_names_test.dart b/runtime/observatory/tests/service/client_resume_approvals_identical_names_test.dart
index 1b746ff4d29..dbcc14a559a 100644
--- a/runtime/observatory/tests/service/client_resume_approvals_identical_names_test.dart
+++ b/runtime/observatory/tests/service/client_resume_approvals_identical_names_test.dart
@@ -16,22 +16,22 @@ void fooBar() {
   print(i);
 }
 
-late WebSocketVM client1;
-late WebSocketVM client2;
+WebSocketVM client1;
+WebSocketVM client2;
 
 final sameClientNamesTest = <IsolateTest>[
   // Multiple clients, same client names.
   (Isolate isolate) async {
     final resumeFuture = waitForResume(isolate);
 
-    client1 = await createClient(isolate.owner as WebSocketVM);
+    client1 = await createClient(isolate.owner);
     await setRequireApprovalForResume(
       client1,
       isolate,
       pauseOnStart: true,
       pauseOnExit: true,
     );
-    client2 = await createClient(isolate.owner as WebSocketVM);
+    client2 = await createClient(isolate.owner);
 
     expect(await isPausedAtStart(isolate), true);
     await resume(client2, isolate);
diff --git a/runtime/observatory/tests/service/client_resume_approvals_multiple_names_test.dart b/runtime/observatory/tests/service/client_resume_approvals_multiple_names_test.dart
index ccd29fdb795..88d217d06f2 100644
--- a/runtime/observatory/tests/service/client_resume_approvals_multiple_names_test.dart
+++ b/runtime/observatory/tests/service/client_resume_approvals_multiple_names_test.dart
@@ -16,14 +16,14 @@ void fooBar() {
   print(i);
 }
 
-late WebSocketVM client1;
-late WebSocketVM client2;
-late WebSocketVM client3;
+WebSocketVM client1;
+WebSocketVM client2;
+WebSocketVM client3;
 
 final multipleClientNamesTest = <IsolateTest>[
   // Multiple clients, different client names.
   (Isolate isolate) async {
-    client1 = await createClient(isolate.owner as WebSocketVM);
+    client1 = await createClient(isolate.owner);
     await setRequireApprovalForResume(
       client1,
       isolate,
@@ -31,11 +31,10 @@ final multipleClientNamesTest = <IsolateTest>[
       pauseOnExit: true,
     );
     client2 = await createClient(
-      isolate.owner as WebSocketVM,
+      isolate.owner,
       clientName: otherClientName,
     );
-    client3 = await createClient(isolate.owner as WebSocketVM,
-        clientName: 'DummyClient');
+    client3 = await createClient(isolate.owner, clientName: 'DummyClient');
 
     final resumeFuture = waitForResume(isolate);
     expect(await isPausedAtStart(isolate), true);
diff --git a/runtime/observatory/tests/service/client_resume_approvals_name_change_test.dart b/runtime/observatory/tests/service/client_resume_approvals_name_change_test.dart
index 6ccdaf7c19b..0e1374e2086 100644
--- a/runtime/observatory/tests/service/client_resume_approvals_name_change_test.dart
+++ b/runtime/observatory/tests/service/client_resume_approvals_name_change_test.dart
@@ -16,9 +16,9 @@ void fooBar() {
   print(i);
 }
 
-late WebSocketVM client1;
-late WebSocketVM client2;
-late WebSocketVM client3;
+WebSocketVM client1;
+WebSocketVM client2;
+WebSocketVM client3;
 
 final nameChangeTest = <IsolateTest>[
   // Remove required approvals via name change.
@@ -26,16 +26,15 @@ final nameChangeTest = <IsolateTest>[
     final resumeFuture = waitForResume(isolate);
 
     // Create two clients with the same name.
-    client1 = await createClient(isolate.owner as WebSocketVM);
-    client2 = await createClient(isolate.owner as WebSocketVM);
+    client1 = await createClient(isolate.owner);
+    client2 = await createClient(isolate.owner);
     await setRequireApprovalForResume(
       client1,
       isolate,
       pauseOnStart: true,
       pauseOnExit: true,
     );
-    client3 = await createClient(isolate.owner as WebSocketVM,
-        clientName: otherClientName);
+    client3 = await createClient(isolate.owner, clientName: otherClientName);
 
     // Check that client3 can't resume the isolate on its own.
     expect(await isPausedAtStart(isolate), true);
diff --git a/runtime/observatory/tests/service/client_resume_approvals_reload_test.dart b/runtime/observatory/tests/service/client_resume_approvals_reload_test.dart
index e4dec0a9ea7..08fdc104cd5 100644
--- a/runtime/observatory/tests/service/client_resume_approvals_reload_test.dart
+++ b/runtime/observatory/tests/service/client_resume_approvals_reload_test.dart
@@ -19,22 +19,22 @@ void fooBar() {
   print(i);
 }
 
-late WebSocketVM client1;
-late WebSocketVM client2;
+WebSocketVM client1;
+WebSocketVM client2;
 
 final hotReloadTest = <IsolateTest>[
   // Multiple clients, hot reload approval.
   (Isolate isolate) async {
     final resumeFuture = waitForResume(isolate);
 
-    client1 = await createClient(isolate.owner as WebSocketVM);
+    client1 = await createClient(isolate.owner);
     await setRequireApprovalForResume(
       client1,
       isolate,
       pauseOnReload: true,
     );
     client2 = await createClient(
-      isolate.owner as WebSocketVM,
+      isolate.owner,
       clientName: otherClientName,
     );
     await setRequireApprovalForResume(
diff --git a/runtime/observatory/tests/service/code_test.dart b/runtime/observatory/tests/service/code_test.dart
index 00bd9daa82e..c08ef61c19c 100644
--- a/runtime/observatory/tests/service/code_test.dart
+++ b/runtime/observatory/tests/service/code_test.dart
@@ -66,15 +66,15 @@ var tests = <IsolateTest>[
 
     List tests = <IsolateTest>[];
     // Load code from frame 0.
-    Code code = await isolate.getObject(codeId0) as Code;
+    Code code = await isolate.getObject(codeId0);
     expect(code.type, equals('Code'));
-    expect(code.function!.name, equals('funcB'));
+    expect(code.function.name, equals('funcB'));
     expect(code.hasDisassembly, equals(true));
 
     // Load code from frame 0.
-    code = await isolate.getObject(codeId1) as Code;
+    code = await isolate.getObject(codeId1);
     expect(code.type, equals('Code'));
-    expect(code.function!.name, equals('funcA'));
+    expect(code.function.name, equals('funcA'));
     expect(code.hasDisassembly, equals(true));
   },
 ];
diff --git a/runtime/observatory/tests/service/command_test.dart b/runtime/observatory/tests/service/command_test.dart
index f77ef0023b3..a23d6b454f1 100644
--- a/runtime/observatory/tests/service/command_test.dart
+++ b/runtime/observatory/tests/service/command_test.dart
@@ -9,17 +9,17 @@ import 'package:test/test.dart';
 
 class TestCommand extends Command {
   TestCommand(this.out, name, children) : super(name, children);
-  StringBuffer? out;
+  StringBuffer out;
 
   Future run(List<String> args) {
-    out!.write('executing ${name}(${args})\n');
+    out.write('executing ${name}(${args})\n');
     return new Future.value(null);
   }
 }
 
 class TestCompleteCommand extends Command {
   TestCompleteCommand(this.out, name, children) : super(name, children);
-  StringBuffer? out;
+  StringBuffer out;
 
   Future<List<String>> complete(List<String> args) {
     var possibles = ['one ', 'two ', 'three '];
@@ -28,7 +28,7 @@ class TestCompleteCommand extends Command {
   }
 
   Future run(List<String> args) {
-    out!.write('executing ${name}(${args})\n');
+    out.write('executing ${name}(${args})\n');
     return new Future.value(null);
   }
 }
diff --git a/runtime/observatory/tests/service/complex_reload_test.dart b/runtime/observatory/tests/service/complex_reload_test.dart
index a9cca6a8c31..f0c8b93e190 100644
--- a/runtime/observatory/tests/service/complex_reload_test.dart
+++ b/runtime/observatory/tests/service/complex_reload_test.dart
@@ -34,9 +34,9 @@ Future<String> invokeTest(Isolate isolate) async {
   await isolate.reload();
   Library lib = isolate.rootLibrary;
   await lib.load();
-  Instance result = await lib.evaluate('test()') as Instance;
+  Instance result = await lib.evaluate('test()');
   expect(result.isString, isTrue);
-  return result.valueAsString as String;
+  return result.valueAsString;
 }
 
 var tests = <IsolateTest>[
diff --git a/runtime/observatory/tests/service/contexts_test.dart b/runtime/observatory/tests/service/contexts_test.dart
index 41afedcdf2c..33021a1d27b 100644
--- a/runtime/observatory/tests/service/contexts_test.dart
+++ b/runtime/observatory/tests/service/contexts_test.dart
@@ -58,58 +58,58 @@ void script() {
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Field field = lib.variables.singleWhere((v) => v.name == 'cleanBlock');
     await field.load();
-    Instance block = await field.staticValue!.load() as Instance;
+    Instance block = await field.staticValue.load();
     expect(block.isClosure, isTrue);
     expect(block.closureContext, isNull);
   },
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Field field = lib.variables.singleWhere((v) => v.name == 'copyingBlock');
     await field.load();
-    Instance block = await field.staticValue!.load() as Instance;
+    Instance block = await field.staticValue.load();
     expect(block.isClosure, isTrue);
-    expect(block.closureContext!.isContext, isTrue);
-    expect(block.closureContext!.length, equals(1));
-    Context ctxt = await block.closureContext!.load() as Context;
-    expect(ctxt.variables!.single.value.asValue!.isString, isTrue);
-    expect(ctxt.variables!.single.value.asValue!.valueAsString,
+    expect(block.closureContext.isContext, isTrue);
+    expect(block.closureContext.length, equals(1));
+    Context ctxt = await block.closureContext.load();
+    expect(ctxt.variables.single.value.asValue.isString, isTrue);
+    expect(ctxt.variables.single.value.asValue.valueAsString,
         equals('I could be copied into the block'));
     expect(ctxt.parentContext, isNull);
   },
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Field field = lib.variables.singleWhere((v) => v.name == 'fullBlock');
     await field.load();
-    Instance block = await field.staticValue!.load() as Instance;
+    Instance block = await field.staticValue.load();
     expect(block.isClosure, isTrue);
-    expect(block.closureContext!.isContext, isTrue);
-    expect(block.closureContext!.length, equals(1));
-    Context ctxt = await block.closureContext!.load() as Context;
-    expect(ctxt.variables!.single.value.asValue!.isInt, isTrue);
-    expect(ctxt.variables!.single.value.asValue!.valueAsString, equals('43'));
+    expect(block.closureContext.isContext, isTrue);
+    expect(block.closureContext.length, equals(1));
+    Context ctxt = await block.closureContext.load();
+    expect(ctxt.variables.single.value.asValue.isInt, isTrue);
+    expect(ctxt.variables.single.value.asValue.valueAsString, equals('43'));
     expect(ctxt.parentContext, isNull);
   },
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Field field =
         lib.variables.singleWhere((v) => v.name == 'fullBlockWithChain');
     await field.load();
-    Instance block = await field.staticValue!.load() as Instance;
+    Instance block = await field.staticValue.load();
     expect(block.isClosure, isTrue);
-    expect(block.closureContext!.isContext, isTrue);
-    expect(block.closureContext!.length, equals(1));
-    Context ctxt = await block.closureContext!.load() as Context;
-    expect(ctxt.variables!.single.value.asValue!.isInt, isTrue);
-    expect(ctxt.variables!.single.value.asValue!.valueAsString, equals('4201'));
-    expect(ctxt.parentContext!.isContext, isTrue);
-    expect(ctxt.parentContext!.length, equals(1));
-    Context outerCtxt = await ctxt.parentContext!.load() as Context;
-    expect(outerCtxt.variables!.single.value.asValue!.isInt, isTrue);
-    expect(outerCtxt.variables!.single.value.asValue!.valueAsString,
-        equals('421'));
+    expect(block.closureContext.isContext, isTrue);
+    expect(block.closureContext.length, equals(1));
+    Context ctxt = await block.closureContext.load();
+    expect(ctxt.variables.single.value.asValue.isInt, isTrue);
+    expect(ctxt.variables.single.value.asValue.valueAsString, equals('4201'));
+    expect(ctxt.parentContext.isContext, isTrue);
+    expect(ctxt.parentContext.length, equals(1));
+    Context outerCtxt = await ctxt.parentContext.load();
+    expect(outerCtxt.variables.single.value.asValue.isInt, isTrue);
+    expect(
+        outerCtxt.variables.single.value.asValue.valueAsString, equals('421'));
     expect(outerCtxt.parentContext, isNull);
   },
 ];
diff --git a/runtime/observatory/tests/service/coverage_const_field_async_closure_test.dart b/runtime/observatory/tests/service/coverage_const_field_async_closure_test.dart
index 957251b45b2..a85a9c3b25b 100644
--- a/runtime/observatory/tests/service/coverage_const_field_async_closure_test.dart
+++ b/runtime/observatory/tests/service/coverage_const_field_async_closure_test.dart
@@ -55,7 +55,7 @@ var tests = <IsolateTest>[
     int match = 0;
     for (var range in ranges) {
       for (int i in range["coverage"]["hits"]) {
-        int? line = script.tokenToLine(i);
+        int line = script.tokenToLine(i);
         if (line == null) {
           throw FormatException('token ${i} was missing source location');
         }
diff --git a/runtime/observatory/tests/service/coverage_leaf_function_test.dart b/runtime/observatory/tests/service/coverage_leaf_function_test.dart
index 5cd8e1224a2..c36441b1324 100644
--- a/runtime/observatory/tests/service/coverage_leaf_function_test.dart
+++ b/runtime/observatory/tests/service/coverage_leaf_function_test.dart
@@ -55,9 +55,9 @@ var tests = <IsolateTest>[
 
     var params = {
       'reports': ['Coverage'],
-      'scriptId': func.location!.script.id,
-      'tokenPos': func.location!.tokenPos,
-      'endTokenPos': func.location!.endTokenPos,
+      'scriptId': func.location.script.id,
+      'tokenPos': func.location.tokenPos,
+      'endTokenPos': func.location.endTokenPos,
       'forceCompile': true
     };
     var report = await isolate.invokeRpcNoUpgrade('getSourceReport', params);
@@ -96,9 +96,9 @@ var tests = <IsolateTest>[
 
     var params = {
       'reports': ['Coverage'],
-      'scriptId': func.location!.script.id,
-      'tokenPos': func.location!.tokenPos,
-      'endTokenPos': func.location!.endTokenPos,
+      'scriptId': func.location.script.id,
+      'tokenPos': func.location.tokenPos,
+      'endTokenPos': func.location.endTokenPos,
       'forceCompile': true
     };
     var report = await isolate.invokeRpcNoUpgrade('getSourceReport', params);
diff --git a/runtime/observatory/tests/service/coverage_optimized_function_test.dart b/runtime/observatory/tests/service/coverage_optimized_function_test.dart
index b6c9823933c..ad57d161d1a 100644
--- a/runtime/observatory/tests/service/coverage_optimized_function_test.dart
+++ b/runtime/observatory/tests/service/coverage_optimized_function_test.dart
@@ -48,9 +48,9 @@ var tests = <IsolateTest>[
 
     var params = {
       'reports': ['Coverage'],
-      'scriptId': func.location!.script.id,
-      'tokenPos': func.location!.tokenPos,
-      'endTokenPos': func.location!.endTokenPos,
+      'scriptId': func.location.script.id,
+      'tokenPos': func.location.tokenPos,
+      'endTokenPos': func.location.endTokenPos,
       'forceCompile': true
     };
     var report = await isolate.invokeRpcNoUpgrade('getSourceReport', params);
diff --git a/runtime/observatory/tests/service/dds_log_history_size_gigantic_test.dart b/runtime/observatory/tests/service/dds_log_history_size_gigantic_test.dart
index d8cd47ae33e..1bd6136c309 100644
--- a/runtime/observatory/tests/service/dds_log_history_size_gigantic_test.dart
+++ b/runtime/observatory/tests/service/dds_log_history_size_gigantic_test.dart
@@ -29,7 +29,7 @@ Future setLogHistorySize(Isolate isolate, int size) async {
 Future<int> getLogHistorySize(Isolate isolate) async {
   final result = await isolate.invokeRpcNoUpgrade('getLogHistorySize', {});
   expect(result['type'], 'Size');
-  return result['size'] as int;
+  return result['size'];
 }
 
 var tests = <IsolateTest>[
@@ -59,7 +59,7 @@ var tests = <IsolateTest>[
     int i = 11;
     await subscribeToStream(isolate.vm, 'Logging', (event) async {
       expect(
-        event.logRecord!['message'].valueAsString,
+        event.logRecord['message'].valueAsString,
         'All work and no play makes Ben a dull boy ($i)',
       );
       i++;
diff --git a/runtime/observatory/tests/service/dds_log_history_size_simple_test.dart b/runtime/observatory/tests/service/dds_log_history_size_simple_test.dart
index fc1e0df3de2..69e0ee2f9c7 100644
--- a/runtime/observatory/tests/service/dds_log_history_size_simple_test.dart
+++ b/runtime/observatory/tests/service/dds_log_history_size_simple_test.dart
@@ -29,7 +29,7 @@ Future setLogHistorySize(Isolate isolate, int size) async {
 Future<int> getLogHistorySize(Isolate isolate) async {
   final result = await isolate.invokeRpcNoUpgrade('getLogHistorySize', {});
   expect(result['type'], 'Size');
-  return result['size'] as int;
+  return result['size'];
 }
 
 var tests = <IsolateTest>[
@@ -45,7 +45,7 @@ var tests = <IsolateTest>[
 
     int i = 1;
     await subscribeToStream(isolate.vm, 'Logging', (event) async {
-      expect(event.logRecord!['message'].valueAsString, 'log$i');
+      expect(event.logRecord['message'].valueAsString, 'log$i');
       i++;
 
       if (i == 10) {
@@ -69,11 +69,11 @@ var tests = <IsolateTest>[
 
     // Create a new client as we want to get log messages from the entire
     // history buffer.
-    final client = await createClient(isolate.vm as WebSocketVM);
+    final client = await createClient(isolate.vm);
 
     int i = 6;
     await subscribeToStream(client, 'Logging', (event) async {
-      expect(event.logRecord!['message'].valueAsString, 'log$i');
+      expect(event.logRecord['message'].valueAsString, 'log$i');
       i++;
 
       if (i == 11) {
@@ -84,7 +84,7 @@ var tests = <IsolateTest>[
       }
     });
     await completer.future;
-    client.disconnect();
+    await client.disconnect();
   },
 ];
 
diff --git a/runtime/observatory/tests/service/dds_log_history_size_test.dart b/runtime/observatory/tests/service/dds_log_history_size_test.dart
index 5186f1ed38f..98cc6e38e34 100644
--- a/runtime/observatory/tests/service/dds_log_history_size_test.dart
+++ b/runtime/observatory/tests/service/dds_log_history_size_test.dart
@@ -33,7 +33,7 @@ Future setLogHistorySize(Isolate isolate, int size) async {
 Future<int> getLogHistorySize(Isolate isolate) async {
   final result = await isolate.invokeRpcNoUpgrade('getLogHistorySize', {});
   expect(result['type'], 'Size');
-  return result['size'] as int;
+  return result['size'];
 }
 
 var tests = <IsolateTest>[
@@ -59,7 +59,7 @@ var tests = <IsolateTest>[
     // With the log history set to 20, the first log message should be 'log11'
     int i = 11;
     await subscribeToStream(isolate.vm, 'Logging', (event) async {
-      expect(event.logRecord!['message'].valueAsString, 'log$i');
+      expect(event.logRecord['message'].valueAsString, 'log$i');
       i++;
 
       if (i == 30) {
diff --git a/runtime/observatory/tests/service/debugger_inspect_test.dart b/runtime/observatory/tests/service/debugger_inspect_test.dart
index 77dc38787c5..1501b329c20 100644
--- a/runtime/observatory/tests/service/debugger_inspect_test.dart
+++ b/runtime/observatory/tests/service/debugger_inspect_test.dart
@@ -24,7 +24,7 @@ var tests = <IsolateTest>[
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
       if (event.kind == ServiceEvent.kInspect) {
-        expect(event.inspectee!.clazz!.name, equals('Point'));
+        expect(event.inspectee.clazz.name, equals('Point'));
         subscription.cancel();
         completer.complete();
       }
diff --git a/runtime/observatory/tests/service/debugger_location_second_test.dart b/runtime/observatory/tests/service/debugger_location_second_test.dart
index ba9420d688a..e8d6739346d 100644
--- a/runtime/observatory/tests/service/debugger_location_second_test.dart
+++ b/runtime/observatory/tests/service/debugger_location_second_test.dart
@@ -41,7 +41,7 @@ class DebuggerLocationTestFoo {
   void method() {}
   void madness() {}
 
-  int? field;
+  int field;
 }
 
 class DebuggerLocationTestBar {}
diff --git a/runtime/observatory/tests/service/debugger_location_test.dart b/runtime/observatory/tests/service/debugger_location_test.dart
index fe66893a8be..77b939c7a5c 100644
--- a/runtime/observatory/tests/service/debugger_location_test.dart
+++ b/runtime/observatory/tests/service/debugger_location_test.dart
@@ -41,7 +41,7 @@ class DebuggerLocationTestFoo {
   void method() {}
   void madness() {}
 
