diff --git a/pkg/compiler/lib/src/js_backend/specialized_checks.dart b/pkg/compiler/lib/src/js_backend/specialized_checks.dart
index 76b6cba7ab3..f86b47f8068 100644
--- a/pkg/compiler/lib/src/js_backend/specialized_checks.dart
+++ b/pkg/compiler/lib/src/js_backend/specialized_checks.dart
@@ -106,7 +106,7 @@ class SpecializedChecks {
     return null;
   }
 
-  static MemberEntity? findAsCheck(DartType dartType,
+  static FunctionEntity? findAsCheck(DartType dartType,
       JCommonElements commonElements, bool useLegacySubtyping) {
     if (dartType is InterfaceType) {
       if (dartType.typeArguments.isNotEmpty) return null;
@@ -147,7 +147,7 @@ class SpecializedChecks {
   ///     String    nullable: false  legacy: true     String    yes
   ///     String    nullable: false  legacy: false    String    no
   ///
-  static MemberEntity? _findAsCheck(
+  static FunctionEntity? _findAsCheck(
       ClassEntity element, JCommonElements commonElements,
       {required bool nullable, required bool legacy}) {
     if (element == commonElements.jsStringClass ||
diff --git a/pkg/compiler/lib/src/ssa/codegen.dart b/pkg/compiler/lib/src/ssa/codegen.dart
index 05e4603e8ea..ec6aac0f518 100644
--- a/pkg/compiler/lib/src/ssa/codegen.dart
+++ b/pkg/compiler/lib/src/ssa/codegen.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.10
-
 import 'dart:math' as math;
 import 'dart:collection' show Queue;
 
@@ -58,8 +56,7 @@ class SsaCodeGeneratorTask extends CompilerTask {
   final _CodegenMetrics _metrics = _CodegenMetrics();
 
   SsaCodeGeneratorTask(
-      Measurer measurer, this._options, this.sourceInformationStrategy)
-      : super(measurer);
+      Measurer super.measurer, this._options, this.sourceInformationStrategy);
 
   @override
   String get name => 'SSA code generator';
@@ -73,7 +70,7 @@ class SsaCodeGeneratorTask extends CompilerTask {
       return js.Fun(parameters, body, asyncModifier: asyncModifier)
           .withSourceInformation(sourceInformationStrategy
               .createBuilderForContext(element)
-              .buildDeclaration(element));
+              .buildDeclaration(element)) as js.Fun;
     }
 
     if (needsAsyncRewrite) {
@@ -101,13 +98,13 @@ class SsaCodeGeneratorTask extends CompilerTask {
     if (member is FieldEntity) {
       code = generateLazyInitializer(
           member, graph, codegen, closedWorld, registry, namer, emitter);
-    } else {
+    } else if (member is FunctionEntity) {
       code = generateMethod(
           member, graph, codegen, closedWorld, registry, namer, emitter);
+    } else {
+      failedAt(member, 'Cannot generate JavaScript for $member');
     }
-    if (code != null) {
-      codegen.tracer.traceJavaScriptText('JavaScript', code.debugPrint);
-    }
+    codegen.tracer.traceJavaScriptText('JavaScript', code.debugPrint);
     return code;
   }
 
@@ -120,7 +117,7 @@ class SsaCodeGeneratorTask extends CompilerTask {
       ModularNamer namer,
       ModularEmitter emitter) {
     return measure(() {
-      SourceInformation sourceInformation = sourceInformationStrategy
+      SourceInformation? sourceInformation = sourceInformationStrategy
           .createBuilderForContext(field)
           .buildDeclaration(field);
       SsaCodeGenerator codeGenerator = SsaCodeGenerator(
@@ -246,12 +243,17 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   final _CodegenMetrics _metrics;
 
   final Set<HInstruction> generateAtUseSite = {};
-  final Set<HInstruction> controlFlowOperators = {};
+  final Set<HIf> controlFlowOperators = {};
   final Set<JumpTarget> breakAction = {};
   final Set<LabelDefinition> continueAction = {};
   final Set<JumpTarget> implicitContinueAction = {};
   final List<js.Parameter> parameters = [];
 