-  late int field;
+  int field;
 }
 
 class DebuggerLocationTestBar {}
diff --git a/runtime/observatory/tests/service/debugging_inlined_finally_test.dart b/runtime/observatory/tests/service/debugging_inlined_finally_test.dart
index e2e1bbb3542..9012cf38796 100644
--- a/runtime/observatory/tests/service/debugging_inlined_finally_test.dart
+++ b/runtime/observatory/tests/service/debugging_inlined_finally_test.dart
@@ -51,8 +51,8 @@ var tests = <IsolateTest>[
       expect(result is Breakpoint, isTrue);
       Breakpoint bpt = result;
       expect(bpt.type, equals('Breakpoint'));
-      expect(bpt.location!.script.id, equals(script.id));
-      expect(bpt.location!.script.tokenToLine(bpt.location!.tokenPos),
+      expect(bpt.location.script.id, equals(script.id));
+      expect(bpt.location.script.tokenToLine(bpt.location.tokenPos),
           equals(LINE_A));
       expect(isolate.breakpoints.length, equals(1));
     }
@@ -62,8 +62,8 @@ var tests = <IsolateTest>[
       expect(result is Breakpoint, isTrue);
       Breakpoint bpt = result;
       expect(bpt.type, equals('Breakpoint'));
-      expect(bpt.location!.script.id, equals(script.id));
-      expect(bpt.location!.script.tokenToLine(bpt.location!.tokenPos),
+      expect(bpt.location.script.id, equals(script.id));
+      expect(bpt.location.script.tokenToLine(bpt.location.tokenPos),
           equals(LINE_B));
       expect(isolate.breakpoints.length, equals(2));
     }
@@ -73,8 +73,8 @@ var tests = <IsolateTest>[
       expect(result is Breakpoint, isTrue);
       Breakpoint bpt = result;
       expect(bpt.type, equals('Breakpoint'));
-      expect(bpt.location!.script.id, equals(script.id));
-      expect(bpt.location!.script.tokenToLine(bpt.location!.tokenPos),
+      expect(bpt.location.script.id, equals(script.id));
+      expect(bpt.location.script.tokenToLine(bpt.location.tokenPos),
           equals(LINE_C));
       expect(isolate.breakpoints.length, equals(3));
     }
diff --git a/runtime/observatory/tests/service/debugging_test.dart b/runtime/observatory/tests/service/debugging_test.dart
index 5d4b6307c76..55a6e058f99 100644
--- a/runtime/observatory/tests/service/debugging_test.dart
+++ b/runtime/observatory/tests/service/debugging_test.dart
@@ -55,7 +55,7 @@ var tests = <IsolateTest>[
 
 // Add breakpoint
   (Isolate isolate) async {
-    await isolate.rootLibrary.load() as Library;
+    await isolate.rootLibrary.load();
 
     // Set up a listener to wait for breakpoint events.
     Completer completer = new Completer();
@@ -77,9 +77,8 @@ var tests = <IsolateTest>[
     expect(result is Breakpoint, isTrue);
     Breakpoint bpt = result;
     expect(bpt.type, equals('Breakpoint'));
-    expect(bpt.location!.script.id, equals(script.id));
-    expect(
-        bpt.location!.script.tokenToLine(bpt.location!.tokenPos), equals(15));
+    expect(bpt.location.script.id, equals(script.id));
+    expect(bpt.location.script.tokenToLine(bpt.location.tokenPos), equals(15));
     expect(isolate.breakpoints.length, equals(1));
 
     await completer.future; // Wait for breakpoint events.
@@ -187,9 +186,8 @@ var tests = <IsolateTest>[
     expect(result is Breakpoint, isTrue);
     Breakpoint bpt = result;
     expect(bpt.type, equals('Breakpoint'));
-    expect(bpt.location!.script.name, equals('debugging_test.dart'));
-    expect(
-        bpt.location!.script.tokenToLine(bpt.location!.tokenPos), equals(12));
+    expect(bpt.location.script.name, equals('debugging_test.dart'));
+    expect(bpt.location.script.tokenToLine(bpt.location.tokenPos), equals(12));
     expect(isolate.breakpoints.length, equals(1));
 
     await completer.future; // Wait for breakpoint events.
diff --git a/runtime/observatory/tests/service/dev_fs_test.dart b/runtime/observatory/tests/service/dev_fs_test.dart
index ebd72db7037..920a1523757 100644
--- a/runtime/observatory/tests/service/dev_fs_test.dart
+++ b/runtime/observatory/tests/service/dev_fs_test.dart
@@ -23,7 +23,7 @@ var tests = <VMTest>[
     expect(result['type'], equals('FileSystemList'));
     expect(result['fsNames'].toString(), equals('[alpha]'));
 
-    bool caughtException = false;
+    bool caughtException;
     try {
       await vm.invokeRpcNoUpgrade('_createDevFS', params);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/developer_extension_test.dart b/runtime/observatory/tests/service/developer_extension_test.dart
index 29aa764ffa2..30ca38dd8f0 100644
--- a/runtime/observatory/tests/service/developer_extension_test.dart
+++ b/runtime/observatory/tests/service/developer_extension_test.dart
@@ -16,7 +16,7 @@ Future<ServiceExtensionResponse> Handler(String method, Map paremeters) {
   print('Invoked extension: $method');
   switch (method) {
     case 'ext..delay':
-      var c = new Completer<ServiceExtensionResponse>();
+      Completer c = new Completer<ServiceExtensionResponse>();
       new Timer(new Duration(seconds: 1), () {
         c.complete(new ServiceExtensionResponse.result(jsonEncode({
           'type': '_delayedType',
@@ -39,7 +39,6 @@ Future<ServiceExtensionResponse> Handler(String method, Map paremeters) {
         'parameters': paremeters,
       })));
   }
-  throw "Unknown extension: $method";
 }
 
 void test() {
@@ -75,7 +74,7 @@ var tests = <IsolateTest>[
     expect(event.kind, equals(ServiceEvent.kExtension));
     expect(event.extensionKind, equals('ALPHA'));
     expect(event.extensionData, new isInstanceOf<Map>());
-    expect(event.extensionData!['cat'], equals('dog'));
+    expect(event.extensionData['cat'], equals('dog'));
   }),
   hasStoppedAtBreakpoint,
   resumeIsolateAndAwaitEvent(VM.kIsolateStream, (ServiceEvent event) {
diff --git a/runtime/observatory/tests/service/developer_server_control_test.dart b/runtime/observatory/tests/service/developer_server_control_test.dart
index d025edffbe7..24347ea2aa3 100644
--- a/runtime/observatory/tests/service/developer_server_control_test.dart
+++ b/runtime/observatory/tests/service/developer_server_control_test.dart
@@ -8,9 +8,9 @@ import 'package:expect/expect.dart';
 import 'package:observatory/service_io.dart' as S;
 import 'test_helper.dart';
 
-int? majorVersion;
-int? minorVersion;
-Uri? serverUri;
+int majorVersion;
+int minorVersion;
+Uri serverUri;
 
 Future<Null> testeeBefore() async {
   print('testee before');
diff --git a/runtime/observatory/tests/service/developer_service_get_isolate_id_test.dart b/runtime/observatory/tests/service/developer_service_get_isolate_id_test.dart
index 11b61da3073..1539e0705c5 100644
--- a/runtime/observatory/tests/service/developer_service_get_isolate_id_test.dart
+++ b/runtime/observatory/tests/service/developer_service_get_isolate_id_test.dart
@@ -12,9 +12,9 @@ import 'service_test_common.dart';
 import 'test_helper.dart';
 
 // testee state.
-late String selfId;
-late Core.Isolate childIsolate;
-late String childId;
+String selfId;
+Core.Isolate childIsolate;
+String childId;
 
 void spawnEntry(int i) {
   dev.debugger();
@@ -26,8 +26,8 @@ Future testeeMain() async {
   childIsolate = await Core.Isolate.spawn(spawnEntry, 0);
   // Assign the id for this isolate and it's child to strings so they can
   // be read by the tester.
-  selfId = dev.Service.getIsolateID(Core.Isolate.current)!;
-  childId = dev.Service.getIsolateID(childIsolate)!;
+  selfId = dev.Service.getIsolateID(Core.Isolate.current);
+  childId = dev.Service.getIsolateID(childIsolate);
   dev.debugger();
 }
 
@@ -38,8 +38,8 @@ getSelfId() => selfId;
 getChildId() => childId;
 
 // tester state:
-late Service.Isolate initialIsolate;
-late Service.Isolate localChildIsolate;
+Service.Isolate initialIsolate;
+Service.Isolate localChildIsolate;
 
 var tests = <VMTest>[
   (Service.VM vm) async {
@@ -67,12 +67,11 @@ var tests = <VMTest>[
     await initialIsolate.reload();
 
     // Grab the root library.
-    Service.Library rootLbirary =
-        await initialIsolate.rootLibrary.load() as Service.Library;
+    Service.Library rootLbirary = await initialIsolate.rootLibrary.load();
 
     // Grab self id.
-    Service.Instance localSelfId = await initialIsolate.invoke(
-        rootLbirary, 'getSelfId') as Service.Instance;
+    Service.Instance localSelfId =
+        await initialIsolate.invoke(rootLbirary, 'getSelfId');
 
     // Check that the id reported from dart:developer matches the id reported
     // from the service protocol.
@@ -80,8 +79,8 @@ var tests = <VMTest>[
     expect(initialIsolate.id, equals(localSelfId.valueAsString));
 
     // Grab the child isolate's id.
-    Service.Instance localChildId = await initialIsolate.invoke(
-        rootLbirary, 'getChildId') as Service.Instance;
+    Service.Instance localChildId =
+        await initialIsolate.invoke(rootLbirary, 'getChildId');
 
     // Check that the id reported from dart:developer matches the id reported
     // from the service protocol.
diff --git a/runtime/observatory/tests/service/echo_test.dart b/runtime/observatory/tests/service/echo_test.dart
index 13c81303f2f..49ebe73216e 100644
--- a/runtime/observatory/tests/service/echo_test.dart
+++ b/runtime/observatory/tests/service/echo_test.dart
@@ -24,10 +24,10 @@ var tests = <IsolateTest>[
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
       assert(event.kind == '_Echo');
-      expect(event.data!.lengthInBytes, equals(3));
-      expect(event.data![0], equals(0));
-      expect(event.data![1], equals(128));
-      expect(event.data![2], equals(255));
+      expect(event.data.lengthInBytes, equals(3));
+      expect(event.data[0], equals(0));
+      expect(event.data[1], equals(128));
+      expect(event.data[2], equals(255));
       subscription.cancel();
       completer.complete();
     });
diff --git a/runtime/observatory/tests/service/enable_service_port_fallback_test.dart b/runtime/observatory/tests/service/enable_service_port_fallback_test.dart
index ab8d23ea612..ea63b8b23d9 100644
--- a/runtime/observatory/tests/service/enable_service_port_fallback_test.dart
+++ b/runtime/observatory/tests/service/enable_service_port_fallback_test.dart
@@ -18,7 +18,7 @@ var tests = <VMTest>[
   }
 ];
 
-late ServerSocket socket;
+ServerSocket socket;
 int portNumber = -1;
 
 main(args) async {
diff --git a/runtime/observatory/tests/service/eval_internal_class_test.dart b/runtime/observatory/tests/service/eval_internal_class_test.dart
index 7c01361b097..98b58401cbb 100644
--- a/runtime/observatory/tests/service/eval_internal_class_test.dart
+++ b/runtime/observatory/tests/service/eval_internal_class_test.dart
@@ -8,9 +8,9 @@ import 'test_helper.dart';
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library root = await isolate.rootLibrary.load() as Library;
+    Library root = await isolate.rootLibrary.load();
 
-    Class classLibrary = await root.clazz!.load() as Class;
+    Class classLibrary = await root.clazz.load();
     print(classLibrary);
     {
       bool caughtExceptions = false;
@@ -24,7 +24,7 @@ var tests = <IsolateTest>[
       expect(caughtExceptions, isTrue);
     }
 
-    Class classClass = await classLibrary.clazz!.load() as Class;
+    Class classClass = await classLibrary.clazz.load();
     print(classClass);
     {
       bool caughtExceptions = false;
@@ -38,11 +38,10 @@ var tests = <IsolateTest>[
       expect(caughtExceptions, isTrue);
     }
 
-    Instance someArray =
-        await root.evaluate("new List<dynamic>.filled(2, null)") as Instance;
+    Instance someArray = await root.evaluate("new List(2)");
     print(someArray);
     expect(someArray is Instance, isTrue);
-    Class classArray = await someArray.clazz!.load() as Class;
+    Class classArray = await someArray.clazz.load();
     print(classArray);
     dynamic result = await classArray.evaluate('3 + 4');
     print(result);
diff --git a/runtime/observatory/tests/service/eval_regression_flutter20255_test.dart b/runtime/observatory/tests/service/eval_regression_flutter20255_test.dart
index 0762ae38b36..28f07d7d69e 100644
--- a/runtime/observatory/tests/service/eval_regression_flutter20255_test.dart
+++ b/runtime/observatory/tests/service/eval_regression_flutter20255_test.dart
@@ -36,7 +36,7 @@ class ISub<T> implements Base<T> {
 }
 
 class Box<T> {
-  late T value;
+  T value;
 
   @pragma('vm:never-inline')
   void setValue(T value) {
diff --git a/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart b/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart
index 621ebd2387d..e1a78c4d647 100644
--- a/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart
+++ b/runtime/observatory/tests/service/evaluate_activation_in_method_class_test.dart
@@ -41,42 +41,42 @@ Future testerDo(Isolate isolate) async {
 
   Instance result;
 
-  result = await isolate.evalFrame(topFrame, '_local') as Instance;
+  result = await isolate.evalFrame(topFrame, '_local');
   print(result);
   expect(result.valueAsString, equals('Klass'));
 
-  result = await isolate.evalFrame(topFrame, '_instVar') as Instance;
+  result = await isolate.evalFrame(topFrame, '_instVar');
   print(result);
   expect(result.valueAsString, equals('Klass'));
 
-  result = await isolate.evalFrame(topFrame, 'instVar') as Instance;
+  result = await isolate.evalFrame(topFrame, 'instVar');
   print(result);
   expect(result.valueAsString, equals('Subclass'));
 
-  result = await isolate.evalFrame(topFrame, 'method()') as Instance;
+  result = await isolate.evalFrame(topFrame, 'method()');
   print(result);
   expect(result.valueAsString, equals('Subclass'));
 
-  result = await isolate.evalFrame(topFrame, 'super._instVar') as Instance;
+  result = await isolate.evalFrame(topFrame, 'super._instVar');
   print(result);
   expect(result.valueAsString, equals('Superclass'));
 
-  result = await isolate.evalFrame(topFrame, 'super.instVar') as Instance;
+  result = await isolate.evalFrame(topFrame, 'super.instVar');
   print(result);
   expect(result.valueAsString, equals('Superclass'));
 
-  result = await isolate.evalFrame(topFrame, 'super.method()') as Instance;
+  result = await isolate.evalFrame(topFrame, 'super.method()');
   print(result);
   expect(result.valueAsString, equals('Superclass'));
 
-  result = await isolate.evalFrame(topFrame, 'staticMethod()') as Instance;
+  result = await isolate.evalFrame(topFrame, 'staticMethod()');
   print(result);
   expect(result.valueAsString, equals('Klass'));
 
   // function.Owner verus function.Origin
   // The mixin of Superclass is in _other.dart and the mixin
   // application is in _test.dart.
-  result = await isolate.evalFrame(topFrame, 'topLevel') as Instance;
+  result = await isolate.evalFrame(topFrame, 'topLevel');
   print(result);
   expect(result.valueAsString, equals('OtherLibrary'));
 }
diff --git a/runtime/observatory/tests/service/evaluate_activation_test.dart b/runtime/observatory/tests/service/evaluate_activation_test.dart
index bf65d96233d..059d769f4ae 100644
--- a/runtime/observatory/tests/service/evaluate_activation_test.dart
+++ b/runtime/observatory/tests/service/evaluate_activation_test.dart
@@ -70,7 +70,7 @@ class C {
 Future testMethod(Isolate isolate) async {
   // silence analyzer.
   expect(math.sqrt(4), equals(2));
-  Library rootLib = await isolate.rootLibrary.load() as Library;
+  Library rootLib = await isolate.rootLibrary.load();
   ServiceFunction function =
       rootLib.functions.singleWhere((f) => f.name == 'breakHere');
   Breakpoint bpt = await isolate.addBreakpointAtEntry(function);
@@ -83,7 +83,7 @@ Future testMethod(Isolate isolate) async {
   sub = stream.listen((ServiceEvent event) async {
     print("Event $event");
     if (event.kind == ServiceEvent.kPauseBreakpoint) {
-      dynamic frameNumber = 1, r;
+      var frameNumber = 1, r;
       r = await isolate.evalFrame(frameNumber, '123');  /// instance: ok
       expect(r.valueAsString, equals('123'));  /// instance: continued
       r = await isolate.evalFrame(frameNumber, 'this');  /// scope: ok
@@ -119,7 +119,7 @@ Future testMethod(Isolate isolate) async {
 }
 
 Future testMethod2(Isolate isolate) async {
-  Library rootLib = await isolate.rootLibrary.load() as Library;
+  Library rootLib = await isolate.rootLibrary.load();
   ServiceFunction function =
       rootLib.functions.singleWhere((f) => f.name == 'breakHere');
   Breakpoint bpt = await isolate.addBreakpointAtEntry(function);
@@ -132,7 +132,7 @@ Future testMethod2(Isolate isolate) async {
   sub = stream.listen((ServiceEvent event) async {
     print("Event $event");
     if (event.kind == ServiceEvent.kPauseBreakpoint) {
-      dynamic frameNumber = 1, r;
+      var frameNumber = 1, r;
       r = await isolate.evalFrame(frameNumber, '123');
       expect(r.valueAsString, equals('123'));
       r = await isolate.evalFrame(frameNumber, 'this');
@@ -168,7 +168,7 @@ Future testMethod2(Isolate isolate) async {
 }
 
 Future testMethod3(Isolate isolate) async {
-  Library rootLib = await isolate.rootLibrary.load() as Library;
+  Library rootLib = await isolate.rootLibrary.load();
   ServiceFunction function =
       rootLib.functions.singleWhere((f) => f.name == 'breakHere');
   Breakpoint bpt = await isolate.addBreakpointAtEntry(function);
@@ -181,7 +181,7 @@ Future testMethod3(Isolate isolate) async {
   sub = stream.listen((ServiceEvent event) async {
     print("Event $event");
     if (event.kind == ServiceEvent.kPauseBreakpoint) {
-      dynamic frameNumber = 1, r;
+      var frameNumber = 1, r;
       r = await isolate.evalFrame(frameNumber, '123');
       expect(r.valueAsString, equals('123'));
       r = await isolate.evalFrame(frameNumber, 'this');
@@ -210,7 +210,7 @@ Future testMethod3(Isolate isolate) async {
 
 
 Future testMethod4(Isolate isolate) async {
-  Library rootLib = await isolate.rootLibrary.load() as Library;
+  Library rootLib = await isolate.rootLibrary.load();
   ServiceFunction function =
       rootLib.functions.singleWhere((f) => f.name == 'breakHere');
   Breakpoint bpt = await isolate.addBreakpointAtEntry(function);
@@ -223,7 +223,7 @@ Future testMethod4(Isolate isolate) async {
   sub = stream.listen((ServiceEvent event) async {
     print("Event $event");
     if (event.kind == ServiceEvent.kPauseBreakpoint) {
-      dynamic frameNumber = 1, r;
+      var frameNumber = 1, r;
       r = await isolate.evalFrame(frameNumber, '123');  /// instance: continued
       expect(r.valueAsString, equals('123'));  /// instance: continued
       r = await isolate.evalFrame(frameNumber, 'this');  /// scope: continued
diff --git a/runtime/observatory/tests/service/evaluate_async_closure_test.dart b/runtime/observatory/tests/service/evaluate_async_closure_test.dart
index 182ee3da515..4b64e63d08d 100644
--- a/runtime/observatory/tests/service/evaluate_async_closure_test.dart
+++ b/runtime/observatory/tests/service/evaluate_async_closure_test.dart
@@ -18,7 +18,7 @@ var tests = <IsolateTest>[
         "  var w = () async { return await k(); }; "
         "  return w(); "
         "}()";
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
 
     var result = await lib.evaluate(test);
     expect("$result", equals("Instance(a _Future)"));
diff --git a/runtime/observatory/tests/service/evaluate_class_type_parameters_test.dart b/runtime/observatory/tests/service/evaluate_class_type_parameters_test.dart
index 38b34fce776..48b65309636 100644
--- a/runtime/observatory/tests/service/evaluate_class_type_parameters_test.dart
+++ b/runtime/observatory/tests/service/evaluate_class_type_parameters_test.dart
@@ -35,7 +35,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 20);
 
-    Instance result = await isolate.evalFrame(topFrame, '"\$S"') as Instance;
+    Instance result = await isolate.evalFrame(topFrame, '"\$S"');
     print(result);
     expect(result.valueAsString, equals("String"));
   },
@@ -48,7 +48,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 14);
 
-    Instance result = await isolate.evalFrame(topFrame, '"\$T"') as Instance;
+    Instance result = await isolate.evalFrame(topFrame, '"\$T"');
     print(result);
     expect(result.valueAsString, equals("int"));
   },
diff --git a/runtime/observatory/tests/service/evaluate_function_type_parameters_test.dart b/runtime/observatory/tests/service/evaluate_function_type_parameters_test.dart
index 1eb0f4f3d51..1f8fcd0e846 100644
--- a/runtime/observatory/tests/service/evaluate_function_type_parameters_test.dart
+++ b/runtime/observatory/tests/service/evaluate_function_type_parameters_test.dart
@@ -49,8 +49,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 14);
 
-    Instance result =
-        await isolate.evalFrame(topFrame, "S.toString()") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "S.toString()");
     print(result);
     expect(result.valueAsString, equals("String"));
   },
@@ -63,30 +62,27 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 16);
 
-    Instance result =
-        await isolate.evalFrame(topFrame, "TBool.toString()") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "TBool.toString()");
     print(result);
     expect(result.valueAsString, equals("bool"));
 
-    result =
-        await isolate.evalFrame(topFrame, "TString.toString()") as Instance;
+    result = await isolate.evalFrame(topFrame, "TString.toString()");
     print(result);
     expect(result.valueAsString, equals("String"));
 
-    result =
-        await isolate.evalFrame(topFrame, "TDouble.toString()") as Instance;
+    result = await isolate.evalFrame(topFrame, "TDouble.toString()");
     print(result);
     expect(result.valueAsString, equals("double"));
 
-    result = await isolate.evalFrame(topFrame, "TInt.toString()") as Instance;
+    result = await isolate.evalFrame(topFrame, "TInt.toString()");
     print(result);
     expect(result.valueAsString, equals("int"));
 
-    result = await isolate.evalFrame(topFrame, "S.toString()") as Instance;
+    result = await isolate.evalFrame(topFrame, "S.toString()");
     print(result);
     expect(result.valueAsString, equals("String"));
 
-    result = await isolate.evalFrame(topFrame, "x") as Instance;
+    result = await isolate.evalFrame(topFrame, "x");
     print(result);
     expect(result.valueAsString, equals("3"));
   },
@@ -99,8 +95,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 22);
 
-    Instance result =
-        await isolate.evalFrame(topFrame, "S.toString()") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "S.toString()");
     print(result);
     expect(result.valueAsString, equals("String"));
   },
@@ -113,12 +108,11 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 30);
 
-    Instance result =
-        await isolate.evalFrame(topFrame, "T.toString()") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "T.toString()");
     print(result);
     expect(result.valueAsString, equals("int"));
 
-    result = await isolate.evalFrame(topFrame, "S.toString()") as Instance;
+    result = await isolate.evalFrame(topFrame, "S.toString()");
     print(result);
     expect(result.valueAsString, equals("bool"));
   },
@@ -131,11 +125,10 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 34);
 
-    Instance result =
-        await isolate.evalFrame(topFrame, "T.toString()") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "T.toString()");
     print(result);
     expect(result.valueAsString, equals("dynamic"));
-    result = await isolate.evalFrame(topFrame, "t") as Instance;
+    result = await isolate.evalFrame(topFrame, "t");
     print(result);
     expect(result.valueAsString, equals("42"));
   },
diff --git a/runtime/observatory/tests/service/evaluate_in_async_activation_test.dart b/runtime/observatory/tests/service/evaluate_in_async_activation_test.dart
index 1c41188767c..6607b89d8db 100644
--- a/runtime/observatory/tests/service/evaluate_in_async_activation_test.dart
+++ b/runtime/observatory/tests/service/evaluate_in_async_activation_test.dart
@@ -27,7 +27,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 16);
 
-    Instance result = await isolate.evalFrame(topFrame, "x") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "x");
     print(result);
     expect(result.valueAsString, equals("3"));
   },
@@ -40,7 +40,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 18);
 
-    Instance result = await isolate.evalFrame(topFrame, "z") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "z");
     print(result);
     expect(result.valueAsString, equals("7"));
   },
diff --git a/runtime/observatory/tests/service/evaluate_in_async_star_activation_test.dart b/runtime/observatory/tests/service/evaluate_in_async_star_activation_test.dart
index 67b7fd1ca72..4d6771adf76 100644
--- a/runtime/observatory/tests/service/evaluate_in_async_star_activation_test.dart
+++ b/runtime/observatory/tests/service/evaluate_in_async_star_activation_test.dart
@@ -31,7 +31,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 15);
 
-    Instance result = await isolate.evalFrame(topFrame, "x") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "x");
     print(result);
     expect(result.valueAsString, equals("3"));
   },
@@ -44,7 +44,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 18);
 
-    Instance result = await isolate.evalFrame(topFrame, "z") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "z");
     print(result);
     expect(result.valueAsString, equals("7"));
   },
diff --git a/runtime/observatory/tests/service/evaluate_in_frame_rpc_test.dart b/runtime/observatory/tests/service/evaluate_in_frame_rpc_test.dart
index 97df7899c74..2e47197ae4e 100644
--- a/runtime/observatory/tests/service/evaluate_in_frame_rpc_test.dart
+++ b/runtime/observatory/tests/service/evaluate_in_frame_rpc_test.dart
@@ -26,17 +26,17 @@ var tests = <IsolateTest>[
 
 // Evaluate against library, class, and instance.
   (Isolate isolate) async {
-    Instance result;
-    result = await isolate.evalFrame(0, 'value') as Instance;
+    var result;
+    result = await isolate.evalFrame(0, 'value');
     expect(result.valueAsString, equals('10000'));
 
-    result = await isolate.evalFrame(0, '_') as Instance;
+    result = await isolate.evalFrame(0, '_');
     expect(result.valueAsString, equals('50'));
 
-    result = await isolate.evalFrame(0, 'value + _') as Instance;
+    result = await isolate.evalFrame(0, 'value + _');
     expect(result.valueAsString, equals('10050'));
 
-    result = await isolate.evalFrame(1, 'i') as Instance;
+    result = await isolate.evalFrame(1, 'i');
     expect(result.valueAsString, equals('100000000'));
   },
 ];
diff --git a/runtime/observatory/tests/service/evaluate_in_frame_with_scope_test.dart b/runtime/observatory/tests/service/evaluate_in_frame_with_scope_test.dart
index 40d596eae24..fecc2453ecd 100644
--- a/runtime/observatory/tests/service/evaluate_in_frame_with_scope_test.dart
+++ b/runtime/observatory/tests/service/evaluate_in_frame_with_scope_test.dart
@@ -34,16 +34,14 @@ var tests = <IsolateTest>[
     expect(stack['frames'].length, greaterThanOrEqualTo(1));
     expect(stack['frames'][0].function.name, equals('foo'));
 
-    Library lib = await isolate.rootLibrary.load() as Library;
-    Field thing1Field = await lib.variables
-        .singleWhere((v) => v.name == "thing1")
-        .load() as Field;
-    var thing1 = thing1Field.staticValue!;
+    Library lib = await isolate.rootLibrary.load();
+    Field thing1Field =
+        await lib.variables.singleWhere((v) => v.name == "thing1").load();
+    var thing1 = thing1Field.staticValue;
     print(thing1);
-    Field thing2Field = await lib.variables
-        .singleWhere((v) => v.name == "thing2")
-        .load() as Field;
-    var thing2 = thing2Field.staticValue!;
+    Field thing2Field =
+        await lib.variables.singleWhere((v) => v.name == "thing2").load();
+    var thing2 = thing2Field.staticValue;
     print(thing2);
 
     final isInstanceOf<Instance> isInstanceOfInstance =
diff --git a/runtime/observatory/tests/service/evaluate_in_sync_star_activation_test.dart b/runtime/observatory/tests/service/evaluate_in_sync_star_activation_test.dart
index ade6955bbde..bbaf78f0981 100644
--- a/runtime/observatory/tests/service/evaluate_in_sync_star_activation_test.dart
+++ b/runtime/observatory/tests/service/evaluate_in_sync_star_activation_test.dart
@@ -31,7 +31,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 15);
 
-    Instance result = await isolate.evalFrame(topFrame, "x") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "x");
     print(result);
     expect(result.valueAsString, equals("3"));
   },
@@ -44,7 +44,7 @@ var tests = <IsolateTest>[
     expect(stack.type, equals('Stack'));
     expect(await stack['frames'][topFrame].location.getLine(), 18);
 
-    Instance result = await isolate.evalFrame(topFrame, "z") as Instance;
+    Instance result = await isolate.evalFrame(topFrame, "z");
     print(result);
     expect(result.valueAsString, equals("7"));
   },
diff --git a/runtime/observatory/tests/service/evaluate_with_escaping_closure_test.dart b/runtime/observatory/tests/service/evaluate_with_escaping_closure_test.dart
index 7320bfa3fd2..7849fc7f702 100644
--- a/runtime/observatory/tests/service/evaluate_with_escaping_closure_test.dart
+++ b/runtime/observatory/tests/service/evaluate_with_escaping_closure_test.dart
@@ -12,15 +12,14 @@ testeeMain() {}
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
 
-    Instance result =
-        await lib.evaluate("escapedClosure = (x, y) => x + y") as Instance;
+    Instance result = await lib.evaluate("escapedClosure = (x, y) => x + y");
     print(result);
-    expect(result.clazz!.name, startsWith('_Closure'));
+    expect(result.clazz.name, startsWith('_Closure'));
 
     for (var i = 0; i < 100; i++) {
-      result = await lib.evaluate("escapedClosure(3, 4)") as Instance;
+      result = await lib.evaluate("escapedClosure(3, 4)");
       print(result);
       expect(result.valueAsString, equals('7'));
     }
diff --git a/runtime/observatory/tests/service/evaluate_with_scope_test.dart b/runtime/observatory/tests/service/evaluate_with_scope_test.dart
index 9cfbdd3a2cf..90ea063aea5 100644
--- a/runtime/observatory/tests/service/evaluate_with_scope_test.dart
+++ b/runtime/observatory/tests/service/evaluate_with_scope_test.dart
@@ -16,26 +16,24 @@ testeeMain() {
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
-    Field thing1Field = await lib.variables
-        .singleWhere((v) => v.name == "thing1")
-        .load() as Field;
-    var thing1 = thing1Field.staticValue!;
+    Library lib = await isolate.rootLibrary.load();
+    Field thing1Field =
+        await lib.variables.singleWhere((v) => v.name == "thing1").load();
+    var thing1 = thing1Field.staticValue;
     print(thing1);
-    Field thing2Field = await lib.variables
-        .singleWhere((v) => v.name == "thing2")
-        .load() as Field;
-    var thing2 = thing2Field.staticValue!;
+    Field thing2Field =
+        await lib.variables.singleWhere((v) => v.name == "thing2").load();
+    var thing2 = thing2Field.staticValue;
     print(thing2);
 
     Instance result = await lib.evaluate("x + y",
-        scope: <String, ServiceObject>{"x": thing1, "y": thing2}) as Instance;
+        scope: <String, ServiceObject>{"x": thing1, "y": thing2});
     expect(result.valueAsString, equals('7'));
 
     bool didThrow = false;
     try {
       result = await lib.evaluate("x + y",
-          scope: <String, ServiceObject>{"x": lib, "y": lib}) as Instance;
+          scope: <String, ServiceObject>{"x": lib, "y": lib});
       print(result);
     } catch (e) {
       didThrow = true;
@@ -47,8 +45,7 @@ var tests = <IsolateTest>[
     didThrow = false;
     try {
       result = await lib.evaluate("x + y",
-              scope: <String, ServiceObject>{"not&an&identifier": thing1})
-          as Instance;
+          scope: <String, ServiceObject>{"not&an&identifier": thing1});
       print(result);
     } catch (e) {
       didThrow = true;
diff --git a/runtime/observatory/tests/service/external_service_asynchronous_invocation_test.dart b/runtime/observatory/tests/service/external_service_asynchronous_invocation_test.dart
index d633c2b5d6e..91f015d50d5 100644
--- a/runtime/observatory/tests/service/external_service_asynchronous_invocation_test.dart
+++ b/runtime/observatory/tests/service/external_service_asynchronous_invocation_test.dart
@@ -11,7 +11,7 @@ import 'dart:async' show Future, Stream, StreamController;
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    VM vm = isolate.owner as VM;
+    VM vm = isolate.owner;
 
     final serviceEvents =
         (await vm.getEventStream('Service')).asBroadcastStream();
@@ -23,7 +23,7 @@ var tests = <IsolateTest>[
 
     // Avoid to manually encode and decode messages from the stream
     Stream<String> stream = socket.stream.map(jsonEncode);
-    stream.cast<dynamic>().pipe(_socket);
+    stream.cast<Object>().pipe(_socket);
     dynamic _decoder(dynamic obj) {
       return jsonDecode(obj);
     }
diff --git a/runtime/observatory/tests/service/external_service_disappear_test.dart b/runtime/observatory/tests/service/external_service_disappear_test.dart
index 1efa80ec3f5..fec1d3170c3 100644
--- a/runtime/observatory/tests/service/external_service_disappear_test.dart
+++ b/runtime/observatory/tests/service/external_service_disappear_test.dart
@@ -11,7 +11,7 @@ import 'dart:async' show Future, Stream, StreamController;
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    VM vm = isolate.owner as VM;
+    VM vm = isolate.owner;
 
     final serviceEvents =
         (await vm.getEventStream('Service')).asBroadcastStream();
@@ -23,7 +23,7 @@ var tests = <IsolateTest>[
 
     // Avoid to manually encode and decode messages from the stream
     Stream<String> stream = socket.stream.map(jsonEncode);
-    stream.cast<dynamic>().pipe(_socket);
+    stream.cast<Object>().pipe(_socket);
     dynamic _decoder(dynamic obj) {
       return jsonDecode(obj);
     }
diff --git a/runtime/observatory/tests/service/external_service_notification_invocation_test.dart b/runtime/observatory/tests/service/external_service_notification_invocation_test.dart
index 89845267a76..2a659a1b2bd 100644
--- a/runtime/observatory/tests/service/external_service_notification_invocation_test.dart
+++ b/runtime/observatory/tests/service/external_service_notification_invocation_test.dart
@@ -11,7 +11,7 @@ import 'dart:async' show Future, Stream, StreamController;
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    VM vm = isolate.owner as VM;
+    VM vm = isolate.owner;
 
     final serviceEvents =
         (await vm.getEventStream('Service')).asBroadcastStream();
@@ -26,10 +26,10 @@ var tests = <IsolateTest>[
 
     // Avoid to manually encode and decode messages from the stream
     Stream<String> socket_stream = socket.stream.map(jsonEncode);
-    socket_stream.cast<dynamic>().pipe(_socket);
+    socket_stream.cast<Object>().pipe(_socket);
     Stream<String> socket_invoker_stream =
         socket_invoker.stream.map(jsonEncode);
-    socket_invoker_stream.cast<dynamic>().pipe(_socket_invoker);
+    socket_invoker_stream.cast<Object>().pipe(_socket_invoker);
     dynamic _decoder(dynamic obj) {
       return jsonDecode(obj);
     }
diff --git a/runtime/observatory/tests/service/external_service_registration_test.dart b/runtime/observatory/tests/service/external_service_registration_test.dart
index 79364095406..469facbb183 100644
--- a/runtime/observatory/tests/service/external_service_registration_test.dart
+++ b/runtime/observatory/tests/service/external_service_registration_test.dart
@@ -11,7 +11,7 @@ import 'dart:async' show Future, Stream, StreamController;
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    VM vm = isolate.owner as VM;
+    VM vm = isolate.owner;
 
     final serviceEvents =
         (await vm.getEventStream('Service')).asBroadcastStream();
@@ -26,7 +26,7 @@ var tests = <IsolateTest>[
 
     // Avoid to manually encode and decode messages from the stream
     Stream<String> socket_stream = socket.stream.map(jsonEncode);
-    socket_stream.cast<dynamic>().pipe(_socket);
+    socket_stream.cast<Object>().pipe(_socket);
     dynamic _decoder(dynamic obj) {
       return jsonDecode(obj);
     }
diff --git a/runtime/observatory/tests/service/external_service_registration_via_notification_test.dart b/runtime/observatory/tests/service/external_service_registration_via_notification_test.dart
index 5b2ca8a7e54..008c221ee9d 100644
--- a/runtime/observatory/tests/service/external_service_registration_via_notification_test.dart
+++ b/runtime/observatory/tests/service/external_service_registration_via_notification_test.dart
@@ -11,7 +11,7 @@ import 'dart:async' show Future, Stream, StreamController;
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    VM vm = isolate.owner as VM;
+    VM vm = isolate.owner;
 
     final serviceEvents =
         (await vm.getEventStream('Service')).asBroadcastStream();
@@ -26,7 +26,7 @@ var tests = <IsolateTest>[
 
     // Avoid to manually encode and decode messages from the stream
     Stream<String> stream = socket.stream.map(jsonEncode);
-    stream.cast<dynamic>().pipe(_socket);
+    stream.cast<Object>().pipe(_socket);
     dynamic _decoder(dynamic obj) {
       return jsonDecode(obj);
     }
diff --git a/runtime/observatory/tests/service/external_service_synchronous_invocation_test.dart b/runtime/observatory/tests/service/external_service_synchronous_invocation_test.dart
index 317408d82bc..a7cee786fa6 100644
--- a/runtime/observatory/tests/service/external_service_synchronous_invocation_test.dart
+++ b/runtime/observatory/tests/service/external_service_synchronous_invocation_test.dart
@@ -11,7 +11,7 @@ import 'dart:async' show Future, Stream, StreamController;
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    VM vm = isolate.owner as VM;
+    VM vm = isolate.owner;
 
     final serviceEvents =
         (await vm.getEventStream('Service')).asBroadcastStream();
@@ -23,7 +23,7 @@ var tests = <IsolateTest>[
 
     // Avoid to manually encode and decode messages from the stream
     Stream<String> stream = socket.stream.map(jsonEncode);
-    stream.cast<dynamic>().pipe(_socket);
+    stream.cast<Object>().pipe(_socket);
     dynamic _decoder(dynamic obj) {
       return jsonDecode(obj);
     }
@@ -121,7 +121,7 @@ var tests = <IsolateTest>[
       } on ServerRpcException catch (e) {
         expect(e.code, equals(errorCode + iteration));
         expect(e.data, isNotNull);
-        expect(e.data![errorKey + end], equals(errorValue + end));
+        expect(e.data[errorKey + end], equals(errorValue + end));
       }
     }
   },
diff --git a/runtime/observatory/tests/service/field_script_test.dart b/runtime/observatory/tests/service/field_script_test.dart
index 6e32b958482..17be1b17cb7 100644
--- a/runtime/observatory/tests/service/field_script_test.dart
+++ b/runtime/observatory/tests/service/field_script_test.dart
@@ -18,13 +18,13 @@ code() {
 var tests = <IsolateTest>[
   hasPausedAtStart,
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     var fields = lib.variables;
     expect(fields.length, 2);
     print(lib.variables);
     for (Field f in fields) {
       await f.load();
-      String locationString = await f.location!.toUserString();
+      String locationString = await f.location.toUserString();
       if (f.name == "tests") {
         expect(locationString, "field_script_test.dart:18:5");
       } else if (f.name == "otherField") {
diff --git a/runtime/observatory/tests/service/get_allocation_profile_public_rpc_test.dart b/runtime/observatory/tests/service/get_allocation_profile_public_rpc_test.dart
index f944dd7d468..d1c81e059c2 100644
--- a/runtime/observatory/tests/service/get_allocation_profile_public_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_allocation_profile_public_rpc_test.dart
@@ -99,14 +99,14 @@ var tests = <IsolateTest>[
     var params = {
       'reset': 'banana',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getAllocationProfile', params);
       expect(false, isTrue, reason: 'Unreachable');
     } on ServerRpcException catch (e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
-      expect(e.data!['details'],
+      expect(e.data['details'],
           "getAllocationProfile: invalid \'reset\' parameter: banana");
     }
     expect(caughtException, isTrue);
@@ -115,14 +115,14 @@ var tests = <IsolateTest>[
     var params = {
       'gc': 'banana',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getAllocationProfile', params);
       expect(false, isTrue, reason: 'Unreachable');
     } on ServerRpcException catch (e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
-      expect(e.data!['details'],
+      expect(e.data['details'],
           "getAllocationProfile: invalid \'gc\' parameter: banana");
     }
     expect(caughtException, isTrue);
diff --git a/runtime/observatory/tests/service/get_allocation_profile_rpc_test.dart b/runtime/observatory/tests/service/get_allocation_profile_rpc_test.dart
index ae4efac5be8..05ab7c82103 100644
--- a/runtime/observatory/tests/service/get_allocation_profile_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_allocation_profile_rpc_test.dart
@@ -99,14 +99,14 @@ var tests = <IsolateTest>[
     var params = {
       'reset': 'banana',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('_getAllocationProfile', params);
       expect(false, isTrue, reason: 'Unreachable');
     } on ServerRpcException catch (e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
-      expect(e.data!['details'],
+      expect(e.data['details'],
           "_getAllocationProfile: invalid \'reset\' parameter: banana");
     }
     expect(caughtException, isTrue);
@@ -115,14 +115,14 @@ var tests = <IsolateTest>[
     var params = {
       'gc': 'banana',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('_getAllocationProfile', params);
       expect(false, isTrue, reason: 'Unreachable');
     } on ServerRpcException catch (e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
-      expect(e.data!['details'],
+      expect(e.data['details'],
           "_getAllocationProfile: invalid \'gc\' parameter: banana");
     }
     expect(caughtException, isTrue);
diff --git a/runtime/observatory/tests/service/get_allocation_samples_test.dart b/runtime/observatory/tests/service/get_allocation_samples_test.dart
index 42461543ed9..2370433e375 100644
--- a/runtime/observatory/tests/service/get_allocation_samples_test.dart
+++ b/runtime/observatory/tests/service/get_allocation_samples_test.dart
@@ -32,7 +32,7 @@ var tests = <IsolateTest>[
   // Initial.
   (Isolate isolate) async {
     // Verify initial state of 'Foo'.
-    var fooClass = await getClassFromRootLib(isolate, 'Foo') as Class;
+    var fooClass = await getClassFromRootLib(isolate, 'Foo');
     expect(fooClass, isNotNull);
     expect(fooClass.name, equals('Foo'));
     print(fooClass.id);
@@ -51,7 +51,7 @@ var tests = <IsolateTest>[
 