+  // Using a Block as the current container allows a statement tree to be
+  // constructed that contains the block, and then have the block filled in
+  // later.
+  // TODO(sra): It would be cleaner if the [js.Block] could be unmodifiable but
+  // that would require deferring the construction of the containing AST.
   js.Block currentContainer = js.Block.empty();
   js.Block get body => currentContainer;
   List<js.Expression> expressionStack = [];
@@ -259,7 +261,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   /// Contains the names of the instructions, as well as the parallel
   /// copies to perform on block transitioning.
-  VariableNames variableNames;
+  late VariableNames variableNames;
 
   /// `true` when we need to generate a `var` declaration at function entry,
   /// `false` if we can generate a `var` declaration at first assignment in the
@@ -275,17 +277,17 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   /// Set of variables and parameters that have already been declared.
   final Set<String> declaredLocals = {};
 
-  HGraph currentGraph;
+  late HGraph currentGraph;
 
   // Records a block-information that is being handled specially.
   // Used to break bad recursion.
-  HBlockInformation currentBlockInformation;
+  HBlockInformation? currentBlockInformation;
   // The subgraph is used to delimit traversal for some constructions, e.g.,
   // if branches.
-  SubGraph subGraph;
+  SubGraph? subGraph;
 
   // Pending blocks than need to be visited as part of current subgraph.
-  Queue<HBasicBlock> blockQueue;
+  Queue<HBasicBlock>? blockQueue;
 
   SsaCodeGenerator(
       this._codegenTask,
@@ -331,16 +333,18 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   // of bits.
   int bitWidth(HInstruction instruction) {
     const int MAX = 32;
-    int constant(HInstruction instruction) {
-      if (instruction is HConstant && instruction.isConstantInteger()) {
-        IntConstantValue constant = instruction.constant;
-        return constant.intValue.toInt();
+    int? constant(HInstruction instruction) {
+      if (instruction is HConstant) {
+        ConstantValue constant = instruction.constant;
+        if (constant is IntConstantValue) {
+          return constant.intValue.toInt();
+        }
       }
       return null;
     }
 
     if (instruction.isConstantInteger()) {
-      int value = constant(instruction);
+      int value = constant(instruction)!;
       if (value < 0) return MAX;
       if (value > ((1 << 31) - 1)) return MAX;
       return value.bitLength;
@@ -348,21 +352,25 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     if (instruction is HBitAnd) {
       return math.min(bitWidth(instruction.left), bitWidth(instruction.right));
     }
-    if (instruction is HBitOr || instruction is HBitXor) {
-      HBinaryBitOp bitOp = instruction;
-      int leftWidth = bitWidth(bitOp.left);
+    if (instruction is HBitOr) {
+      int leftWidth = bitWidth(instruction.left);
       if (leftWidth == MAX) return MAX;
-      return math.max(leftWidth, bitWidth(bitOp.right));
+      return math.max(leftWidth, bitWidth(instruction.right));
+    }
+    if (instruction is HBitXor) {
+      int leftWidth = bitWidth(instruction.left);
+      if (leftWidth == MAX) return MAX;
+      return math.max(leftWidth, bitWidth(instruction.right));
     }
     if (instruction is HShiftLeft) {
-      int shiftCount = constant(instruction.right);
+      int? shiftCount = constant(instruction.right);
       if (shiftCount == null || shiftCount < 0 || shiftCount > 31) return MAX;
       int leftWidth = bitWidth(instruction.left);
       int width = leftWidth + shiftCount;
       return math.min(width, MAX);
     }
     if (instruction is HShiftRight) {
-      int shiftCount = constant(instruction.right);
+      int? shiftCount = constant(instruction.right);
       if (shiftCount == null || shiftCount < 0 || shiftCount > 31) return MAX;
       int leftWidth = bitWidth(instruction.left);
       if (leftWidth >= MAX) return MAX;
@@ -400,14 +408,14 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   /// If the [instruction] is not `null` it will be used to attach the position
   /// to the [expression].
   pushExpressionAsStatement(
-      js.Expression expression, SourceInformation sourceInformation) {
+      js.Expression expression, SourceInformation? sourceInformation) {
     pushStatement(js.ExpressionStatement(expression)
         .withSourceInformation(sourceInformation));
   }
 
   /// If the [instruction] is not `null` it will be used to attach the position
   /// to the [expression].
-  push(js.Expression /*!*/ expression) {
+  push(js.Expression expression) {
     expressionStack.add(expression);
   }
 
@@ -448,7 +456,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     shouldGroupVarDeclarations = allocator.names.numberOfVariables > 1;
   }
 
-  void handleDelayedVariableDeclarations(SourceInformation sourceInformation) {
+  void handleDelayedVariableDeclarations(SourceInformation? sourceInformation) {
     // Create 'var' list at the start of function.  Move assignment statements
     // from the top of the body into the variable initializers.
     if (collectedVariableDeclarations.isEmpty) return;
@@ -469,9 +477,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
             js.Expression value = expression.value;
             if (_safeInInitializer(value) &&
                 collectedVariableDeclarations.remove(name)) {
-              var initialization =
-                  js.VariableInitialization(js.VariableDeclaration(name), value)
-                      .withSourceInformation(expression.sourceInformation);
+              var initialization = js.VariableInitialization(
+                  js.VariableDeclaration(name), value,
+                  sourceInformation: expression.sourceInformation);
               declarations.add(initialization);
               ++nextStatement;
               continue;
@@ -506,12 +514,14 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     handleDelayedVariableDeclarations(graph.sourceInformation);
   }
 
-  void visitSubGraph(SubGraph newSubGraph) {
-    SubGraph oldSubGraph = subGraph;
-    Queue<HBasicBlock> oldBlockQueue = blockQueue;
+  void visitSubGraph(SubGraph? newSubGraph) {
+    final oldSubGraph = subGraph;
+    final oldBlockQueue = blockQueue;
+
     subGraph = newSubGraph;
-    blockQueue = Queue<HBasicBlock>();
-    enterSubGraph(subGraph.start);
+    blockQueue = Queue();
+    enterSubGraph(subGraph!.start);
+
     blockQueue = oldBlockQueue;
     subGraph = oldSubGraph;
   }
@@ -529,8 +539,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     // here. If we start using the other HExpressionInformation types too,
     // this code should be generalized.
     assert(info is HSubExpressionBlockInformation);
+    info as HSubExpressionBlockInformation;
     HSubExpressionBlockInformation expressionInfo = info;
-    SubGraph limits = expressionInfo.subExpression;
+    SubGraph limits = expressionInfo.subExpression!;
 
     // Start assuming that we can generate declarations. If we find a
     // counter-example, we degrade our assumption to either expression or
@@ -540,7 +551,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     int result = TYPE_DECLARATION;
     HBasicBlock basicBlock = limits.start;
     do {
-      HInstruction current = basicBlock.first;
+      HInstruction current = basicBlock.first!;
       while (current != basicBlock.last) {
         // E.g, bounds check.
         if (current.isControlFlow()) {
@@ -551,7 +562,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         if (current.usedBy.isEmpty || current is HFieldSet) {
           result = TYPE_EXPRESSION;
         }
-        current = current.next;
+        current = current.next!;
       }
       if (current is HGoto) {
         basicBlock = basicBlock.successors[0];
@@ -578,27 +589,30 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     return !identical(expressionType(info), TYPE_STATEMENT);
   }
 
-  bool isJSCondition(HExpressionInformation info) {
-    HSubExpressionBlockInformation graph = info;
-    SubExpression limits = graph.subExpression;
+  bool isJSCondition(HExpressionInformation? info) {
+    // Currently we only handle sub-expression graphs.
+    info as HSubExpressionBlockInformation;
+
+    SubExpression? limits = info.subExpression;
     return !identical(expressionType(info), TYPE_STATEMENT) &&
-        (limits.end.last is HConditionalBranch);
+        (limits!.end.last is HConditionalBranch);
   }
 
   /// Generate statements from block information.
   /// If the block information contains expressions, generate only
   /// assignments, and if it ends in a conditional branch, don't generate
   /// the condition.
-  void generateStatements(HBlockInformation block) {
+  void generateStatements(HBlockInformation? block) {
     if (block is HStatementInformation) {
       block.accept(this);
+    } else if (block is HSubExpressionBlockInformation) {
+      visitSubGraph(block.subExpression);
     } else {
-      HSubExpressionBlockInformation expression = block;
-      visitSubGraph(expression.subExpression);
+      failedAt(CURRENT_ELEMENT_SPANNABLE, 'Unexpected block: $block');
     }
   }
 