   // Allocation profile.
   (Isolate isolate) async {
-    var fooClass = await getClassFromRootLib(isolate, 'Foo') as Class;
+    var fooClass = await getClassFromRootLib(isolate, 'Foo');
     await fooClass.reload();
     expect(fooClass.traceAllocations, isTrue);
     dynamic profileResponse = await fooClass.getAllocationSamples();
@@ -65,7 +65,7 @@ var tests = <IsolateTest>[
     cpuProfile.buildCodeCallerAndCallees();
     cpuProfile.buildFunctionCallerAndCallees();
     var tree = cpuProfile.loadCodeTree(M.ProfileTreeDirection.exclusive);
-    CodeCallTreeNode? node = tree.root;
+    var node = tree.root;
     var expected = [
       'Root',
       'DRT_AllocateObject',
@@ -75,7 +75,7 @@ var tests = <IsolateTest>[
       '_Closure.call'
     ];
     for (var i = 0; i < expected.length; i++) {
-      expect(node!.profileCode.code.name, equals(expected[i]));
+      expect(node.profileCode.code.name, equals(expected[i]));
       // Depth first traversal.
       if (node.children.length == 0) {
         node = null;
diff --git a/runtime/observatory/tests/service/get_client_name_rpc_test.dart b/runtime/observatory/tests/service/get_client_name_rpc_test.dart
index 89382f732dc..25b7828fd6c 100644
--- a/runtime/observatory/tests/service/get_client_name_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_client_name_rpc_test.dart
@@ -13,7 +13,7 @@ void fooBar() {}
 
 Future<String> getClientName(Isolate isolate) async {
   final result = await isolate.vm.invokeRpcNoUpgrade('getClientName', {});
-  return result['name'] as String;
+  return result['name'];
 }
 
 Future<void> setClientName(Isolate isolate, String name) async =>
diff --git a/runtime/observatory/tests/service/get_flag_list_rpc_test.dart b/runtime/observatory/tests/service/get_flag_list_rpc_test.dart
index 83f6f0c2989..f1c2c544f22 100644
--- a/runtime/observatory/tests/service/get_flag_list_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_flag_list_rpc_test.dart
@@ -85,7 +85,7 @@ var tests = <VMTest>[
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
       if (event.kind == ServiceEvent.kVMFlagUpdate) {
-        expect(event.owner!.type, 'VM');
+        expect(event.owner.type, 'VM');
         expect(event.flag, kProfilePeriod);
         expect(event.newValue, kValue.toString());
         subscription.cancel();
diff --git a/runtime/observatory/tests/service/get_isolate_after_async_error_test.dart b/runtime/observatory/tests/service/get_isolate_after_async_error_test.dart
index 2272969f07a..814cb7bb0b3 100644
--- a/runtime/observatory/tests/service/get_isolate_after_async_error_test.dart
+++ b/runtime/observatory/tests/service/get_isolate_after_async_error_test.dart
@@ -16,7 +16,7 @@ var tests = <IsolateTest>[
   (Isolate isolate) async {
     await isolate.reload();
     expect(isolate.error, isNotNull);
-    expect(isolate.error!.message!.contains('oh no'), isTrue);
+    expect(isolate.error.message.contains('oh no'), isTrue);
   }
 ];
 
diff --git a/runtime/observatory/tests/service/get_isolate_after_stack_overflow_error_test.dart b/runtime/observatory/tests/service/get_isolate_after_stack_overflow_error_test.dart
index 0b0159996c2..ab1c05deae7 100644
--- a/runtime/observatory/tests/service/get_isolate_after_stack_overflow_error_test.dart
+++ b/runtime/observatory/tests/service/get_isolate_after_stack_overflow_error_test.dart
@@ -21,7 +21,7 @@ var tests = <IsolateTest>[
   (Isolate isolate) async {
     await isolate.reload();
     expect(isolate.error, isNotNull);
-    expect(isolate.error!.message!.contains('Stack Overflow'), isTrue);
+    expect(isolate.error.message.contains('Stack Overflow'), isTrue);
   }
 ];
 
diff --git a/runtime/observatory/tests/service/get_isolate_after_sync_error_test.dart b/runtime/observatory/tests/service/get_isolate_after_sync_error_test.dart
index 7165a8c681d..8d0db3c69a3 100644
--- a/runtime/observatory/tests/service/get_isolate_after_sync_error_test.dart
+++ b/runtime/observatory/tests/service/get_isolate_after_sync_error_test.dart
@@ -16,7 +16,7 @@ var tests = <IsolateTest>[
   (Isolate isolate) async {
     await isolate.reload();
     expect(isolate.error, isNotNull);
-    expect(isolate.error!.message!.contains('oh no'), isTrue);
+    expect(isolate.error.message.contains('oh no'), isTrue);
   }
 ];
 
diff --git a/runtime/observatory/tests/service/get_isolate_group_memory_usage.dart b/runtime/observatory/tests/service/get_isolate_group_memory_usage.dart
index ebd2194f314..1bdb6bdf35b 100644
--- a/runtime/observatory/tests/service/get_isolate_group_memory_usage.dart
+++ b/runtime/observatory/tests/service/get_isolate_group_memory_usage.dart
@@ -23,7 +23,7 @@ var tests = <VMTest>[
     final params = {
       'isolateGroupId': 'badid',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await vm.invokeRpcNoUpgrade('getIsolateGroupMemoryUsage', params);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/get_isolate_rpc_test.dart b/runtime/observatory/tests/service/get_isolate_rpc_test.dart
index da57e6dfced..2346282925a 100644
--- a/runtime/observatory/tests/service/get_isolate_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_isolate_rpc_test.dart
@@ -36,7 +36,7 @@ var tests = <VMTest>[
     var params = {
       'isolateId': 'badid',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await vm.invokeRpcNoUpgrade('getIsolate', params);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/get_memory_usage.dart b/runtime/observatory/tests/service/get_memory_usage.dart
index 5b9a1c9fb5f..19ce0d23a28 100644
--- a/runtime/observatory/tests/service/get_memory_usage.dart
+++ b/runtime/observatory/tests/service/get_memory_usage.dart
@@ -22,7 +22,7 @@ var tests = <VMTest>[
     var params = {
       'isolateId': 'badid',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await vm.invokeRpcNoUpgrade('getMemoryUsage', params);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/get_native_allocation_samples_test.dart b/runtime/observatory/tests/service/get_native_allocation_samples_test.dart
index ca41a91b18f..31f532559ba 100644
--- a/runtime/observatory/tests/service/get_native_allocation_samples_test.dart
+++ b/runtime/observatory/tests/service/get_native_allocation_samples_test.dart
@@ -25,8 +25,8 @@ void verifyHelper(var root, bool exclusive) {
   int exclusiveAllocations = 0;
 
   for (int i = 0; i < root.children.length; i++) {
-    inclusiveAllocations += root.children[i].inclusiveNativeAllocations as int;
-    exclusiveAllocations += root.children[i].exclusiveNativeAllocations as int;
+    inclusiveAllocations += root.children[i].inclusiveNativeAllocations;
+    exclusiveAllocations += root.children[i].exclusiveNativeAllocations;
   }
 
   int rootMemory;
@@ -57,8 +57,7 @@ var tests = <VMTest>[
   // Verify inclusive tries.
   (VM vm) async {
     var response =
-        await vm.invokeRpc('_getNativeAllocationSamples', {'_code': true})
-            as ServiceMap;
+        await vm.invokeRpc('_getNativeAllocationSamples', {'_code': true});
     SampleProfile cpuProfile = new SampleProfile();
     await cpuProfile.load(vm, response);
     var codeTree = cpuProfile.loadCodeTree(M.ProfileTreeDirection.inclusive);
@@ -70,8 +69,7 @@ var tests = <VMTest>[
   // Verify exclusive tries.
   (VM vm) async {
     var response =
-        await vm.invokeRpc('_getNativeAllocationSamples', {'_code': true})
-            as ServiceMap;
+        await vm.invokeRpc('_getNativeAllocationSamples', {'_code': true});
     SampleProfile cpuProfile = new SampleProfile();
     await cpuProfile.load(vm, response);
     var codeTreeExclusive =
diff --git a/runtime/observatory/tests/service/get_object_rpc_test.dart b/runtime/observatory/tests/service/get_object_rpc_test.dart
index 09b39d06954..faf5eca4288 100644
--- a/runtime/observatory/tests/service/get_object_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_object_rpc_test.dart
@@ -13,7 +13,7 @@ import 'test_helper.dart';
 
 class _DummyClass {
   static var dummyVar = 11;
-  final List<String> dummyList = new List<String>.filled(20, '');
+  final List<String> dummyList = new List<String>.filled(20, null);
   void dummyFunction() {}
 }
 
@@ -693,7 +693,7 @@ var tests = <IsolateTest>[
     var params = {
       'objectId': 'libraries/9999999',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason: 'Unreachable');
@@ -736,7 +736,7 @@ var tests = <IsolateTest>[
     var params = {
       'objectId': 'scripts/9999999',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason: 'Unreachable');
@@ -783,7 +783,7 @@ var tests = <IsolateTest>[
     var params = {
       'objectId': 'classes/9999999',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason: 'Unreachable');
@@ -823,7 +823,7 @@ var tests = <IsolateTest>[
     var params = {
       'objectId': id,
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason: 'Unreachable');
@@ -868,7 +868,7 @@ var tests = <IsolateTest>[
     var params = {
       'objectId': id,
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason: 'Unreachable');
@@ -944,7 +944,7 @@ var tests = <IsolateTest>[
     var params = {
       'objectId': id,
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason: 'Unreachable');
@@ -992,7 +992,7 @@ var tests = <IsolateTest>[
     var params = {
       'objectId': 'code/0',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getObject', params);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/get_retaining_path_rpc_test.dart b/runtime/observatory/tests/service/get_retaining_path_rpc_test.dart
index 5a87e8b7619..1a036e590b0 100644
--- a/runtime/observatory/tests/service/get_retaining_path_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_retaining_path_rpc_test.dart
@@ -11,20 +11,20 @@ class _TestClass {
   _TestClass();
   // Make sure these fields are not removed by the tree shaker.
   @pragma("vm:entry-point")
-  dynamic x;
+  var x;
   @pragma("vm:entry-point")
-  dynamic y;
+  var y;
 }
 
-dynamic target1 = new _TestClass();
-dynamic target2 = new _TestClass();
-dynamic target3 = new _TestClass();
-dynamic target4 = new _TestClass();
-dynamic target5 = new _TestClass();
-dynamic globalObject = new _TestClass();
-dynamic globalList = new List<dynamic>.filled(100, null);
-dynamic globalMap1 = new Map();
-dynamic globalMap2 = new Map();
+var target1 = new _TestClass();
+var target2 = new _TestClass();
+var target3 = new _TestClass();
+var target4 = new _TestClass();
+var target5 = new _TestClass();
+var globalObject = new _TestClass();
+var globalList = new List(100);
+var globalMap1 = new Map();
+var globalMap2 = new Map();
 
 void warmup() {
   globalObject.x = target1;
@@ -104,14 +104,14 @@ var tests = <IsolateTest>[
     var params = {
       'targetId': obj['id'],
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('getRetainingPath', params);
       expect(false, isTrue, reason: 'Unreachable');
     } on ServerRpcException catch (e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
-      expect(e.data!['details'],
+      expect(e.data['details'],
           "getRetainingPath expects the \'limit\' parameter");
     }
     expect(caughtException, isTrue);
diff --git a/runtime/observatory/tests/service/get_scripts_rpc_test.dart b/runtime/observatory/tests/service/get_scripts_rpc_test.dart
index c27358bff4c..9331adb368f 100644
--- a/runtime/observatory/tests/service/get_scripts_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_scripts_rpc_test.dart
@@ -22,7 +22,7 @@ var tests = <VMTest>[
     var params = {
       'isolateId': 'badid',
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await vm.invokeRpcNoUpgrade('getScripts', params);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/get_source_report_with_mixin_test.dart b/runtime/observatory/tests/service/get_source_report_with_mixin_test.dart
index 061d85fc51b..db85b1a039b 100644
--- a/runtime/observatory/tests/service/get_source_report_with_mixin_test.dart
+++ b/runtime/observatory/tests/service/get_source_report_with_mixin_test.dart
@@ -34,7 +34,7 @@ var tests = <IsolateTest>[
     expect(stack['frames'][0].function.name, equals('testFunction'));
 
     final List<Script> scripts = await isolate.getScripts();
-    Script? foundScript;
+    Script foundScript;
     for (Script script in scripts) {
       if (script.uri.contains(lib1Filename)) {
         foundScript = script;
@@ -58,7 +58,7 @@ var tests = <IsolateTest>[
       // Now get report for the lib1 only.
       final Map<String, Object> params = {
         'reports': ['Coverage'],
-        'scriptId': foundScript!.id!
+        'scriptId': foundScript.id
       };
       final coverage =
           await isolate.invokeRpcNoUpgrade('getSourceReport', params);
diff --git a/runtime/observatory/tests/service/get_stack_rpc_test.dart b/runtime/observatory/tests/service/get_stack_rpc_test.dart
index 1c7a6e271a0..db1695ff76e 100644
--- a/runtime/observatory/tests/service/get_stack_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_stack_rpc_test.dart
@@ -84,7 +84,7 @@ var tests = <IsolateTest>[
     expect(msgHandlerObjectId, isNotNull);
 
     // Get object.
-    Instance object = await isolate.getObject(msgHandlerObjectId) as Instance;
+    Instance object = await isolate.getObject(msgHandlerObjectId);
     expect(object.valueAsString, equals('34'));
   }
 ];
diff --git a/runtime/observatory/tests/service/get_vm_timeline_rpc_test.dart b/runtime/observatory/tests/service/get_vm_timeline_rpc_test.dart
index a0d219c6f44..b149a7e5837 100644
--- a/runtime/observatory/tests/service/get_vm_timeline_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_vm_timeline_rpc_test.dart
@@ -43,7 +43,7 @@ bool mapContains(Map map, Map submap) {
   return true;
 }
 
-bool eventsContains(List events, String phase, String name, [Map? arguments]) {
+bool eventsContains(List events, String phase, String name, [Map arguments]) {
   for (Map event in events) {
     if ((event['ph'] == phase) && (event['name'] == name)) {
       if (arguments == null) {
diff --git a/runtime/observatory/tests/service/http_get_isolate_group_rpc_common.dart b/runtime/observatory/tests/service/http_get_isolate_group_rpc_common.dart
index f85e8d974cc..ebe21f51577 100644
--- a/runtime/observatory/tests/service/http_get_isolate_group_rpc_common.dart
+++ b/runtime/observatory/tests/service/http_get_isolate_group_rpc_common.dart
@@ -26,9 +26,9 @@ Future<String> getIsolateGroupId(
       .cast<List<int>>()
       .transform(utf8.decoder)
       .transform(json.decoder)
-      .first as Map;
+      .first;
   final result = response['result'];
-  return result['isolateGroups'][0]['id'] as String;
+  return result['isolateGroups'][0]['id'];
 }
 
 Future<Null> testeeBefore() async {
@@ -41,17 +41,17 @@ Future<Null> testeeBefore() async {
 
   // Build the request.
   final params = <String, String>{
-    'isolateGroupId': await getIsolateGroupId(httpClient, info.serverUri!),
+    'isolateGroupId': await getIsolateGroupId(httpClient, info.serverUri),
   };
 
   const method = 'getIsolateGroup';
-  final pathSegments = <String>[]..addAll(info.serverUri!.pathSegments);
+  final pathSegments = <String>[]..addAll(info.serverUri.pathSegments);
   if (pathSegments.isNotEmpty) {
     pathSegments[pathSegments.length - 1] = method;
   } else {
     pathSegments.add(method);
   }
-  final requestUri = info.serverUri!
+  final requestUri = info.serverUri
       .replace(pathSegments: pathSegments, queryParameters: params);
 
   try {
@@ -61,7 +61,7 @@ Future<Null> testeeBefore() async {
         .cast<List<int>>()
         .transform(utf8.decoder)
         .transform(json.decoder)
-        .first as Map;
+        .first;
     final result = jsonResponse['result'];
     Expect.equals(result['type'], 'IsolateGroup');
     Expect.isTrue(result['id'].startsWith('isolateGroups/'));
diff --git a/runtime/observatory/tests/service/http_get_isolate_rpc_common.dart b/runtime/observatory/tests/service/http_get_isolate_rpc_common.dart
index 3bbc12ed387..6d487c21b36 100644
--- a/runtime/observatory/tests/service/http_get_isolate_rpc_common.dart
+++ b/runtime/observatory/tests/service/http_get_isolate_rpc_common.dart
@@ -25,9 +25,9 @@ Future<String> getIsolateId(io.HttpClient httpClient, Uri serverUri) async {
       .cast<List<int>>()
       .transform(utf8.decoder)
       .transform(json.decoder)
-      .first as Map;
+      .first;
   Map result = response['result'];
-  return result['isolates'][0]['id'] as String;
+  return result['isolates'][0]['id'];
 }
 
 Future<Null> testeeBefore() async {
@@ -40,17 +40,17 @@ Future<Null> testeeBefore() async {
 
   // Build the request.
   final params = <String, String>{
-    'isolateId': await getIsolateId(httpClient, info.serverUri!),
+    'isolateId': await getIsolateId(httpClient, info.serverUri),
   };
 
   String method = 'getIsolate';
-  final pathSegments = <String>[]..addAll(info.serverUri!.pathSegments);
+  final pathSegments = <String>[]..addAll(info.serverUri.pathSegments);
   if (pathSegments.isNotEmpty) {
     pathSegments[pathSegments.length - 1] = method;
   } else {
     pathSegments.add(method);
   }
-  final requestUri = info.serverUri!
+  final requestUri = info.serverUri
       .replace(pathSegments: pathSegments, queryParameters: params);
 
   try {
@@ -59,7 +59,7 @@ Future<Null> testeeBefore() async {
         .cast<List<int>>()
         .transform(utf8.decoder)
         .transform(json.decoder)
-        .first as Map;
+        .first;
     Map result = response['result'];
     Expect.equals(result['type'], 'Isolate');
     Expect.isTrue(result['id'].startsWith('isolates/'));
diff --git a/runtime/observatory/tests/service/http_get_vm_rpc_common.dart b/runtime/observatory/tests/service/http_get_vm_rpc_common.dart
index 481f567cff1..b9b405e02d6 100644
--- a/runtime/observatory/tests/service/http_get_vm_rpc_common.dart
+++ b/runtime/observatory/tests/service/http_get_vm_rpc_common.dart
@@ -19,14 +19,14 @@ Future<Null> testeeBefore() async {
   var httpClient = new io.HttpClient();
 
   // Build the request.
-  final pathSegments = <String>[]..addAll(info.serverUri!.pathSegments);
+  final pathSegments = <String>[]..addAll(info.serverUri.pathSegments);
   String method = 'getVM';
   if (pathSegments.isNotEmpty) {
     pathSegments[pathSegments.length - 1] = method;
   } else {
     pathSegments.add(method);
   }
-  final requestUri = info.serverUri!.replace(pathSegments: pathSegments);
+  final requestUri = info.serverUri.replace(pathSegments: pathSegments);
 
   try {
     var request = await httpClient.getUrl(requestUri);
@@ -34,7 +34,7 @@ Future<Null> testeeBefore() async {
         .cast<List<int>>()
         .transform(utf8.decoder)
         .transform(json.decoder)
-        .first as Map;
+        .first;
     Map result = response['result'];
     Expect.equals(result['type'], 'VM');
     Expect.equals(result['name'], 'vm');
diff --git a/runtime/observatory/tests/service/implicit_getter_setter_test.dart b/runtime/observatory/tests/service/implicit_getter_setter_test.dart
index 4b7f96ffe05..30710d52252 100644
--- a/runtime/observatory/tests/service/implicit_getter_setter_test.dart
+++ b/runtime/observatory/tests/service/implicit_getter_setter_test.dart
@@ -20,12 +20,12 @@ script() {
 }
 
 Future testGetter(Isolate isolate) async {
-  Library rootLibrary = await isolate.rootLibrary.load() as Library;
+  Library rootLibrary = await isolate.rootLibrary.load();
   expect(rootLibrary.classes.length, equals(1));
-  Class classA = await rootLibrary.classes[0].load() as Class;
+  Class classA = await rootLibrary.classes[0].load();
   expect(classA.name, equals('A'));
   // Find getter.
-  ServiceFunction? getterFunc;
+  ServiceFunction getterFunc;
   for (ServiceFunction function in classA.functions) {
     if (function.name == 'field') {
       getterFunc = function;
@@ -33,21 +33,21 @@ Future testGetter(Isolate isolate) async {
     }
   }
   expect(getterFunc, isNotNull);
-  await getterFunc!.load();
-  Field field = await getterFunc.field!.load() as Field;
+  await getterFunc.load();
+  Field field = await getterFunc.field.load();
   expect(field, isNotNull);
   expect(field.name, equals('field'));
-  Class classDouble = await field.guardClass!.load() as Class;
+  Class classDouble = await field.guardClass.load();
   expect(classDouble.name, equals('_Double'));
 }
 