-  js.Block generateStatementsInNewBlock(HBlockInformation block) {
+  js.Block generateStatementsInNewBlock(HBlockInformation? block) {
     js.Block result = js.Block.empty();
     js.Block oldContainer = currentContainer;
     currentContainer = result;
@@ -623,9 +637,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   /// Generate expressions from block information.
-  js.Expression generateExpression(HExpressionInformation expression) {
+  js.Expression? generateExpression(HExpressionInformation expression) {
     // Currently we only handle sub-expression graphs.
-    assert(expression is HSubExpressionBlockInformation);
+    expression as HSubExpressionBlockInformation;
 
     bool oldIsGeneratingExpression = isGeneratingExpression;
     isGeneratingExpression = true;
@@ -654,13 +668,10 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   List<js.Expression> visitArguments(List<HInstruction> inputs,
       {int start = HInvoke.ARGUMENTS_OFFSET}) {
     assert(inputs.length >= start);
-    List<js.Expression> result =
-        List<js.Expression>.filled(inputs.length - start, null);
-    for (int i = start; i < inputs.length; i++) {
-      use(inputs[i]);
-      result[i - start] = pop();
-    }
-    return result;
+    return List.generate(inputs.length - start, (i) {
+      use(inputs[i + start]);
+      return pop();
+    }, growable: false);
   }
 
   bool isVariableDeclared(String variableName) {
@@ -669,7 +680,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   js.Expression generateExpressionAssignment(String variableName,
-      js.Expression value, SourceInformation sourceInformation) {
+      js.Expression value, SourceInformation? sourceInformation) {
     // TODO(johnniwinther): Introduce a DeferredVariableUse to handle this
     // in the SSA codegen or let the JS printer handle it fully and remove it
     // here.
@@ -703,7 +714,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void assignVariable(String variableName, js.Expression value,
-      SourceInformation sourceInformation) {
+      SourceInformation? sourceInformation) {
     if (isGeneratingExpression) {
       // If we are in an expression then we can't declare the variable here.
       // We have no choice, but to use it and then declare it separately.
@@ -749,7 +760,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
           instruction is HBoolConversion ||
           instruction is HNullCheck ||
           instruction is HLateReadCheck) {
-        String inputName = variableNames.getName(instruction.checkedInput);
+        String? inputName = variableNames.getName(instruction.checkedInput);
         if (variableNames.getName(instruction) == inputName) {
           needsAssignment = false;
         }
@@ -763,7 +774,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         !instruction.isControlFlow() &&
         variableNames.hasName(instruction)) {
       visitExpression(instruction);
-      assignVariable(variableNames.getName(instruction), pop(),
+      assignVariable(variableNames.getName(instruction)!, pop(),
           instruction.sourceInformation);
       return;
     }
@@ -804,7 +815,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       use(skipGenerateAtUseCheckInputs(check));
     } else {
       assert(variableNames.hasName(argument));
-      push(js.VariableUse(variableNames.getName(argument)));
+      push(js.VariableUse(variableNames.getName(argument)!));
     }
   }
 
@@ -864,16 +875,16 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     }
 
     if (isExpression) {
-      push(generateExpression(info.expression));
+      push(generateExpression(info.expression)!);
     } else {
-      use(info.expression.conditionExpression);
+      use(info.expression.conditionExpression!);
     }
     js.Expression key = pop();
     bool handledDefault = false;
     List<js.SwitchClause> cases = [];
-    HSwitch switchInstruction = info.expression.end.last;
+    HSwitch switchInstruction = info.expression.end.last as HSwitch;
     List<HInstruction> inputs = switchInstruction.inputs;
-    List<HBasicBlock> successors = switchInstruction.block.successors;
+    List<HBasicBlock> successors = switchInstruction.block!.successors;
 
     js.Block oldContainer = currentContainer;
     for (int inputIndex = 1, statementIndex = 0;
@@ -944,30 +955,23 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     return false;
   }
 
-  @override
-  bool visitAndOrInfo(HAndOrBlockInformation info) {
-    return false;
-  }
-
   @override
   bool visitTryInfo(HTryBlockInformation info) {
     js.Block body = generateStatementsInNewBlock(info.body);
-    js.Catch catchPart = null;
-    js.Block finallyPart = null;
+    js.Catch? catchPart = null;
+    js.Block? finallyPart = null;
     if (info.catchBlock != null) {
       void register(ClassEntity classElement) {
-        if (classElement != null) {
-          _registry
-              // ignore:deprecated_member_use_from_same_package
-              .registerInstantiatedClass(classElement);
-        }
+        _registry
+            // ignore:deprecated_member_use_from_same_package
+            .registerInstantiatedClass(classElement);
       }
 
       register(_commonElements.jsPlainJavaScriptObjectClass);
       register(_commonElements.jsUnknownJavaScriptObjectClass);
 
-      HLocalValue exception = info.catchVariable;
-      String name = variableNames.getName(exception);
+      HLocalValue? exception = info.catchVariable;
+      String name = variableNames.getName(exception)!;
       js.VariableDeclaration decl = js.VariableDeclaration(name);
       js.Block catchBlock = generateStatementsInNewBlock(info.catchBlock);
       catchPart = js.Catch(decl, catchBlock);
@@ -980,9 +984,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void visitBodyIgnoreLabels(HLoopBlockInformation info) {
-    if (info.body.start.isLabeledBlock()) {
-      HBlockInformation oldInfo = currentBlockInformation;
-      currentBlockInformation = info.body.start.blockFlow.body;
+    if (info.body!.start.isLabeledBlock()) {
+      HBlockInformation? oldInfo = currentBlockInformation;
+      currentBlockInformation = info.body!.start.blockFlow!.body;
       generateStatements(info.body);
       currentBlockInformation = oldInfo;
     } else {
@@ -992,10 +996,10 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   bool visitLoopInfo(HLoopBlockInformation info) {
-    HExpressionInformation condition = info.condition;
+    HExpressionInformation? condition = info.condition;
     bool isConditionExpression = isJSCondition(condition);
 
-    js.Loop loop;
+    late js.Loop loop;
 
     switch (info.kind) {
       // Treat all three "test-first" loops the same way.
@@ -1003,7 +1007,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       case HLoopBlockInformation.WHILE_LOOP:
       case HLoopBlockInformation.FOR_IN_LOOP:
       case HLoopBlockInformation.SWITCH_CONTINUE_LOOP:
-        HBlockInformation initialization = info.initializer;
+        HExpressionInformation? initialization = info.initializer;
         int initializationType = TYPE_STATEMENT;
         if (initialization != null) {
           initializationType = expressionType(initialization);
@@ -1018,17 +1022,17 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         js.Block oldContainer = currentContainer;
         js.Block avoidContainer = js.Block.empty();
         currentContainer = avoidContainer;
-        assignPhisOfSuccessors(condition.end.successors.last);
+        assignPhisOfSuccessors(condition!.end.successors.last);
         bool hasPhiUpdates = !avoidContainer.statements.isEmpty;
         currentContainer = oldContainer;
 
         if (isConditionExpression &&
             !hasPhiUpdates &&
             info.updates != null &&
-            isJSExpression(info.updates)) {
+            isJSExpression(info.updates!)) {
           // If we have an updates graph, and it's expressible as an
           // expression, generate a for-loop.
-          js.Expression jsInitialization = null;
+          js.Expression? jsInitialization = null;
           if (initialization != null) {
             int delayedVariablesCount = collectedVariableDeclarations.length;
             jsInitialization = generateExpression(initialization);
@@ -1040,30 +1044,30 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
               // expressions and see if they are all assignments that can be
               // converted into declarations.
 