 Future testSetter(Isolate isolate) async {
-  Library rootLibrary = await isolate.rootLibrary.load() as Library;
+  Library rootLibrary = await isolate.rootLibrary.load();
   expect(rootLibrary.classes.length, equals(1));
-  Class classA = await rootLibrary.classes[0].load() as Class;
+  Class classA = await rootLibrary.classes[0].load();
   expect(classA.name, equals('A'));
   // Find setter.
-  ServiceFunction? setterFunc;
+  ServiceFunction setterFunc;
   for (ServiceFunction function in classA.functions) {
     if (function.name == 'field=') {
       setterFunc = function;
@@ -55,11 +55,11 @@ Future testSetter(Isolate isolate) async {
     }
   }
   expect(setterFunc, isNotNull);
-  await setterFunc!.load();
-  Field field = await setterFunc.field!.load() as Field;
+  await setterFunc.load();
+  Field field = await setterFunc.field.load();
   expect(field, isNotNull);
   expect(field.name, equals('field'));
-  Class classDouble = await field.guardClass!.load() as Class;
+  Class classDouble = await field.guardClass.load();
   expect(classDouble.name, equals('_Double'));
 }
 
diff --git a/runtime/observatory/tests/service/inbound_references_test.dart b/runtime/observatory/tests/service/inbound_references_test.dart
index 05c7bc0f4d3..267f1b1cd57 100644
--- a/runtime/observatory/tests/service/inbound_references_test.dart
+++ b/runtime/observatory/tests/service/inbound_references_test.dart
@@ -22,19 +22,18 @@ void script() {
   n = new Node();
   e = new Edge();
   n.edge = e;
-  array = new List<dynamic>.filled(2, null);
+  array = new List(2);
   array[0] = n;
   array[1] = e;
 }
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Field field = lib.variables.where((v) => v.name == 'e').single;
     await field.load();
-    Instance e = field.staticValue as Instance;
-    ServiceMap response =
-        await isolate.getInboundReferences(e, 100) as ServiceMap;
+    Instance e = field.staticValue;
+    ServiceMap response = await isolate.getInboundReferences(e, 100);
     List references = response['references'];
     hasReferenceSuchThat(predicate) {
       expect(references.any(predicate), isTrue);
diff --git a/runtime/observatory/tests/service/invoke_test.dart b/runtime/observatory/tests/service/invoke_test.dart
index d1c8466c06d..4d198a7abd7 100644
--- a/runtime/observatory/tests/service/invoke_test.dart
+++ b/runtime/observatory/tests/service/invoke_test.dart
@@ -51,13 +51,13 @@ var tests = <IsolateTest>[
     Field field =
         lib.variables.singleWhere((field) => field.name == "instance");
     await field.load();
-    Instance instance = field.staticValue as Instance;
+    Instance instance = field.staticValue;
     field = lib.variables.singleWhere((field) => field.name == "apple");
     await field.load();
-    Instance apple = field.staticValue as Instance;
+    Instance apple = field.staticValue;
     field = lib.variables.singleWhere((field) => field.name == "banana");
     await field.load();
-    Instance banana = field.staticValue as Instance;
+    Instance banana = field.staticValue;
 
     dynamic result = await isolate.invokeRpc("invoke",
         {"targetId": lib.id, "selector": "libraryFunction", "argumentIds": []});
diff --git a/runtime/observatory/tests/service/isolate_lifecycle_test.dart b/runtime/observatory/tests/service/isolate_lifecycle_test.dart
index 8b452f19a18..a4a364b060f 100644
--- a/runtime/observatory/tests/service/isolate_lifecycle_test.dart
+++ b/runtime/observatory/tests/service/isolate_lifecycle_test.dart
@@ -110,7 +110,7 @@ var tests = <VMTest>[
     var resumesIssued = 0;
     var isolateList = vm.isolates.toList();
     for (var isolate in isolateList) {
-      if (isolate.name!.endsWith('main')) {
+      if (isolate.name.endsWith('main')) {
         continue;
       }
       try {
diff --git a/runtime/observatory/tests/service/issue_25465_test.dart b/runtime/observatory/tests/service/issue_25465_test.dart
index 2375be6d9d8..4b17e14f2ce 100644
--- a/runtime/observatory/tests/service/issue_25465_test.dart
+++ b/runtime/observatory/tests/service/issue_25465_test.dart
@@ -28,8 +28,8 @@ var tests = <IsolateTest>[
 
     var bpt1 = await isolate.addBreakpoint(script, LINE_A);
     var bpt2 = await isolate.addBreakpoint(script, LINE_B);
-    expect(await bpt1.location!.getLine(), equals(LINE_A));
-    expect(await bpt2.location!.getLine(), equals(LINE_B));
+    expect(await bpt1.location.getLine(), equals(LINE_A));
+    expect(await bpt2.location.getLine(), equals(LINE_B));
 
     var stream = await isolate.vm.getEventStream(VM.kDebugStream);
     Completer completer = new Completer();
@@ -41,7 +41,7 @@ var tests = <IsolateTest>[
         print('break count is $breakCount');
         if (breakCount == 1) {
           // We are stopped at breakpoint 1.
-          expect(event.breakpoint!.number, equals(bpt1.number));
+          expect(event.breakpoint.number, equals(bpt1.number));
 
           // Remove both breakpoints
           var result = await isolate.removeBreakpoint(bpt1);
diff --git a/runtime/observatory/tests/service/issue_30555_test.dart b/runtime/observatory/tests/service/issue_30555_test.dart
index 535e01b59e7..22f6a2bf7ec 100644
--- a/runtime/observatory/tests/service/issue_30555_test.dart
+++ b/runtime/observatory/tests/service/issue_30555_test.dart
@@ -38,36 +38,36 @@ var tests = <IsolateTest>[
     var sub;
     final Isolate firstIsolate = isolate;
     print("First isolate is: ${firstIsolate.id}");
-    Isolate? secondIsolate;
+    Isolate secondIsolate;
     sub = await isolate.vm.listenEventStream(VM.kDebugStream, (ServiceEvent c) {
-      print("Event ${c.kind} on ${c.isolate!.id}");
+      print("Event ${c.kind} on ${c.isolate.id}");
       switch (step) {
         case 0:
           expect(c.kind, equals(ServiceEvent.kResume),
               reason: "First isolate should resume");
-          expect(c.isolate!.id, equals(firstIsolate.id),
+          expect(c.isolate.id, equals(firstIsolate.id),
               reason: "First isolate should resume");
           break;
         case 1:
           expect(c.kind, equals(ServiceEvent.kPauseStart),
               reason: "Second isolate should pause on start");
-          expect(c.isolate!.id, equals(isolate.vm.isolates[1].id),
+          expect(c.isolate.id, equals(isolate.vm.isolates[1].id),
               reason: "Second isolate should pause on start");
-          secondIsolate = c.isolate!;
-          print("Second isolate is: ${secondIsolate!.id}");
+          secondIsolate = c.isolate;
+          print("Second isolate is: ${secondIsolate.id}");
           print("Resuming second isolate");
-          secondIsolate!.resume();
+          secondIsolate.resume();
           break;
         case 2:
           expect(c.kind, equals(ServiceEvent.kResume),
               reason: "Second isolate should resume");
-          expect(c.isolate!.id, equals(secondIsolate!.id),
+          expect(c.isolate.id, equals(secondIsolate.id),
               reason: "Second isolate should resume");
           break;
         case 3:
           expect(c.kind, equals(ServiceEvent.kPauseBreakpoint),
               reason: "First isolate should stop at debugger()");
-          expect(c.isolate!.id, equals(firstIsolate.id),
+          expect(c.isolate.id, equals(firstIsolate.id),
               reason: "First isolate should stop at debugger()");
           print("Resuming first isolate");
           firstIsolate.resume();
@@ -75,7 +75,7 @@ var tests = <IsolateTest>[
         case 4:
           expect(c.kind, equals(ServiceEvent.kResume),
               reason: "First isolate should resume (1)");
-          expect(c.isolate!.id, equals(firstIsolate.id),
+          expect(c.isolate.id, equals(firstIsolate.id),
               reason: "First isolate should resume (1)");
           break;
         case 5:
@@ -86,18 +86,18 @@ var tests = <IsolateTest>[
           expect(c.kind, equals(ServiceEvent.kPauseBreakpoint),
               reason: "First & Second isolate should stop at debugger()");
           print("Resuming second isolate");
-          secondIsolate!.resume();
+          secondIsolate.resume();
           break;
         case 7:
           expect(c.kind, equals(ServiceEvent.kResume),
               reason: "Second isolate should resume before the exception");
-          expect(c.isolate!.id, equals(secondIsolate!.id),
+          expect(c.isolate.id, equals(secondIsolate.id),
               reason: "Second isolate should resume before the exception");
           break;
         case 8:
           expect(c.kind, equals(ServiceEvent.kPauseExit),
               reason: "Second isolate should exit at the exception");
-          expect(c.isolate!.id, equals(secondIsolate!.id),
+          expect(c.isolate.id, equals(secondIsolate.id),
               reason: "Second isolate should exit at the exception");
           print("Resuming first isolate");
           firstIsolate.resume();
@@ -105,11 +105,11 @@ var tests = <IsolateTest>[
         case 9:
           expect(c.kind, equals(ServiceEvent.kResume),
               reason: "First isolate should resume after the exception");
-          expect(c.isolate!.id, equals(firstIsolate.id),
+          expect(c.isolate.id, equals(firstIsolate.id),
               reason: "First isolate should resume after the exception");
           break;
         case 10:
-          expect(c.isolate!.id, equals(firstIsolate.id),
+          expect(c.isolate.id, equals(firstIsolate.id),
               reason: "First "
                   "isolate should stop at debugger() after exception.\n"
                   "Probably the second resumed even though it was not expect "
diff --git a/runtime/observatory/tests/service/local_variable_declaration_test.dart b/runtime/observatory/tests/service/local_variable_declaration_test.dart
index 162f311a096..f7e696ad492 100644
--- a/runtime/observatory/tests/service/local_variable_declaration_test.dart
+++ b/runtime/observatory/tests/service/local_variable_declaration_test.dart
@@ -9,12 +9,12 @@ import 'service_test_common.dart';
 import 'test_helper.dart';
 import 'dart:developer';
 
-testParameters(int jjjj, int oooo, [int? hhhh, int? nnnn]) {
+testParameters(int jjjj, int oooo, [int hhhh, int nnnn]) {
   debugger();
 }
 
 testMain() {
-  int? xxx, yyyy, zzzzz;
+  int xxx, yyyy, zzzzz;
   for (int i = 0; i < 1; i++) {
     var foo = () {};
     debugger();
@@ -38,7 +38,7 @@ var tests = <IsolateTest>[
     // Grab the top frame.
     Frame frame = stack['frames'][0];
     // Grab the script.
-    Script script = frame.location!.script;
+    Script script = frame.location.script;
     await script.load();
 
     // Ensure that the token at each declaration position is the name of the
@@ -46,7 +46,7 @@ var tests = <IsolateTest>[
     for (var variable in frame.variables) {
       final int declarationTokenPos = variable['declarationTokenPos'];
       final String name = variable['name'];
-      final String? token = script.getToken(declarationTokenPos);
+      final String token = script.getToken(declarationTokenPos);
       // When running from an appjit snapshot, sources aren't available so the returned token will
       // be null.
       if (token != null) {
@@ -65,14 +65,14 @@ var tests = <IsolateTest>[
     // Grab the top frame.
     Frame frame = stack['frames'][0];
     // Grab the script.
-    Script script = frame.location!.script;
+    Script script = frame.location.script;
     await script.load();
     print(frame);
     expect(frame.variables.length, greaterThanOrEqualTo(1));
     for (var variable in frame.variables) {
       final int declarationTokenPos = variable['declarationTokenPos'];
       final String name = variable['name'];
-      final String? token = script.getToken(declarationTokenPos);
+      final String token = script.getToken(declarationTokenPos);
       // When running from an appjit snapshot, sources aren't available so the returned token will
       // be null.
       if (token != null) {
@@ -90,7 +90,7 @@ var tests = <IsolateTest>[
     // Grab the top frame.
     Frame frame = stack['frames'][0];
     // Grab the script.
-    Script script = frame.location!.script;
+    Script script = frame.location.script;
     await script.load();
 
     // Ensure that the token at each declaration position is the name of the
@@ -99,7 +99,7 @@ var tests = <IsolateTest>[
     for (var variable in frame.variables) {
       final int declarationTokenPos = variable['declarationTokenPos'];
       final String name = variable['name'];
-      final String? token = script.getToken(declarationTokenPos);
+      final String token = script.getToken(declarationTokenPos);
       // When running from an appjit snapshot, sources aren't available so the returned token will
       // be null.
       if (token != null) {
diff --git a/runtime/observatory/tests/service/logging_test.dart b/runtime/observatory/tests/service/logging_test.dart
index 87c8cdbcfd9..4acf8b0df5e 100644
--- a/runtime/observatory/tests/service/logging_test.dart
+++ b/runtime/observatory/tests/service/logging_test.dart
@@ -34,18 +34,18 @@ var tests = <IsolateTest>[
   hasStoppedAtBreakpoint,
   resumeIsolateAndAwaitEvent(Isolate.kLoggingStream, (ServiceEvent event) {
     expect(event.kind, equals(ServiceEvent.kLogging));
-    expect(event.logRecord!['sequenceNumber'], equals(0));
-    expect(event.logRecord!['message'].valueAsString, equals('Hey Buddy!'));
-    expect(event.logRecord!['level'], equals(Level.FINE));
-    expect(event.logRecord!['time'], new isInstanceOf<DateTime>());
+    expect(event.logRecord['sequenceNumber'], equals(0));
+    expect(event.logRecord['message'].valueAsString, equals('Hey Buddy!'));
+    expect(event.logRecord['level'], equals(Level.FINE));
+    expect(event.logRecord['time'], new isInstanceOf<DateTime>());
   }),
   hasStoppedAtBreakpoint,
   resumeIsolateAndAwaitEvent(Isolate.kLoggingStream, (ServiceEvent event) {
     expect(event.kind, equals(ServiceEvent.kLogging));
-    expect(event.logRecord!['sequenceNumber'], equals(1));
-    expect(event.logRecord!['level'], equals(Level.INFO));
-    expect(event.logRecord!['message'].valueAsString, equals('YES'));
-    expect(event.logRecord!['time'], new isInstanceOf<DateTime>());
+    expect(event.logRecord['sequenceNumber'], equals(1));
+    expect(event.logRecord['level'], equals(Level.INFO));
+    expect(event.logRecord['message'].valueAsString, equals('YES'));
+    expect(event.logRecord['time'], new isInstanceOf<DateTime>());
   }),
 ];
 
diff --git a/runtime/observatory/tests/service/malformed_test.dart b/runtime/observatory/tests/service/malformed_test.dart
index 27b14560ef1..9e277c8400b 100644
--- a/runtime/observatory/tests/service/malformed_test.dart
+++ b/runtime/observatory/tests/service/malformed_test.dart
@@ -8,7 +8,7 @@ import 'test_helper.dart';
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpc('_respondWithMalformedObject', {});
       expect(false, isTrue, reason: 'Unreachable');
@@ -21,7 +21,7 @@ var tests = <IsolateTest>[
 
   // Do this test last... it kills the vm connection.
   (Isolate isolate) async {
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpc('_respondWithMalformedJson', {});
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/metrics_test.dart b/runtime/observatory/tests/service/metrics_test.dart
index 95c0e1f0708..b35806c88cf 100644
--- a/runtime/observatory/tests/service/metrics_test.dart
+++ b/runtime/observatory/tests/service/metrics_test.dart
@@ -25,12 +25,12 @@ var tests = <IsolateTest>[
   (Isolate isolate) async {
     var params = {'metricId': 'metrics/a.b.c'};
     ServiceMetric counter =
-        await isolate.invokeRpc('_getIsolateMetric', params) as ServiceMetric;
+        await isolate.invokeRpc('_getIsolateMetric', params);
     expect(counter.name, equals('a.b.c'));
     expect(counter.value, equals(1234.5));
   },
   (Isolate isolate) async {
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate
           .invokeRpc('_getIsolateMetric', {'metricId': 'metrics/a.b.d'});
diff --git a/runtime/observatory/tests/service/mirror_references_test.dart b/runtime/observatory/tests/service/mirror_references_test.dart
index 9dc5fb70891..1567d2d4527 100644
--- a/runtime/observatory/tests/service/mirror_references_test.dart
+++ b/runtime/observatory/tests/service/mirror_references_test.dart
@@ -11,14 +11,14 @@ import 'test_helper.dart';
 
 class Foo {}
 
-dynamic /*Foo*/ foo;
-dynamic /*MirrorReference*/ ref;
+Foo foo;
+var /*MirrorReference*/ ref;
 
 void script() {
   foo = new Foo();
   ClassMirror fooClassMirror = reflectClass(Foo);
   InstanceMirror fooClassMirrorMirror = reflect(fooClassMirror);
-  LibraryMirror libmirrors = fooClassMirrorMirror.type.owner as LibraryMirror;
+  LibraryMirror libmirrors = fooClassMirrorMirror.type.owner;
   ref = reflect(fooClassMirror)
       .getField(MirrorSystem.getSymbol('_reflectee', libmirrors))
       .reflectee;
@@ -26,20 +26,20 @@ void script() {
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Field fooField = lib.variables.singleWhere((v) => v.name == 'foo');
     await fooField.load();
-    Instance foo = fooField.staticValue as Instance;
+    Instance foo = fooField.staticValue;
     Field refField = lib.variables.singleWhere((v) => v.name == 'ref');
     await refField.load();
-    Instance ref = refField.staticValue as Instance;
+    Instance ref = refField.staticValue;
 
     expect(foo.isMirrorReference, isFalse);
     expect(ref.isMirrorReference, isTrue);
     expect(ref.referent, isNull);
-    Instance loadedRef = await ref.load() as Instance;
+    Instance loadedRef = await ref.load();
     expect(loadedRef.referent, isNotNull);
-    expect(loadedRef.referent!.name, equals('Foo'));
+    expect(loadedRef.referent.name, equals('Foo'));
     expect(loadedRef.referent, equals(foo.clazz));
   },
 ];
diff --git a/runtime/observatory/tests/service/native_metrics_test.dart b/runtime/observatory/tests/service/native_metrics_test.dart
index c200a4ad550..0cb1394507d 100644
--- a/runtime/observatory/tests/service/native_metrics_test.dart
+++ b/runtime/observatory/tests/service/native_metrics_test.dart
@@ -26,12 +26,12 @@ var tests = <IsolateTest>[
   (Isolate isolate) async {
     var params = {'metricId': 'metrics/native/heap.old.used'};
     ServiceMetric counter =
-        await isolate.invokeRpc('_getIsolateMetric', params) as ServiceMetric;
+        await isolate.invokeRpc('_getIsolateMetric', params);
     expect(counter.type, equals('Counter'));
     expect(counter.name, equals('heap.old.used'));
   },
   (Isolate isolate) async {
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpc(
           '_getIsolateMetric', {'metricId': 'metrics/native/doesnotexist'});
diff --git a/runtime/observatory/tests/service/network_profiling_test.dart b/runtime/observatory/tests/service/network_profiling_test.dart
index cfffe33dcf3..4da599542a2 100644
--- a/runtime/observatory/tests/service/network_profiling_test.dart
+++ b/runtime/observatory/tests/service/network_profiling_test.dart
@@ -32,14 +32,14 @@ Future<void> socketTest() async {
   var socket = await io.Socket.connect(localhost, serverSocket.port);
   socket.write(content);
   await socket.flush();
-  socket.destroy();
+  await socket.destroy();
 
   // rawDatagram
   final doneCompleter = Completer<void>();
   var server = await io.RawDatagramSocket.bind(localhost, 0);
   server.listen((io.RawSocketEvent event) {
     if (event == io.RawSocketEvent.read) {
-      io.Datagram dg = server.receive()!;
+      io.Datagram dg = server.receive();
       if (!doneCompleter.isCompleted) {
         doneCompleter.complete();
       }
@@ -56,11 +56,11 @@ Future<void> socketTest() async {
   postEvent('socketTest', {'socket': 'test'});
 }
 
-void checkFinishEvent(ServiceEvent event) {
+Future<void> checkFinishEvent(ServiceEvent event) {
   expect(event.kind, equals(ServiceEvent.kExtension));
   expect(event.extensionKind, equals('socketTest'));
   expect(event.extensionData, isInstanceOf<Map>());
-  expect(event.extensionData!['socket'], equals('test'));
+  expect(event.extensionData['socket'], equals('test'));
 }
 
 var tests = <IsolateTest>[
diff --git a/runtime/observatory/tests/service/next_through_assign_call_test.dart b/runtime/observatory/tests/service/next_through_assign_call_test.dart
index 439d2ebac6e..3b815d2df8b 100644
--- a/runtime/observatory/tests/service/next_through_assign_call_test.dart
+++ b/runtime/observatory/tests/service/next_through_assign_call_test.dart
@@ -9,16 +9,16 @@ const int LINE_A = 12;
 const String file = "next_through_assign_call_test.dart";
 
 code() {
-  int? a;
-  int? b;
+  int a;
+  int b;
   a = b = foo();
   print(a);
   print(b);
   a = foo();
   print(a);
-  int? d = foo();
+  int d = foo();
   print(d);
-  int? e = foo(), f, g = foo();
+  int e = foo(), f, g = foo();
   print(e);
   print(f);
   print(g);
@@ -30,18 +30,18 @@ foo() {
 
 List<String> stops = [];
 List<String> expected = [
-  "$file:${LINE_A+0}:8", // on variable 'a'
-  "$file:${LINE_A+1}:8", // on variable 'b'
+  "$file:${LINE_A+0}:7", // on variable 'a'
+  "$file:${LINE_A+1}:7", // on variable 'b'
   "$file:${LINE_A+2}:11", // on call to 'foo'
   "$file:${LINE_A+3}:3", // on call to 'print'
   "$file:${LINE_A+4}:3", // on call to 'print'
   "$file:${LINE_A+5}:7", // on call to 'foo'
   "$file:${LINE_A+6}:3", // on call to 'print'
-  "$file:${LINE_A+7}:12", // on call to 'foo'
+  "$file:${LINE_A+7}:11", // on call to 'foo'
   "$file:${LINE_A+8}:3", // on call to 'print'
-  "$file:${LINE_A+9}:12", // on first call to 'foo'
-  "$file:${LINE_A+9}:19", // on variable 'f'
-  "$file:${LINE_A+9}:26", // on second call to 'foo'
+  "$file:${LINE_A+9}:11", // on first call to 'foo'
+  "$file:${LINE_A+9}:18", // on variable 'f'
+  "$file:${LINE_A+9}:25", // on second call to 'foo'
   "$file:${LINE_A+10}:3", // on call to 'print'
   "$file:${LINE_A+11}:3", // on call to 'print'
   "$file:${LINE_A+12}:3", // on call to 'print'
diff --git a/runtime/observatory/tests/service/next_through_assign_int_test.dart b/runtime/observatory/tests/service/next_through_assign_int_test.dart
index d5029e24262..c57fd84a4ff 100644
--- a/runtime/observatory/tests/service/next_through_assign_int_test.dart
+++ b/runtime/observatory/tests/service/next_through_assign_int_test.dart
@@ -9,16 +9,16 @@ const int LINE_A = 12;
 const String file = "next_through_assign_int_test.dart";
 
 code() {
-  int? a;
-  int? b;
+  int a;
+  int b;
   a = b = 42;
   print(a);
   print(b);
   a = 42;
   print(a);
-  int? d = 42;
+  int d = 42;
   print(d);
-  int? e = 41, f, g = 42;
+  int e = 41, f, g = 42;
   print(e);
   print(f);
   print(g);
@@ -26,18 +26,18 @@ code() {
 