-              List<js.Assignment> assignments;
+              List<js.Assignment>? assignments;
 
               bool allSimpleAssignments(js.Expression expression) {
                 if (expression is js.Assignment) {
                   js.Assignment assignment = expression;
                   if (assignment.leftHandSide is js.VariableUse &&
                       !assignment.isCompound) {
-                    assignments ??= [];
-                    assignments.add(expression);
+                    (assignments ??= []).add(expression);
                     return true;
                   }
-                } else if (expression.isCommaOperator) {
-                  js.Binary binary = expression;
-                  return allSimpleAssignments(binary.left) &&
-                      allSimpleAssignments(binary.right);
+                } else if (expression is js.Binary &&
+                    expression.isCommaOperator) {
+                  return allSimpleAssignments(expression.left) &&
+                      allSimpleAssignments(expression.right);
                 }
                 return false;
               }
 
-              if (allSimpleAssignments(jsInitialization)) {
+              if (jsInitialization != null &&
+                  allSimpleAssignments(jsInitialization)) {
                 List<js.VariableInitialization> inits = [];
-                for (js.Assignment assignment in assignments) {
-                  String id = (assignment.leftHandSide as js.VariableUse).name;
-                  js.Node declaration = js.VariableDeclaration(id);
+                for (js.Assignment assignment in assignments!) {
+                  final id = (assignment.leftHandSide as js.VariableUse).name;
+                  final declaration = js.VariableDeclaration(id);
                   inits.add(
                       js.VariableInitialization(declaration, assignment.value));
                   collectedVariableDeclarations.remove(id);
@@ -1073,29 +1077,29 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
               }
             }
           }
-          js.Expression jsCondition = generateExpression(condition);
-          js.Expression jsUpdates = generateExpression(info.updates);
+          js.Expression? jsCondition = generateExpression(condition);
+          js.Expression? jsUpdates = generateExpression(info.updates!);
           // The body might be labeled. Ignore this when recursing on the
           // subgraph.
           // TODO(lrn): Remove this extra labeling when handling all loops
           // using subgraphs.
           oldContainer = currentContainer;
-          js.Statement body = js.Block.empty();
+          js.Block body = js.Block.empty();
           currentContainer = body;
           visitBodyIgnoreLabels(info);
           currentContainer = oldContainer;
-          body = unwrapStatement(body);
-          loop = js.For(jsInitialization, jsCondition, jsUpdates, body)
-              .withSourceInformation(info.sourceInformation);
+          loop = js.For(
+              jsInitialization, jsCondition, jsUpdates, unwrapStatement(body),
+              sourceInformation: info.sourceInformation);
         } else {
           // We have either no update graph, or it's too complex to
           // put in an expression.
           if (initialization != null) {
             generateStatements(initialization);
           }
-          js.Expression jsCondition;
+          js.Expression? jsCondition;
           js.Block oldContainer = currentContainer;
-          js.Statement body = js.Block.empty();
+          js.Block body = js.Block.empty();
           if (isConditionExpression && !hasPhiUpdates) {
             jsCondition = generateExpression(condition);
             currentContainer = body;
@@ -1103,7 +1107,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
             jsCondition = newLiteralBool(true, info.sourceInformation);
             currentContainer = body;
             generateStatements(condition);
-            use(condition.conditionExpression);
+            use(condition.conditionExpression!);
             js.Expression ifTest = js.Prefix("!", pop());
             js.Statement jsBreak = js.Break(null);
             js.Statement exitLoop;
@@ -1122,9 +1126,8 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
             visitBodyIgnoreLabels(info);
           }
           currentContainer = oldContainer;
-          body = unwrapStatement(body);
-          loop = js.While(jsCondition, body)
-              .withSourceInformation(info.sourceInformation);
+          loop = js.While(jsCondition!, unwrapStatement(body),
+              sourceInformation: info.sourceInformation);
         }
         break;
       case HLoopBlockInformation.DO_WHILE_LOOP:
@@ -1136,7 +1139,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         js.Block oldContainer = currentContainer;
         js.Block exitAvoidContainer = js.Block.empty();
         currentContainer = exitAvoidContainer;
-        assignPhisOfSuccessors(condition.end.successors.last);
+        assignPhisOfSuccessors(condition!.end.successors.last);
         bool hasExitPhiUpdates = !exitAvoidContainer.statements.isEmpty;
         currentContainer = oldContainer;
 