 List<String> stops = [];
 List<String> expected = [
-  "$file:${LINE_A+0}:8", // on variable 'a'
-  "$file:${LINE_A+1}:8", // on variable 'b'
+  "$file:${LINE_A+0}:7", // on variable 'a'
+  "$file:${LINE_A+1}:7", // on variable 'b'
   "$file:${LINE_A+2}:7", // on 'b'
   "$file:${LINE_A+3}:3", // on call to 'print'
   "$file:${LINE_A+4}:3", // on call to 'print'
   "$file:${LINE_A+5}:3", // on 'a'
   "$file:${LINE_A+6}:3", // on call to 'print'
-  "$file:${LINE_A+7}:10", // on '='
+  "$file:${LINE_A+7}:9", // on '='
   "$file:${LINE_A+8}:3", // on call to 'print'
-  "$file:${LINE_A+9}:10", // on first '='
-  "$file:${LINE_A+9}:16", // on 'f'
-  "$file:${LINE_A+9}:21", // on second '='
+  "$file:${LINE_A+9}:9", // on first '='
+  "$file:${LINE_A+9}:15", // on 'f'
+  "$file:${LINE_A+9}:20", // on second '='
   "$file:${LINE_A+10}:3", // on call to 'print'
   "$file:${LINE_A+11}:3", // on call to 'print'
   "$file:${LINE_A+12}:3", // on call to 'print'
diff --git a/runtime/observatory/tests/service/next_through_create_list_and_map_test.dart b/runtime/observatory/tests/service/next_through_create_list_and_map_test.dart
index 5dddf49c696..523ef135b2b 100644
--- a/runtime/observatory/tests/service/next_through_create_list_and_map_test.dart
+++ b/runtime/observatory/tests/service/next_through_create_list_and_map_test.dart
@@ -37,7 +37,7 @@ code() {
   };
   print(myList);
   print(myConstList);
-  int lookup = myMap[1]!;
+  int lookup = myMap[1];
   print(lookup);
   print(myMap);
   print(myConstMap);
diff --git a/runtime/observatory/tests/service/object_graph_vm_test.dart b/runtime/observatory/tests/service/object_graph_vm_test.dart
index a4bf3fec15f..94147099e7c 100644
--- a/runtime/observatory/tests/service/object_graph_vm_test.dart
+++ b/runtime/observatory/tests/service/object_graph_vm_test.dart
@@ -16,9 +16,9 @@ class Foo {
   dynamic right;
 }
 
-late Foo r;
+Foo r;
 
-late List lst;
+List lst;
 
 void script() {
   // Create 3 instances of Foo, with out-degrees
@@ -30,10 +30,10 @@ void script() {
   r.right = b;
   a.left = b;
 
-  lst = new List<dynamic>.filled(2, null);
+  lst = new List(2);
   lst[0] = lst; // Self-loop.
   // Larger than any other fixed-size list in a fresh heap.
-  lst[1] = new List<dynamic>.filled(1234569, null);
+  lst[1] = new List(1234569);
 }
 
 var tests = <IsolateTest>[
diff --git a/runtime/observatory/tests/service/observatory_test_package/pubspec.yaml b/runtime/observatory/tests/service/observatory_test_package/pubspec.yaml
index 20169fdf4df..db61a01f499 100644
--- a/runtime/observatory/tests/service/observatory_test_package/pubspec.yaml
+++ b/runtime/observatory/tests/service/observatory_test_package/pubspec.yaml
@@ -1,4 +1,4 @@
 name: observatory_test_package
 publish_to: none
 environment:
-  sdk: '>=2.9.0 <3.0.0'
+  sdk: '^2.7.0'
diff --git a/runtime/observatory/tests/service/parameters_in_scope_at_entry_test.dart b/runtime/observatory/tests/service/parameters_in_scope_at_entry_test.dart
index 48cacd2d72a..03285f4a8b8 100644
--- a/runtime/observatory/tests/service/parameters_in_scope_at_entry_test.dart
+++ b/runtime/observatory/tests/service/parameters_in_scope_at_entry_test.dart
@@ -43,7 +43,7 @@ var tests = <IsolateTest>[
     var stack = await isolate.getStack();
     Frame top = stack['frames'][0];
     print(top);
-    expect(top.function!.name, equals("foo"));
+    expect(top.function.name, equals("foo"));
     print(top.variables);
     expect(top.variables.length, equals(1));
     var param = top.variables[0];
@@ -59,7 +59,7 @@ var tests = <IsolateTest>[
     var stack = await isolate.getStack();
     Frame top = stack['frames'][0];
     print(top);
-    expect(top.function!.name, equals("theClosureFunction"));
+    expect(top.function.name, equals("theClosureFunction"));
     print(top.variables);
     expect(top.variables.length, equals(1));
     var param = top.variables[0];
diff --git a/runtime/observatory/tests/service/pause_on_exceptions_test.dart b/runtime/observatory/tests/service/pause_on_exceptions_test.dart
index 35bd59e5783..05dae04297d 100644
--- a/runtime/observatory/tests/service/pause_on_exceptions_test.dart
+++ b/runtime/observatory/tests/service/pause_on_exceptions_test.dart
@@ -25,7 +25,7 @@ doUncaught() {
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.reload() as Library;
+    Library lib = await isolate.rootLibrary.reload();
 
     var onPaused = null;
     var onResume = null;
diff --git a/runtime/observatory/tests/service/pause_on_start_and_exit_test.dart b/runtime/observatory/tests/service/pause_on_start_and_exit_test.dart
index 508d7a8bf42..f1aff65e5a4 100644
--- a/runtime/observatory/tests/service/pause_on_start_and_exit_test.dart
+++ b/runtime/observatory/tests/service/pause_on_start_and_exit_test.dart
@@ -41,13 +41,13 @@ var tests = <IsolateTest>[
     print('Done waiting for pause event.');
 
     // Grab the timestamp.
-    var pausetime1 = isolate.pauseEvent!.timestamp;
+    var pausetime1 = isolate.pauseEvent.timestamp;
     expect(pausetime1, isNotNull);
     // Reload the isolate.
     await isolate.reload();
     // Verify that it is the same.
     expect(pausetime1.millisecondsSinceEpoch,
-        equals(isolate.pauseEvent!.timestamp.millisecondsSinceEpoch));
+        equals(isolate.pauseEvent.timestamp.millisecondsSinceEpoch));
 
     completer = new Completer();
     stream = await isolate.vm.getEventStream(VM.kDebugStream);
@@ -66,13 +66,13 @@ var tests = <IsolateTest>[
     await completer.future;
 
     // Grab the timestamp.
-    var pausetime2 = isolate.pauseEvent!.timestamp;
+    var pausetime2 = isolate.pauseEvent.timestamp;
     expect(pausetime2, isNotNull);
     // Reload the isolate.
     await isolate.reload();
     // Verify that it is the same.
     expect(pausetime2.millisecondsSinceEpoch,
-        equals(isolate.pauseEvent!.timestamp.millisecondsSinceEpoch));
+        equals(isolate.pauseEvent.timestamp.millisecondsSinceEpoch));
     expect(pausetime2.millisecondsSinceEpoch,
         greaterThan(pausetime1.millisecondsSinceEpoch));
   },
diff --git a/runtime/observatory/tests/service/pause_on_start_then_step_test.dart b/runtime/observatory/tests/service/pause_on_start_then_step_test.dart
index 17707c6ee25..93cefb4c20a 100644
--- a/runtime/observatory/tests/service/pause_on_start_then_step_test.dart
+++ b/runtime/observatory/tests/service/pause_on_start_then_step_test.dart
@@ -41,13 +41,13 @@ var tests = <IsolateTest>[
     print('Done waiting for pause event.');
 
     // Grab the timestamp.
-    var pausetime1 = isolate.pauseEvent!.timestamp;
+    var pausetime1 = isolate.pauseEvent.timestamp;
     expect(pausetime1, isNotNull);
     // Reload the isolate.
     await isolate.reload();
     // Verify that it is the same.
     expect(pausetime1.millisecondsSinceEpoch,
-        equals(isolate.pauseEvent!.timestamp.millisecondsSinceEpoch));
+        equals(isolate.pauseEvent.timestamp.millisecondsSinceEpoch));
 
     completer = new Completer();
     stream = await isolate.vm.getEventStream(VM.kDebugStream);
@@ -69,7 +69,7 @@ var tests = <IsolateTest>[
 
     // Grab the timestamp.
     print('Getting pausevent timestamp');
-    var pausetime2 = isolate.pauseEvent!.timestamp;
+    var pausetime2 = isolate.pauseEvent.timestamp;
     expect(pausetime2, isNotNull);
     // Reload the isolate.
     print('Reloading isolate');
@@ -77,7 +77,7 @@ var tests = <IsolateTest>[
     print('Reload finished');
     // Verify that it is the same.
     expect(pausetime2.millisecondsSinceEpoch,
-        equals(isolate.pauseEvent!.timestamp.millisecondsSinceEpoch));
+        equals(isolate.pauseEvent.timestamp.millisecondsSinceEpoch));
 
     expect(pausetime2.millisecondsSinceEpoch,
         greaterThan(pausetime1.millisecondsSinceEpoch));
diff --git a/runtime/observatory/tests/service/reachable_size_test.dart b/runtime/observatory/tests/service/reachable_size_test.dart
index 8cf9f9ebcbe..bca88d81c9e 100644
--- a/runtime/observatory/tests/service/reachable_size_test.dart
+++ b/runtime/observatory/tests/service/reachable_size_test.dart
@@ -11,34 +11,34 @@ import 'service_test_common.dart';
 class Pair {
   // Make sure these fields are not removed by the tree shaker.
   @pragma("vm:entry-point")
-  dynamic x;
+  var x;
   @pragma("vm:entry-point")
-  dynamic y;
+  var y;
 }
 
-dynamic p1;
-dynamic p2;
+var p1;
+var p2;
 
 buildGraph() {
   p1 = new Pair();
   p2 = new Pair();
 
   // Adds to both reachable and retained size.
-  p1.x = <dynamic>[];
-  p2.x = <dynamic>[];
+  p1.x = new List();
+  p2.x = new List();
 
   // Adds to reachable size only.
-  p1.y = p2.y = <dynamic>[];
+  p1.y = p2.y = new List();
 }
 
 Future<int> getReachableSize(ServiceObject obj) async {
-  Instance size = await obj.isolate!.getReachableSize(obj) as Instance;
-  return int.parse(size.valueAsString!);
+  Instance size = await obj.isolate.getReachableSize(obj);
+  return int.parse(size.valueAsString);
 }
 
 Future<int> getRetainedSize(ServiceObject obj) async {
-  Instance size = await obj.isolate!.getRetainedSize(obj) as Instance;
-  return int.parse(size.valueAsString!);
+  Instance size = await obj.isolate.getRetainedSize(obj);
+  return int.parse(size.valueAsString);
 }
 
 var tests = <IsolateTest>[
@@ -49,7 +49,7 @@ var tests = <IsolateTest>[
     // In general, shallow <= retained <= reachable. In this program,
     // 0 < shallow < retained < reachable.
 
-    int p1_shallow = p1.size!;
+    int p1_shallow = p1.size;
     int p1_retained = await getRetainedSize(p1);
     int p1_reachable = await getReachableSize(p1);
 
@@ -57,7 +57,7 @@ var tests = <IsolateTest>[
     expect(p1_shallow, lessThan(p1_retained));
     expect(p1_retained, lessThan(p1_reachable));
 
-    int p2_shallow = p2.size!;
+    int p2_shallow = p2.size;
     int p2_retained = await getRetainedSize(p2);
     int p2_reachable = await getReachableSize(p2);
 
diff --git a/runtime/observatory/tests/service/regexp_function_test.dart b/runtime/observatory/tests/service/regexp_function_test.dart
index bf2e5bedf04..d11abe0d149 100644
--- a/runtime/observatory/tests/service/regexp_function_test.dart
+++ b/runtime/observatory/tests/service/regexp_function_test.dart
@@ -34,26 +34,26 @@ var tests = <IsolateTest>[
 
     Field field = lib.variables.singleWhere((v) => v.name == 'regex');
     await field.load();
-    Instance regex = field.staticValue as Instance;
+    Instance regex = field.staticValue;
     expect(regex.isInstance, isTrue);
     expect(regex.isRegExp, isTrue);
     await regex.load();
 
     if (regex.oneByteFunction == null) {
       // Running with interpreted regexp.
-      var b1 = await regex.oneByteBytecode!.load();
+      var b1 = await regex.oneByteBytecode.load();
       expect(b1.isTypedData, isTrue);
-      var b2 = await regex.twoByteBytecode!.load();
+      var b2 = await regex.twoByteBytecode.load();
       expect(b2.isTypedData, isFalse); // No two-byte string subject was used.
     } else {
       // Running with compiled regexp.
-      var f1 = await regex.oneByteFunction!.load();
+      var f1 = await regex.oneByteFunction.load();
       expect(f1 is ServiceFunction, isTrue);
-      var f2 = await regex.twoByteFunction!.load();
+      var f2 = await regex.twoByteFunction.load();
       expect(f2 is ServiceFunction, isTrue);
-      var f3 = await regex.externalOneByteFunction!.load();
+      var f3 = await regex.externalOneByteFunction.load();
       expect(f3 is ServiceFunction, isTrue);
-      var f4 = await regex.externalTwoByteFunction!.load();
+      var f4 = await regex.externalTwoByteFunction.load();
       expect(f4 is ServiceFunction, isTrue);
     }
   }
diff --git a/runtime/observatory/tests/service/regress_34841_test.dart b/runtime/observatory/tests/service/regress_34841_test.dart
index 6a7310e1b28..05bb56a5475 100644
--- a/runtime/observatory/tests/service/regress_34841_test.dart
+++ b/runtime/observatory/tests/service/regress_34841_test.dart
@@ -57,8 +57,8 @@ var tests = <IsolateTest>[
 
     // Make sure we can translate it all.
     for (int place in coveragePlaces) {
-      int? line = script.tokenToLine(place);
-      int? column = script.tokenToCol(place);
+      int line = script.tokenToLine(place);
+      int column = script.tokenToCol(place);
       if (line == null || column == null) {
         throw "Token $place translated to $line:$column";
       }
diff --git a/runtime/observatory/tests/service/rewind_optimized_out_test.dart b/runtime/observatory/tests/service/rewind_optimized_out_test.dart
index 21cf92a181e..97aca1cdbcd 100644
--- a/runtime/observatory/tests/service/rewind_optimized_out_test.dart
+++ b/runtime/observatory/tests/service/rewind_optimized_out_test.dart
@@ -16,7 +16,7 @@ int LINE_D = 43;
 int global = 0;
 
 @pragma('vm:never-inline')
-b3(int x) {
+b3(x) {
   int sum = 0;
   try {
     for (int i = 0; i < x; i++) {
@@ -49,13 +49,13 @@ var tests = <IsolateTest>[
   stoppedAtLine(LINE_A),
   (Isolate isolate) async {
     // We are at our breakpoint with global=100.
-    Instance result = await isolate.rootLibrary.evaluate('global') as Instance;
+    Instance result = await isolate.rootLibrary.evaluate('global');
     print('global is $result');
     expect(result.type, equals('Instance'));
     expect(result.valueAsString, equals('100'));
 
     // Rewind the top stack frame.
-    bool caughtException = false;
+    bool caughtException;
     try {
       result = await isolate.rewind(1);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/rewind_test.dart b/runtime/observatory/tests/service/rewind_test.dart
index 5ec932d1c06..87e46ec4823 100644
--- a/runtime/observatory/tests/service/rewind_test.dart
+++ b/runtime/observatory/tests/service/rewind_test.dart
@@ -16,7 +16,7 @@ int LINE_D = 43;
 int global = 0;
 
 @pragma('vm:never-inline')
-b3(int x) {
+b3(x) {
   int sum = 0;
   try {
     for (int i = 0; i < x; i++) {
@@ -49,7 +49,7 @@ var tests = <IsolateTest>[
   stoppedAtLine(LINE_A),
   (Isolate isolate) async {
     // We are not able to rewind frame 0.
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.rewind(0);
       expect(false, isTrue, reason: 'Unreachable');
@@ -62,7 +62,7 @@ var tests = <IsolateTest>[
   },
   (Isolate isolate) async {
     // We are not able to rewind frame 13.
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.rewind(13);
       expect(false, isTrue, reason: 'Unreachable');
@@ -75,7 +75,7 @@ var tests = <IsolateTest>[
   },
   (Isolate isolate) async {
     // We are at our breakpoint with global=100.
-    Instance result = await isolate.rootLibrary.evaluate('global') as Instance;
+    Instance result = await isolate.rootLibrary.evaluate('global');
     print('global is $result');
     expect(result.type, equals('Instance'));
     expect(result.valueAsString, equals('100'));
@@ -94,7 +94,7 @@ var tests = <IsolateTest>[
   stoppedAtLine(LINE_A),
   (Isolate isolate) async {
     // global still is equal to 100.  We did not execute "global++".
-    Instance result = await isolate.rootLibrary.evaluate('global') as Instance;
+    Instance result = await isolate.rootLibrary.evaluate('global');
     print('global is $result');
     expect(result.type, equals('Instance'));
     expect(result.valueAsString, equals('100'));
@@ -107,8 +107,7 @@ var tests = <IsolateTest>[
   stoppedAtLine(LINE_D),
   (Isolate isolate) async {
     // Reset global to 0 and start again.
-    Instance result =
-        await isolate.rootLibrary.evaluate('global=0') as Instance;
+    Instance result = await isolate.rootLibrary.evaluate('global=0');
     print('set global to $result');
     expect(result.type, equals('Instance'));
     expect(result.valueAsString, equals('0'));
@@ -120,7 +119,7 @@ var tests = <IsolateTest>[
   stoppedAtLine(LINE_A),
   (Isolate isolate) async {
     // We are at our breakpoint with global=100.
-    Instance result = await isolate.rootLibrary.evaluate('global') as Instance;
+    Instance result = await isolate.rootLibrary.evaluate('global');
     print('global is $result');
     expect(result.type, equals('Instance'));
     expect(result.valueAsString, equals('100'));
diff --git a/runtime/observatory/tests/service/service_test_common.dart b/runtime/observatory/tests/service/service_test_common.dart
index 74ffc5fb210..673f58992a6 100644
--- a/runtime/observatory/tests/service/service_test_common.dart
+++ b/runtime/observatory/tests/service/service_test_common.dart
@@ -29,7 +29,7 @@ Future subscribeToStream(
 }
 
 Future cancelStreamSubscription(String streamName) async {
-  StreamSubscription subscription = streamSubscriptions[streamName]!;
+  StreamSubscription subscription = streamSubscriptions[streamName];
   subscription.cancel();
   streamSubscriptions.remove(streamName);
 }
@@ -73,12 +73,12 @@ Future syncNext(Isolate isolate) async {
 
 Future asyncStepOver(Isolate isolate) async {
   final Completer pausedAtSyntheticBreakpoint = new Completer();
-  StreamSubscription? subscription;
+  StreamSubscription subscription;
 
   // Cancel the subscription.
   cancelSubscription() {
     if (subscription != null) {
-      subscription!.cancel();
+      subscription.cancel();
       subscription = null;
     }
   }
@@ -99,13 +99,13 @@ Future asyncStepOver(Isolate isolate) async {
     return pausedAtSyntheticBreakpoint.future;
   }
 
-  Breakpoint? syntheticBreakpoint;
+  Breakpoint syntheticBreakpoint;
 
   subscription = stream.listen((ServiceEvent event) async {
     // Synthetic breakpoint add event. This is the first event we will
     // receive.
     bool isAdd = (event.kind == ServiceEvent.kBreakpointAdded) &&
-        (event.breakpoint!.isSyntheticAsyncContinuation!) &&
+        (event.breakpoint.isSyntheticAsyncContinuation) &&
         (event.owner == isolate);
     // Resume after synthetic breakpoint added. This is the second event
     // we will receive.
@@ -140,7 +140,7 @@ Future asyncStepOver(Isolate isolate) async {
   return pausedAtSyntheticBreakpoint.future;
 }
 
-bool isEventOfKind(M.Event? event, String kind) {
+bool isEventOfKind(M.Event event, String kind) {
   switch (kind) {
     case ServiceEvent.kPauseBreakpoint:
       return event is M.PauseBreakpointEvent;
@@ -159,7 +159,7 @@ bool isEventOfKind(M.Event? event, String kind) {
 
 Future hasPausedFor(Isolate isolate, String kind) {
   // Set up a listener to wait for breakpoint events.
-  Completer? completer = new Completer();
+  Completer completer = new Completer();
   isolate.vm.getEventStream(VM.kDebugStream).then((stream) {
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
@@ -168,7 +168,7 @@ Future hasPausedFor(Isolate isolate, String kind) {
           // Reload to update isolate.pauseEvent.
           print('Paused with $kind');
           subscription.cancel();
-          completer!.complete(isolate.reload());
+          completer.complete(isolate.reload());
           completer = null;
         }
       }
@@ -177,19 +177,19 @@ Future hasPausedFor(Isolate isolate, String kind) {
     // Pause may have happened before we subscribed.
     isolate.reload().then((_) {
       if ((isolate.pauseEvent != null) &&
-          isEventOfKind(isolate.pauseEvent!, kind)) {
+          isEventOfKind(isolate.pauseEvent, kind)) {
         // Already waiting at a breakpoint.
         if (completer != null) {
           print('Paused with $kind');
           subscription.cancel();
-          completer!.complete(isolate);
+          completer.complete(isolate);
           completer = null;
         }
       }
     });
   });
 
-  return completer!.future; // Will complete when breakpoint hit.
+  return completer.future; // Will complete when breakpoint hit.
 }
 
 Future hasStoppedAtBreakpoint(Isolate isolate) {
@@ -216,12 +216,13 @@ Future markDartColonLibrariesDebuggable(Isolate isolate) async {
   await isolate.reload();
   for (Library lib in isolate.libraries) {
     await lib.load();
-    if (lib.uri!.startsWith('dart:') && !lib.uri!.startsWith('dart:_')) {
+    if (lib.uri.startsWith('dart:') && !lib.uri.startsWith('dart:_')) {
       var setDebugParams = {
         'libraryId': lib.id,
         'isDebuggable': true,
       };
-      await isolate.invokeRpcNoUpgrade('setLibraryDebuggable', setDebugParams);
+      Map<String, dynamic> result = await isolate.invokeRpcNoUpgrade(
+          'setLibraryDebuggable', setDebugParams);
     }
   }
   return isolate;
@@ -241,7 +242,7 @@ IsolateTest reloadSources([bool pause = false]) {
 IsolateTest setBreakpointAtLine(int line) {
   return (Isolate isolate) async {
     print("Setting breakpoint for line $line");
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Script script = lib.scripts.firstWhere((s) => s.uri == lib.uri);
 
     Breakpoint bpt = await isolate.addBreakpoint(script, line);
@@ -254,7 +255,7 @@ IsolateTest setBreakpointAtLine(int line) {
 IsolateTest setBreakpointAtLineColumn(int line, int column) {
   return (Isolate isolate) async {
     print("Setting breakpoint for line $line column $column");
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Script script = lib.scripts.firstWhere((s) => s.uri == lib.uri);
 
     Breakpoint bpt = await isolate.addBreakpoint(script, line, column);
@@ -289,14 +290,14 @@ IsolateTest stoppedAtLine(int line) {
     expect(frames.length, greaterThanOrEqualTo(1));
 
     Frame top = frames[0];
-    Script script = await top.location!.script.load() as Script;
-    int? actualLine = script.tokenToLine(top.location!.tokenPos);
+    Script script = await top.location.script.load();
+    int actualLine = script.tokenToLine(top.location.tokenPos);
     if (actualLine != line) {
       StringBuffer sb = new StringBuffer();
       sb.write("Expected to be at line $line but actually at line $actualLine");
       sb.write("\nFull stack trace:\n");
       for (Frame f in stack['frames']) {
-        sb.write(" $f [${await f.location!.getLine()}]\n");
+        sb.write(" $f [${await f.location.getLine()}]\n");
       }
       throw sb.toString();
     } else {
@@ -317,12 +318,11 @@ IsolateTest stoppedInFunction(String functionName,
     expect(frames.length, greaterThanOrEqualTo(1));
 
     Frame topFrame = frames[0];
-    ServiceFunction function =
-        await topFrame.function!.load() as ServiceFunction;
-    String name = function.name!;
+    ServiceFunction function = await topFrame.function.load();
+    String name = function.name;
     if (includeOwner) {
       ServiceFunction owner =
-          await (function.dartOwner as ServiceObject).load() as ServiceFunction;
+          await (function.dartOwner as ServiceObject).load();
       name = '${owner.name}.$name';
     }
     final bool matches =
@@ -333,10 +333,10 @@ IsolateTest stoppedInFunction(String functionName,
           "actually in function $name");
       sb.write("\nFull stack trace:\n");
       for (Frame f in frames) {
-        await f.function!.load();
-        await (f.function!.dartOwner as ServiceObject).load();
-        String name = f.function!.name!;
-        String ownerName = (f.function!.dartOwner as ServiceObject).name!;
+        await f.function.load();
+        await (f.function.dartOwner as ServiceObject).load();
+        String name = f.function.name;
+        String ownerName = (f.function.dartOwner as ServiceObject).name;
         sb.write(" $f [$name] [$ownerName]\n");
       }
       throw sb.toString();
@@ -402,8 +402,8 @@ Future isolateIsRunning(Isolate isolate) async {
   expect(isolate.running, true);
 }
 
-Future<Class?> getClassFromRootLib(Isolate isolate, String className) async {
-  Library rootLib = await isolate.rootLibrary.load() as Library;
+Future<Class> getClassFromRootLib(Isolate isolate, String className) async {
+  Library rootLib = await isolate.rootLibrary.load();
   for (Class cls in rootLib.classes) {
     if (cls.name == className) {
       return cls;
@@ -414,10 +414,10 @@ Future<Class?> getClassFromRootLib(Isolate isolate, String className) async {
 
 Future<Instance> rootLibraryFieldValue(
     Isolate isolate, String fieldName) async {
-  Library rootLib = await isolate.rootLibrary.load() as Library;
+  Library rootLib = await isolate.rootLibrary.load();
   Field field = rootLib.variables.singleWhere((v) => v.name == fieldName);
   await field.load();
-  Instance value = field.staticValue as Instance;
+  Instance value = field.staticValue;
   await value.load();
   return value;
 }
@@ -431,9 +431,9 @@ IsolateTest runStepThroughProgramRecordingStops(List<String> recordStops) {
       if (event.kind == ServiceEvent.kPauseBreakpoint) {
         await isolate.reload();
         // We are paused: Step further.
-        Frame frame = isolate.topFrame!;
-        recordStops.add(await frame.location!.toUserString());
-        if (event.atAsyncSuspension!) {
+        Frame frame = isolate.topFrame;
+        recordStops.add(await frame.location.toUserString());
+        if (event.atAsyncSuspension) {
           isolate.stepOverAsyncSuspension();
         } else {
           isolate.stepOver();
@@ -464,10 +464,10 @@ IsolateTest resumeProgramRecordingStops(
         List frames = stack['frames'];
         expect(frames.length, greaterThanOrEqualTo(2));
         Frame frame = frames[0];
-        String brokeAt = await frame.location!.toUserString();
+        String brokeAt = await frame.location.toUserString();
         if (includeCaller) {
           frame = frames[1];
-          String calledFrom = await frame.location!.toUserString();
+          String calledFrom = await frame.location.toUserString();
           recordStops.add("$brokeAt ($calledFrom)");
         } else {
           recordStops.add(brokeAt);
@@ -495,8 +495,8 @@ IsolateTest runStepIntoThroughProgramRecordingStops(List<String> recordStops) {
       if (event.kind == ServiceEvent.kPauseBreakpoint) {
         await isolate.reload();
         // We are paused: Step into further.
-        Frame frame = isolate.topFrame!;
-        recordStops.add(await frame.location!.toUserString());
+        Frame frame = isolate.topFrame;
+        recordStops.add(await frame.location.toUserString());
         isolate.stepInto();
       } else if (event.kind == ServiceEvent.kPauseExit) {
         // We are at the exit: The test is done.
@@ -513,8 +513,8 @@ IsolateTest checkRecordedStops(
     List<String> recordStops, List<String> expectedStops,
     {bool removeDuplicates = false,
     bool debugPrint = false,
-    String? debugPrintFile,
-    int? debugPrintLine}) {
+    String debugPrintFile,
+    int debugPrintLine}) {
   return (Isolate isolate) async {
     if (debugPrint) {
       for (int i = 0; i < recordStops.length; i++) {
@@ -574,7 +574,7 @@ IsolateTest checkRecordedStops(
 
 List<String> removeAdjacentDuplicates(List<String> fromList) {
   List<String> result = <String>[];
-  String? latestLine;
+  String latestLine;
   for (String s in fromList) {
     if (s == latestLine) continue;
     latestLine = s;
diff --git a/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart b/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart
index c94aa2d057d..1e2753e163b 100644
--- a/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart
+++ b/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart
@@ -40,7 +40,7 @@ var tests = <IsolateTest>[
       'libraryId': 'libraries/9999999',
       'isDebuggable': false,
     };
-    bool caughtException = false;
+    bool caughtException;
     try {
       await isolate.invokeRpcNoUpgrade('setLibraryDebuggable', params);
       expect(false, isTrue, reason: 'Unreachable');
diff --git a/runtime/observatory/tests/service/set_library_debuggable_test.dart b/runtime/observatory/tests/service/set_library_debuggable_test.dart
index 74317786e57..4ffbcb67a78 100644
--- a/runtime/observatory/tests/service/set_library_debuggable_test.dart
+++ b/runtime/observatory/tests/service/set_library_debuggable_test.dart
@@ -49,8 +49,8 @@ var tests = <IsolateTest>[
       'libraryId': dartCore.id,
       'isDebuggable': false,
     };
-    var result = await isolate.invokeRpcNoUpgrade(
-        'setLibraryDebuggable', setDebugParams) as Map<String, dynamic>;
+    Map<String, dynamic> result = await isolate.invokeRpcNoUpgrade(
+        'setLibraryDebuggable', setDebugParams);
     expect(result['type'], equals('Success'));
     await dartCore.reload();
     expect(dartCore.debuggable, equals(false));
diff --git a/runtime/observatory/tests/service/set_name_rpc_test.dart b/runtime/observatory/tests/service/set_name_rpc_test.dart
index 521483251f0..1e4253db3e4 100644
--- a/runtime/observatory/tests/service/set_name_rpc_test.dart
+++ b/runtime/observatory/tests/service/set_name_rpc_test.dart
@@ -16,8 +16,8 @@ var tests = <IsolateTest>[
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
       if (event.kind == ServiceEvent.kIsolateUpdate) {
-        expect(event.owner!.type, equals('Isolate'));
-        expect(event.owner!.name, equals('Barbara'));
+        expect(event.owner.type, equals('Isolate'));
+        expect(event.owner.name, equals('Barbara'));
         subscription.cancel();
         completer.complete();
       }
diff --git a/runtime/observatory/tests/service/set_vm_name_rpc_test.dart b/runtime/observatory/tests/service/set_vm_name_rpc_test.dart
index 0254209e02f..67ad4616437 100644
--- a/runtime/observatory/tests/service/set_vm_name_rpc_test.dart
+++ b/runtime/observatory/tests/service/set_vm_name_rpc_test.dart
@@ -17,8 +17,8 @@ var tests = <VMTest>[
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
       if (event.kind == ServiceEvent.kVMUpdate) {
-        expect(event.owner!.type, equals('VM'));
-        expect(event.owner!.name, equals('Barbara'));
+        expect(event.owner.type, equals('VM'));
+        expect(event.owner.name, equals('Barbara'));
         subscription.cancel();
         completer.complete();
       }
diff --git a/runtime/observatory/tests/service/simple_reload_test.dart b/runtime/observatory/tests/service/simple_reload_test.dart
index e67c4295123..f67c95de631 100644
--- a/runtime/observatory/tests/service/simple_reload_test.dart
+++ b/runtime/observatory/tests/service/simple_reload_test.dart
@@ -34,9 +34,9 @@ Future<String> invokeTest(Isolate isolate) async {
   await isolate.reload();
   Library lib = isolate.rootLibrary;
   await lib.load();
-  Instance result = await lib.evaluate('test()') as Instance;
+  Instance result = await lib.evaluate('test()');
   expect(result.isString, isTrue);
-  return result.valueAsString as String;
+  return result.valueAsString;
 }
 
 var tests = <IsolateTest>[
diff --git a/runtime/observatory/tests/service/string_escaping_test.dart b/runtime/observatory/tests/service/string_escaping_test.dart
index 37e02a5af5f..8a11dfbcb8c 100644
--- a/runtime/observatory/tests/service/string_escaping_test.dart
+++ b/runtime/observatory/tests/service/string_escaping_test.dart
@@ -57,14 +57,14 @@ Future testStrings(Isolate isolate) async {
 
   expectFullString(String varName, String varValueAsString) {
     Field field = lib.variables.singleWhere((v) => v.name == varName);
-    Instance value = field.staticValue as Instance;
+    Instance value = field.staticValue;
     expect(value.valueAsString, equals(varValueAsString));
     expect(value.valueAsStringIsTruncated, isFalse);
   }
 
   expectTruncatedString(String varName, String varValueAsString) {
     Field field = lib.variables.singleWhere((v) => v.name == varName);
-    Instance value = field.staticValue as Instance;
+    Instance value = field.staticValue;
     print(value.valueAsString);
     expect(varValueAsString, startsWith(value.valueAsString));
     expect(value.valueAsStringIsTruncated, isTrue);
diff --git a/runtime/observatory/tests/service/tcp_socket_closing_service_test.dart b/runtime/observatory/tests/service/tcp_socket_closing_service_test.dart
index f482ea10476..8996577880e 100644
--- a/runtime/observatory/tests/service/tcp_socket_closing_service_test.dart
+++ b/runtime/observatory/tests/service/tcp_socket_closing_service_test.dart
@@ -36,7 +36,7 @@ Future setup() async {
   var server = await io.RawDatagramSocket.bind('127.0.0.1', 0);
   server.listen((io.RawSocketEvent event) {
     if (event == io.RawSocketEvent.read) {
-      io.Datagram dg = server.receive()!;
+      io.Datagram dg = server.receive();
       dg.data.forEach((x) => true);
       server.close();
     }
diff --git a/runtime/observatory/tests/service/test_helper.dart b/runtime/observatory/tests/service/test_helper.dart
index 3cf48c74197..4c78a05ba56 100644
--- a/runtime/observatory/tests/service/test_helper.dart
+++ b/runtime/observatory/tests/service/test_helper.dart
@@ -32,8 +32,8 @@ List<IsolateTest> ifLazyAsyncStacks(List<IsolateTest> lazyTests) {
 
 /// Will be set to the http address of the VM's service protocol before
 /// any tests are invoked.
-late String serviceHttpAddress;
-late String serviceWebsocketAddress;
+String serviceHttpAddress;
+String serviceWebsocketAddress;
 
 const String _TESTEE_ENV_KEY = 'SERVICE_TEST_TESTEE';
 const Map<String, String> _TESTEE_SPAWN_ENV = const {_TESTEE_ENV_KEY: 'true'};
@@ -46,14 +46,14 @@ bool _shouldLaunchSkyShell() {
   return Platform.environment.containsKey(_SKY_SHELL_ENV_KEY);
 }
 
-String? _skyShellPath() {
+String _skyShellPath() {
   return Platform.environment[_SKY_SHELL_ENV_KEY];
 }
 
 class _ServiceTesteeRunner {
   Future run(
-      {Function? testeeBefore: null,
-      Function? testeeConcurrent: null,
+      {testeeBefore(): null,
+      testeeConcurrent(): null,
       bool pause_on_start: false,
       bool pause_on_exit: false}) async {
     if (!pause_on_start) {
@@ -78,8 +78,8 @@ class _ServiceTesteeRunner {
   }
 
   void runSync(
-      {Function? testeeBeforeSync: null,
-      Function? testeeConcurrentSync: null,
+      {void testeeBeforeSync(): null,
+      void testeeConcurrentSync(): null,
       bool pause_on_start: false,
       bool pause_on_exit: false}) {
     if (!pause_on_start) {
@@ -99,7 +99,7 @@ class _ServiceTesteeRunner {
 }
 
 class _ServiceTesteeLauncher {
-  Process? process;
+  Process process;
   final List<String> args;
   Future<void> get exited => _processCompleter.future;
   final _processCompleter = Completer<void>();
@@ -116,8 +116,8 @@ class _ServiceTesteeLauncher {
       bool testeeControlsServer,
       Uri serviceInfoUri,
       int port,
-      List<String>? extraArgs,
-      List<String>? executableArgs) {
+      List<String> extraArgs,
+      List<String> executableArgs) {
     assert(pause_on_start != null);
     assert(pause_on_exit != null);
     assert(pause_on_unhandled_exceptions != null);
@@ -153,8 +153,8 @@ class _ServiceTesteeLauncher {
       bool testeeControlsServer,
       Uri serviceInfoUri,
       int port,
-      List<String>? extraArgs,
-      List<String>? executableArgs) {
+      List<String> extraArgs,
+      List<String> executableArgs) {
     assert(!_shouldLaunchSkyShell());
 
     final String dartExecutable = Platform.executable;
@@ -180,9 +180,7 @@ class _ServiceTesteeLauncher {
     if (extraArgs != null) {
       fullArgs.addAll(extraArgs);
     }
-    if (executableArgs != null) {
-      fullArgs.addAll(executableArgs);
-    }
+    fullArgs.addAll(executableArgs);
     if (!testeeControlsServer) {
       fullArgs.add('--enable-vm-service:$port');
     }
@@ -196,11 +194,11 @@ class _ServiceTesteeLauncher {
       bool pause_on_exit,
       bool pause_on_unhandled_exceptions,
       bool testeeControlsServer,
-      List<String>? extraArgs,
-      List<String>? executableArgs) {
+      List<String> extraArgs,
+      List<String> executableArgs) {
     assert(_shouldLaunchSkyShell());
 
-    final String dartExecutable = _skyShellPath()!;
+    final String dartExecutable = _skyShellPath();
 
     final dartFlags = <String>[];
     final fullArgs = <String>[];
@@ -220,9 +218,7 @@ class _ServiceTesteeLauncher {
     if (extraArgs != null) {
       fullArgs.addAll(extraArgs);
     }
-    if (executableArgs != null) {
-      fullArgs.addAll(executableArgs);
-    }
+    fullArgs.addAll(executableArgs);
     if (!testeeControlsServer) {
       fullArgs.add('--observatory-port=0');
     }
@@ -253,8 +249,8 @@ class _ServiceTesteeLauncher {
       bool enable_service_port_fallback,
       bool testeeControlsServer,
       int port,
-      List<String>? extraArgs,
-      List<String>? executableArgs) async {
+      List<String> extraArgs,
+      List<String> executableArgs) async {
     final completer = new Completer<Uri>();
     final serviceInfoDir =
         await Directory.systemTemp.createTemp('dart_service');
@@ -275,7 +271,7 @@ class _ServiceTesteeLauncher {
       Uri uri;
       final blankCompleter = Completer();
       bool blankLineReceived = false;
-      p.stdout
+      process.stdout
           .transform(utf8.decoder)
           .transform(new LineSplitter())
           .listen((line) {
@@ -286,13 +282,13 @@ class _ServiceTesteeLauncher {
         }
         print('>testee>out> $line');
       });
-      p.stderr
+      process.stderr
           .transform(utf8.decoder)
           .transform(new LineSplitter())
           .listen((line) {
         print('>testee>err> $line');
       });
-      p.exitCode.then((exitCode) async {
+      process.exitCode.then((exitCode) async {
         await serviceInfoDir.delete(recursive: true);
         if ((exitCode != 0) && !killedByTester) {
           throw "Testee exited with $exitCode";
@@ -325,7 +321,7 @@ class _ServiceTesteeLauncher {
   void requestExit() {
     if (process != null) {
       print('** Killing script');
-      if (process!.kill()) {
+      if (process.kill()) {
         killedByTester = true;
       }
     }
@@ -340,12 +336,12 @@ void setupAddresses(Uri serverAddress) {
 
 class _ServiceTesterRunner {
   void run({
-    List<String>? mainArgs,
-    List<String>? extraArgs,
-    List<String>? executableArgs,
-    List<DDSTest>? ddsTests,
-    List<IsolateTest>? isolateTests,
-    List<VMTest>? vmTests,
+    List<String> mainArgs,
+    List<String> extraArgs,
+    List<String> executableArgs,
+    List<DDSTest> ddsTests,
+    List<IsolateTest> isolateTests,
+    List<VMTest> vmTests,
     bool pause_on_start: false,
     bool pause_on_exit: false,
     bool verbose_vm: false,
@@ -359,9 +355,9 @@ class _ServiceTesterRunner {
     if (executableArgs == null) {
       executableArgs = Platform.executableArguments;
     }
-    late DartDevelopmentService dds;
-    late WebSocketVM vm;
-    late _ServiceTesteeLauncher process;
+    DartDevelopmentService dds;
+    WebSocketVM vm;
+    _ServiceTesteeLauncher process;
     bool testsDone = false;
 
     ignoreLateException(Function f) async {
@@ -392,8 +388,8 @@ class _ServiceTesterRunner {
                   extraArgs,
                   executableArgs)
               .then((Uri serverAddress) async {
-            if (mainArgs != null && mainArgs.contains("--gdb")) {
-              final pid = process.process!.pid;
+            if (mainArgs.contains("--gdb")) {
+              final pid = process.process.pid;
               final wait = new Duration(seconds: 10);
               print("Testee has pid $pid, waiting $wait before continuing");
               sleep(wait);
@@ -421,7 +417,7 @@ class _ServiceTesterRunner {
       () => ignoreLateException(
         () async {
           if (useDds) {
-            await dds.shutdown();
+            await dds?.shutdown();
           }
           process.requestExit();
         },
@@ -487,7 +483,7 @@ class _ServiceTesterRunner {
       }
     }
 
-    Completer<Isolate>? completer = new Completer<Isolate>();
+    Completer completer = new Completer();
     vm.getEventStream(VM.kIsolateStream).then((stream) {
       var subscription;
       subscription = stream.listen((ServiceEvent event) async {
@@ -500,7 +496,7 @@ class _ServiceTesterRunner {
             vm.isolates.first.load().then((result) {
               if (result is Isolate) {
                 subscription.cancel();
-                completer!.complete(result);
+                completer.complete(result);
                 completer = null;
               }
             });
@@ -513,12 +509,12 @@ class _ServiceTesterRunner {
     if (vm.isolates.isNotEmpty) {
       vm.isolates.first.reload().then((result) async {
         if (completer != null && result is Isolate) {
-          completer!.complete(result);
+          completer.complete(result);
           completer = null;
         }
       });
     }
-    return await completer!.future;
+    return await completer.future;
   }
 }
 