@@ -1163,20 +1166,21 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         if (info.updates != null) {
           generateStatements(info.updates);
         }
+        js.Expression? jsCondition;
         if (isConditionExpression) {
-          push(generateExpression(condition));
+          jsCondition = generateExpression(condition);
         } else {
           generateStatements(condition);
-          use(condition.conditionExpression);
+          use(condition.conditionExpression!);
+          jsCondition = pop();
         }
-        js.Expression jsCondition = pop();
         if (jsCondition == null) {
           // If the condition is dead code, we turn the do-while into
           // a simpler while because we will never reach the condition
           // at the end of the loop anyway.
           loop = js.While(newLiteralBool(true, info.sourceInformation),
-                  unwrapStatement(body))
-              .withSourceInformation(info.sourceInformation);
+              unwrapStatement(body),
+              sourceInformation: info.sourceInformation);
         } else {
           if (hasPhiUpdates || hasExitPhiUpdates) {
             updateBody.statements.add(js.Continue(null));
@@ -1191,19 +1195,19 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
             body.statements.add(js.If(jsCondition, updateBody, exitLoop));
             jsCondition = newLiteralBool(true, info.sourceInformation);
           }
-          loop = js.Do(unwrapStatement(body), jsCondition)
-              .withSourceInformation(info.sourceInformation);
+          loop = js.Do(unwrapStatement(body), jsCondition,
+              sourceInformation: info.sourceInformation);
         }
         currentContainer = oldContainer;
         break;
       default:
-        failedAt(condition.conditionExpression,
+        failedAt(condition!.conditionExpression!,
             'Unexpected loop kind: ${info.kind}.');
     }
     js.Statement result = loop;
     if (info.kind == HLoopBlockInformation.SWITCH_CONTINUE_LOOP) {
       String continueLabelString =
-          _namer.implicitContinueLabelName(info.target);
+          _namer.implicitContinueLabelName(info.target!);
       result = js.LabeledStatement(continueLabelString, result);
     }
     pushStatement(wrapIntoLabels(result, info.labels));
@@ -1236,7 +1240,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       // For handling unlabeled continues from the body of a loop.
       // TODO(lrn): Consider recording whether the target is in fact
       // a target of an unlabeled continue, and not generate this if it isn't.
-      JumpTarget target = labeledBlockInfo.target;
+      JumpTarget target = labeledBlockInfo.target!;
       String labelName = _namer.implicitContinueLabelName(target);
       result = js.LabeledStatement(labelName, result);
       implicitContinueAction.add(target);
@@ -1249,7 +1253,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         }
       }
     }
-    JumpTarget target = labeledBlockInfo.target;
+    JumpTarget target = labeledBlockInfo.target!;
     if (target.isSwitch) {
       // This is an extra block around a switch that is generated
       // as a nested if/else chain. We add an extra break target
@@ -1266,7 +1270,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       while (!continueOverrides.isEmpty) {
         continueAction.remove(continueOverrides.head);
         implicitContinueAction.remove(continueOverrides.head);
-        continueOverrides = continueOverrides.tail;
+        continueOverrides = continueOverrides.tail!;
       }
     } else {
       breakAction.remove(labeledBlockInfo.target);
@@ -1280,7 +1284,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   // Wraps a loop body in a block to make continues have a target to break
   // to (if necessary).
   void wrapLoopBodyForContinue(HLoopBlockInformation info) {
-    JumpTarget target = info.target;
+    JumpTarget? target = info.target;
     if (target != null && target.isContinueTarget) {
       js.Block oldContainer = currentContainer;
       js.Block body = js.Block.empty();
@@ -1295,9 +1299,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       }
       String labelName = _namer.implicitContinueLabelName(target);
       result = js.LabeledStatement(labelName, result);
-      implicitContinueAction.add(info.target);
+      implicitContinueAction.add(target);
       visitBodyIgnoreLabels(info);
-      implicitContinueAction.remove(info.target);
+      implicitContinueAction.remove(target);
       for (LabelDefinition label in info.labels) {
         if (label.isContinueTarget) {
           continueAction.remove(label);
@@ -1321,13 +1325,13 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     // "if" and its condition).
     if (identical(info, currentBlockInformation)) return false;
 