@@ -527,8 +523,8 @@ class _ServiceTesterRunner {
 /// concurrently with the tests. Uses [mainArgs] to determine whether
 /// to run tests or testee in this invocation of the script.
 Future runIsolateTests(List<String> mainArgs, List<IsolateTest> tests,
-    {Function? testeeBefore,
-    Function? testeeConcurrent,
+    {testeeBefore(),
+    testeeConcurrent(),
     bool pause_on_start: false,
     bool pause_on_exit: false,
     bool verbose_vm: false,
@@ -536,7 +532,7 @@ Future runIsolateTests(List<String> mainArgs, List<IsolateTest> tests,
     bool testeeControlsServer: false,
     bool enableDds: true,
     bool enableService: true,
-    List<String>? extraArgs}) async {
+    List<String> extraArgs}) async {
   assert(!pause_on_start || testeeBefore == null);
   if (_isTestee()) {
     new _ServiceTesteeRunner().run(
@@ -570,13 +566,13 @@ Future runIsolateTests(List<String> mainArgs, List<IsolateTest> tests,
 /// in an async context (because exceptions are *always* handled in async
 /// functions).
 void runIsolateTestsSynchronous(List<String> mainArgs, List<IsolateTest> tests,
-    {Function? testeeBefore,
-    Function? testeeConcurrent,
+    {void testeeBefore(),
+    void testeeConcurrent(),
     bool pause_on_start: false,
     bool pause_on_exit: false,
     bool verbose_vm: false,
     bool pause_on_unhandled_exceptions: false,
-    List<String>? extraArgs}) {
+    List<String> extraArgs}) {
   assert(!pause_on_start || testeeBefore == null);
   if (_isTestee()) {
     new _ServiceTesteeRunner().runSync(
@@ -601,8 +597,8 @@ void runIsolateTestsSynchronous(List<String> mainArgs, List<IsolateTest> tests,
 /// concurrently with the tests. Uses [mainArgs] to determine whether
 /// to run tests or testee in this invocation of the script.
 Future runVMTests(List<String> mainArgs, List<VMTest> tests,
-    {Function? testeeBefore,
-    Function? testeeConcurrent,
+    {testeeBefore(),
+    testeeConcurrent(),
     bool pause_on_start: false,
     bool pause_on_exit: false,
     bool verbose_vm: false,
@@ -611,8 +607,8 @@ Future runVMTests(List<String> mainArgs, List<VMTest> tests,
     bool enableDds: true,
     bool enableService: true,
     int port = 0,
-    List<String>? extraArgs,
-    List<String>? executableArgs}) async {
+    List<String> extraArgs,
+    List<String> executableArgs}) async {
   if (_isTestee()) {
     new _ServiceTesteeRunner().run(
         testeeBefore: testeeBefore,
@@ -643,16 +639,16 @@ Future runVMTests(List<String> mainArgs, List<VMTest> tests,
 /// determine whether to run tests or testee in this invocation of the
 /// script.
 Future runDDSTests(List<String> mainArgs, List<DDSTest> tests,
-    {Function? testeeBefore,
-    Function? testeeConcurrent,
+    {testeeBefore(),
+    testeeConcurrent(),
     bool pause_on_start: false,
     bool pause_on_exit: false,
     bool verbose_vm: false,
     bool pause_on_unhandled_exceptions: false,
     bool enable_service_port_fallback: false,
     int port = 0,
-    List<String>? extraArgs,
-    List<String>? executableArgs}) async {
+    List<String> extraArgs,
+    List<String> executableArgs}) async {
   if (_isTestee()) {
     new _ServiceTesteeRunner().run(
         testeeBefore: testeeBefore,
diff --git a/runtime/observatory/tests/service/type_arguments_test.dart b/runtime/observatory/tests/service/type_arguments_test.dart
index 92ec78f783d..1c2a6acc715 100644
--- a/runtime/observatory/tests/service/type_arguments_test.dart
+++ b/runtime/observatory/tests/service/type_arguments_test.dart
@@ -36,7 +36,7 @@ var tests = <IsolateTest>[
           // Check that we can 'get' this object again.
           var firstType = allTypeArgsList[0];
           return isolate.getObject(firstType.id).then((ServiceObject object) {
-            TypeArguments type = object as TypeArguments;
+            TypeArguments type = object;
             expect(firstType.name, type.name);
           });
         });
diff --git a/runtime/observatory/tests/service/typed_data_test.dart b/runtime/observatory/tests/service/typed_data_test.dart
index 9c4ba3b2246..d96583adf27 100644
--- a/runtime/observatory/tests/service/typed_data_test.dart
+++ b/runtime/observatory/tests/service/typed_data_test.dart
@@ -78,18 +78,18 @@ void script() {
 var tests = <IsolateTest>[
   (Isolate isolate) async {
     script();
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
 
     // Pre-load all the fields so we don't use await below and get better
     // stacktraces.
     for (var v in lib.variables) {
       await v.load();
-      await v.staticValue!.load();
+      await v.staticValue.load();
     }
 
     expectTypedData(name, expectedValue) {
       var variable = lib.variables.singleWhere((v) => v.name == name);
-      var actualValue = (variable.staticValue as Instance).typedElements!;
+      var actualValue = (variable.staticValue as Instance).typedElements;
       if (expectedValue is Int32x4List) {
         expect(actualValue.length, equals(expectedValue.length));
         for (var i = 0; i < actualValue.length; i++) {
diff --git a/runtime/observatory/tests/service/udp_socket_service_test.dart b/runtime/observatory/tests/service/udp_socket_service_test.dart
index e4459d1e0fc..e49aa97a459 100644
--- a/runtime/observatory/tests/service/udp_socket_service_test.dart
+++ b/runtime/observatory/tests/service/udp_socket_service_test.dart
@@ -20,7 +20,7 @@ Future setupUDP() async {
   var server = await io.RawDatagramSocket.bind('127.0.0.1', 0);
   server.listen((io.RawSocketEvent event) {
     if (event == io.RawSocketEvent.read) {
-      io.Datagram dg = server.receive()!;
+      io.Datagram dg = server.receive();
       dg.data.forEach((x) => true);
       if (!doneCompleter.isCompleted) {
         doneCompleter.complete(null);
diff --git a/runtime/observatory/tests/service/valid_source_locations_test.dart b/runtime/observatory/tests/service/valid_source_locations_test.dart
index d4c47711445..85124798257 100644
--- a/runtime/observatory/tests/service/valid_source_locations_test.dart
+++ b/runtime/observatory/tests/service/valid_source_locations_test.dart
@@ -14,16 +14,16 @@ void testFunction() {
   debugger();
 }
 
-Future validateLocation(Location? location, Object object) async {
+Future validateLocation(Location location, Object object) async {
   if (location == null) return;
-  if (location.tokenPos! < 0) return;
+  if (location.tokenPos < 0) return;
   if (location.script.uri == 'dart:_internal-patch/class_id_fasta.dart') {
     // Injected fields from this script cannot be reloaded.
     return;
   }
 
   // Ensure the script is loaded.
-  final Script script = await location.script.load() as Script;
+  final Script script = await location.script.load();
 
   // Use the more low-level functions.
   final line = script.tokenToLine(location.tokenPos);
@@ -39,17 +39,17 @@ Future validateLocation(Location? location, Object object) async {
 }
 
 Future validateFieldLocation(Field field) async {
-  field = await field.load() as Field;
+  field = await field.load();
   await validateLocation(field.location, field);
 }
 
 Future validateFunctionLocation(ServiceFunction fun) async {
-  fun = await fun.load() as ServiceFunction;
+  fun = await fun.load();
   await validateLocation(fun.location, fun);
 }
 
 Future validateClassLocation(Class klass) async {
-  klass = await klass.load() as Class;
+  klass = await klass.load();
   await validateLocation(klass.location, klass);
 
   for (Field field in klass.fields) {
@@ -72,7 +72,7 @@ var tests = <IsolateTest>[
 
     // Loop over all libraries, classes, functions and fields to ensure .
     for (Library lib in isolate.libraries) {
-      lib = await lib.load() as Library;
+      lib = await lib.load();
 
       for (Field field in lib.variables) {
         await validateFieldLocation(field);
diff --git a/runtime/observatory/tests/service/verify_http_timeline_test.dart b/runtime/observatory/tests/service/verify_http_timeline_test.dart
index 74de89a9f0c..22fae9bd929 100644
--- a/runtime/observatory/tests/service/verify_http_timeline_test.dart
+++ b/runtime/observatory/tests/service/verify_http_timeline_test.dart
@@ -173,10 +173,10 @@ bool hasCompletedEvents(List traceEvents) {
     if (isStartEvent(event)) {
       final id = event['id'];
       events.putIfAbsent(id, () => 0);
-      events[id] = events[id]! + 1;
+      events[id]++;
     } else if (isFinishEvent(event)) {
       final id = event['id'];
-      events[id] = events[id]! - 1;
+      events[id]--;
     }
   }
   bool valid = true;
diff --git a/runtime/observatory/tests/service/vm_test.dart b/runtime/observatory/tests/service/vm_test.dart
index cad42345345..862e54f96a3 100644
--- a/runtime/observatory/tests/service/vm_test.dart
+++ b/runtime/observatory/tests/service/vm_test.dart
@@ -7,8 +7,8 @@ import 'package:test/test.dart';
 import 'test_helper.dart';
 
 var tests = <IsolateTest>[
-  (Isolate isolate) async {
-    VM vm = isolate.owner as VM;
+  (Isolate isolate) {
+    VM vm = isolate.owner;
     expect(vm.targetCPU, isNotNull);
     expect(vm.architectureBits == 32 || vm.architectureBits == 64, isTrue);
     expect(vm.embedder, equals("Dart VM"));
diff --git a/runtime/observatory/tests/service/vm_timeline_events_test.dart b/runtime/observatory/tests/service/vm_timeline_events_test.dart
index 062a0c1ea99..5ae3e8b3852 100644
--- a/runtime/observatory/tests/service/vm_timeline_events_test.dart
+++ b/runtime/observatory/tests/service/vm_timeline_events_test.dart
@@ -26,15 +26,15 @@ List filterEvents(List events, filter) {
   return events.where(filter).toList();
 }
 
-late Completer<void> completer;
-int eventCount = 0;
+Completer completer;
+int eventCount;
 
 onTimelineEvent(ServiceEvent event) {
   if (event.kind != ServiceEvent.kTimelineEvents) {
     return;
   }
   eventCount++;
-  expect(filterEvents(event.timelineEvents!, isDart).length, greaterThan(0));
+  expect(filterEvents(event.timelineEvents, isDart).length, greaterThan(0));
   if (eventCount == 5) {
     completer.complete(eventCount);
   }
diff --git a/runtime/observatory/tests/service/vm_timeline_flags_test.dart b/runtime/observatory/tests/service/vm_timeline_flags_test.dart
index bfdefee0feb..72608f614d2 100644
--- a/runtime/observatory/tests/service/vm_timeline_flags_test.dart
+++ b/runtime/observatory/tests/service/vm_timeline_flags_test.dart
@@ -26,7 +26,7 @@ List filterEvents(List events, filter) {
   return events.where(filter).toList();
 }
 
-int dartEventCount = 0;
+int dartEventCount;
 
 var tests = <IsolateTest>[
   hasStoppedAtBreakpoint,
@@ -51,8 +51,8 @@ var tests = <IsolateTest>[
     final completer = Completer<void>();
     await subscribeToStream(isolate.vm, 'Timeline', (event) async {
       expect(event.kind, ServiceEvent.kTimelineStreamSubscriptionsUpdate);
-      expect(event.updatedStreams!.length, 1);
-      expect(event.updatedStreams!.first, 'Dart');
+      expect(event.updatedStreams.length, 1);
+      expect(event.updatedStreams.first, 'Dart');
       await cancelStreamSubscription('Timeline');
       completer.complete();
     });
@@ -90,7 +90,7 @@ var tests = <IsolateTest>[
     final completer = Completer<void>();
     await subscribeToStream(isolate.vm, 'Timeline', (event) async {
       expect(event.kind, ServiceEvent.kTimelineStreamSubscriptionsUpdate);
-      expect(event.updatedStreams!.length, 0);
+      expect(event.updatedStreams.length, 0);
       await cancelStreamSubscription('Timeline');
       completer.complete();
     });
diff --git a/runtime/observatory/tests/service/weak_properties_test.dart b/runtime/observatory/tests/service/weak_properties_test.dart
index 7e557d1d057..bc08b352b38 100644
--- a/runtime/observatory/tests/service/weak_properties_test.dart
+++ b/runtime/observatory/tests/service/weak_properties_test.dart
@@ -25,7 +25,7 @@ void script() {
   expando[key] = value;
 
   InstanceMirror expandoMirror = reflect(expando);
-  LibraryMirror libcore = expandoMirror.type.owner as LibraryMirror;
+  LibraryMirror libcore = expandoMirror.type.owner;
 
   var entries = expandoMirror
       .getField(MirrorSystem.getSymbol('_data', libcore))
@@ -36,31 +36,31 @@ void script() {
 
 var tests = <IsolateTest>[
   (Isolate isolate) async {
-    Library lib = await isolate.rootLibrary.load() as Library;
+    Library lib = await isolate.rootLibrary.load();
     Field keyField = lib.variables.singleWhere((v) => v.name == 'key');
     await keyField.load();
-    Instance key = keyField.staticValue as Instance;
+    Instance key = keyField.staticValue;
     Field valueField = lib.variables.singleWhere((v) => v.name == 'value');
     await valueField.load();
-    Instance value = valueField.staticValue as Instance;
+    Instance value = valueField.staticValue;
     Field propField =
         lib.variables.singleWhere((v) => v.name == 'weak_property');
     await propField.load();
-    Instance prop = propField.staticValue as Instance;
+    Instance prop = propField.staticValue;
 
     expect(key.isWeakProperty, isFalse);
     expect(value.isWeakProperty, isFalse);
     expect(prop.isWeakProperty, isTrue);
     expect(prop.key, isNull);
     expect(prop.value, isNull);
-    Instance loadedProp = await prop.load() as Instance;
+    Instance loadedProp = await prop.load();
     // Object ids are not canonicalized, so we rely on the key and value
     // being the sole instances of their classes to test we got the objects
     // we expect.
     expect(loadedProp.key, isNotNull);
-    expect(loadedProp.key!.clazz, equals(key.clazz));
+    expect(loadedProp.key.clazz, equals(key.clazz));
     expect(loadedProp.value, isNotNull);
-    expect(loadedProp.value!.clazz, equals(value.clazz));
+    expect(loadedProp.value.clazz, equals(value.clazz));
   },
 ];
 
diff --git a/runtime/observatory/tests/ui/inspector.dart b/runtime/observatory/tests/ui/inspector.dart
index e8fc90a4a7f..7849381ef20 100644
--- a/runtime/observatory/tests/ui/inspector.dart
+++ b/runtime/observatory/tests/ui/inspector.dart
@@ -18,8 +18,9 @@ var node;
 var uninitialized = new Object();
 
 extractPrivateField(obj, name) {
-  var lib = reflect(obj).type.owner as LibraryMirror;
-  return reflect(obj).getField(MirrorSystem.getSymbol(name, lib)).reflectee;
+  return reflect(obj)
+      .getField(MirrorSystem.getSymbol(name, reflect(obj).type.owner))
+      .reflectee;
 }
 
 class A<T> {}
@@ -137,7 +138,7 @@ class Node {
     mixedType = "2";
     mixedType = false;
 
-    array = new List<dynamic>.filled(3, null);
+    array = new List(3);
     array[0] = 1;
     array[1] = 2;
     array[2] = 3;
@@ -155,7 +156,7 @@ class Node {
     float64 = 3.14;
     float64x2 = new Float64x2(0.0, 3.14);
     gauge = new Gauge("GaugeName", "Gauge description", 0.0, 100.0);
-    growableList = <dynamic>[];
+    growableList = new List();
     int32x4 = new Int32x4(0, 1, 10, 11);
     map = {
       "x-key": "x-value",
diff --git a/runtime/observatory/tests/ui/retainingPath.dart b/runtime/observatory/tests/ui/retainingPath.dart
index 238ab4c3879..0a2c30fbd50 100644
--- a/runtime/observatory/tests/ui/retainingPath.dart
+++ b/runtime/observatory/tests/ui/retainingPath.dart
@@ -11,7 +11,7 @@ class Foo {
 }
 
 main() {
-  var list = new List<dynamic>.filled(10, null);
+  var list = new List<Foo>.filled(10, null);
   list[5] = new Foo(42.toString(), new Foo(87.toString(), 17.toString()));
   while (true) {}
 }
diff --git a/runtime/observatory/web/main.dart b/runtime/observatory/web/main.dart
index 820c74aeac0..b09362dbdee 100644
--- a/runtime/observatory/web/main.dart
+++ b/runtime/observatory/web/main.dart
@@ -14,7 +14,7 @@ main() async {
       print('${rec.level.name}: ${rec.time}: ${rec.message}');
     });
     Logger.root.info('Starting Observatory');
-    document.body!.children
+    document.body.children
         .insert(0, new ObservatoryApplicationElement.created().element);
   });
 }
diff --git a/tools/bots/test_matrix.json b/tools/bots/test_matrix.json
index a716c21fc08..a58e637a779 100644
--- a/tools/bots/test_matrix.json
+++ b/tools/bots/test_matrix.json
@@ -1206,8 +1206,7 @@
             "corelib",
             "ffi",
             "language",
-            "lib",
-            "service"
+            "lib"
           ]
         },
         {
@@ -1473,8 +1472,7 @@
             "ffi",
             "language",
             "lib",
-            "standalone/io",
-            "service"
+            "standalone/io"
           ]
         },
         {
@@ -1664,8 +1662,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-${sanitizer}-${system}-${mode}-${arch}",
-            "--exclude_suite=service"
+            "-ndartk-${sanitizer}-${system}-${mode}-${arch}"
           ]
         }
       ]
@@ -1817,8 +1814,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-${system}-${mode}-${arch}",
-            "--exclude_suite=service"
+            "-ndartk-${system}-${mode}-${arch}"
           ],
           "fileset": "vm-kernel",
           "shards": 10
@@ -1851,7 +1847,6 @@
             "language",
             "lib",
             "co19",
-            "service",
             "standalone/io"
           ]
         },
@@ -1893,8 +1888,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-${system}-${mode}-${arch}",
-            "--exclude_suite=service"
+            "-ndartk-${system}-${mode}-${arch}"
           ],
           "fileset": "vm-kernel",
           "shards": 10
@@ -1928,8 +1922,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-checked-linux-release-x64",
-            "--exclude_suite=service"
+            "-ndartk-checked-linux-release-x64"
           ],
           "fileset": "vm-kernel",
           "shards": 10
@@ -1954,8 +1947,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-${system}-${mode}-${arch}",
-            "--exclude_suite=service"
+            "-ndartk-${system}-${mode}-${arch}"
           ],
           "fileset": "vm-kernel",
           "shards": 1
@@ -2306,8 +2298,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-optcounter-linux-release-${arch}",
-            "--exclude_suite=service"
+            "-ndartk-optcounter-linux-release-${arch}"
           ],
           "fileset": "vm-kernel",
           "shards": 10
@@ -2333,8 +2324,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-reload-${system}-${mode}-${arch}",
-            "--exclude_suite=service"
+            "-ndartk-reload-${system}-${mode}-${arch}"
           ],
           "fileset": "vm-kernel",
           "shards": 10
@@ -2360,8 +2350,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartk-reload-rollback-linux-${mode}-x64",
-            "--exclude_suite=service"
+            "-ndartk-reload-rollback-linux-${mode}-x64"
           ],
           "fileset": "vm-kernel",
           "shards": 10
@@ -2451,8 +2440,6 @@
           "arguments": [
             "-ndart2js-hostasserts-linux-ia32-d8",
             "--dart2js-batch",
-            "--enable-experiment=non-nullable",
-            "--dart2js-options=--no-sound-null-safety",
             "observatory_ui"
           ]
         },
@@ -2558,8 +2545,6 @@
             "-ndart2js-${system}-${runtime}",
             "--dart2js-batch",
             "--reset-browser-configuration",
-            "--enable-experiment=non-nullable",
-            "--dart2js-options=--no-sound-null-safety",
             "observatory_ui"
           ]
         },
@@ -2613,8 +2598,6 @@
           "arguments": [
             "-ndart2js-minified-linux-d8",
             "--dart2js-batch",
-            "--enable-experiment=non-nullable",
-            "--dart2js-options=--no-sound-null-safety",
             "observatory_ui"
           ]
         },
@@ -2689,8 +2672,6 @@
             "-ndart2js-minified-csp-linux-chrome",
             "--dart2js-batch",
             "--reset-browser-configuration",
-            "--enable-experiment=non-nullable",
-            "--dart2js-options=--no-sound-null-safety",
             "observatory_ui"
           ]
         },
@@ -3048,8 +3029,7 @@
         {
           "name": "analyze tests enable-asserts",
           "arguments": [
-            "-nanalyzer-asserts-${system}",
-            "--exclude-suite=service"
+            "-nanalyzer-asserts-${system}"
           ]
         },
         {
@@ -3141,7 +3121,6 @@
             "ffi",
             "language",
             "lib",
-            "service",
             "standalone"
           ]
         },
@@ -3413,15 +3392,14 @@
           "name": "analyze runtime/observatory",
           "script": "out/ReleaseX64/dart-sdk/bin/dartanalyzer",
           "arguments": [
-            "--enable-experiment=non-nullable",
+            "--fatal-warnings",
             "runtime/observatory"
           ]
         },
         {
           "name": "dartanalyzer --batch tests",
           "arguments": [
-            "-nanalyzer-${system}",
-            "--exclude-suite=service"
+            "-nanalyzer-${system}"
           ]
         },
         {