-    HBlockInformation oldBlockInformation = currentBlockInformation;
+    HBlockInformation? oldBlockInformation = currentBlockInformation;
     currentBlockInformation = info;
     bool success = info.accept(this);
     currentBlockInformation = oldBlockInformation;
 
     if (success) {
-      HBasicBlock continuation = block.continuation;
+      HBasicBlock? continuation = block.continuation;
       if (continuation != null) {
         continueSubGraph(continuation);
       }
@@ -1336,17 +1340,16 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void enterSubGraph(HBasicBlock node) {
-    assert(blockQueue.isEmpty);
-    assert(node != null);
+    assert(blockQueue!.isEmpty);
     continueSubGraph(node);
-    while (blockQueue.isNotEmpty) {
-      node = blockQueue.removeFirst();
+    while (blockQueue!.isNotEmpty) {
+      node = blockQueue!.removeFirst();
       assert(node.isLive);
-      assert(subGraph.contains(node));
+      assert(subGraph!.contains(node));
 
       // If this node has block-structure based information attached,
       // try using that to traverse from here.
-      if (node.blockFlow != null && handleBlockFlow(node.blockFlow)) {
+      if (node.blockFlow != null && handleBlockFlow(node.blockFlow!)) {
         continue;
       }
 
@@ -1357,12 +1360,12 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   void continueSubGraph(HBasicBlock node) {
     if (!node.isLive) return;
     // Don't follow edges out of the current sub-graph.
-    if (!subGraph.contains(node)) return;
-    blockQueue.add(node);
+    if (!subGraph!.contains(node)) return;
+    blockQueue!.add(node);
   }
 
   void emitAssignment(
-      String destination, String source, SourceInformation sourceInformation) {
+      String destination, String source, SourceInformation? sourceInformation) {
     assignVariable(destination, js.VariableUse(source), sourceInformation);
   }
 
@@ -1372,15 +1375,15 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       Iterable<Copy<HInstruction>> instructionCopies,
       String tempName,
       void doAssignment(
-          String target, String source, SourceInformation sourceInformation)) {
-    Map<String, SourceInformation> sourceInformationMap = {};
+          String target, String source, SourceInformation? sourceInformation)) {
+    Map<String, SourceInformation?> sourceInformationMap = {};
 
     // Map the instructions to strings.
     Iterable<Copy<String>> copies =
         instructionCopies.map((Copy<HInstruction> copy) {
-      String sourceName = variableNames.getName(copy.source);
+      String sourceName = variableNames.getName(copy.source)!;
       sourceInformationMap[sourceName] = copy.source.sourceInformation;
-      String destinationName = variableNames.getName(copy.destination);
+      String destinationName = variableNames.getName(copy.destination)!;
       sourceInformationMap[sourceName] = copy.destination.sourceInformation;
       return Copy<String>(sourceName, destinationName);
     });
@@ -1428,10 +1431,10 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     while (!worklist.isEmpty) {
       while (!ready.isEmpty) {
         String destination = ready.removeLast();
-        String source = initialValue[destination];
+        String source = initialValue[destination]!;
         // Since [source] might have been updated, use the current
         // location of [source]
-        String copy = currentLocation[source];
+        String copy = currentLocation[source]!;
         doAssignment(destination, copy,
             sourceInformationMap[copy] ?? sourceInformationMap[destination]);
         // Now [destination] is the current location of [source].
@@ -1461,14 +1464,14 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void assignPhisOfSuccessors(HBasicBlock node) {
-    CopyHandler handler = variableNames.getCopyHandler(node);
+    CopyHandler? handler = variableNames.getCopyHandler(node);
     if (handler == null) return;
 
     sequentializeCopies(
         handler.copies, variableNames.getSwapTemp(), emitAssignment);
 
     for (Copy<HInstruction> copy in handler.assignments) {
-      String name = variableNames.getName(copy.destination);
+      String name = variableNames.getName(copy.destination)!;
       use(copy.source);
       assignVariable(name, pop(),
           copy.source.sourceInformation ?? copy.destination.sourceInformation);
@@ -1476,19 +1479,19 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void iterateBasicBlock(HBasicBlock node) {
-    HInstruction instruction = node.first;
+    HInstruction instruction = node.first!;
     while (!identical(instruction, node.last)) {
       if (!isGenerateAtUseSite(instruction)) {
         define(instruction);
       }
-      instruction = instruction.next;
+      instruction = instruction.next!;
     }
     assignPhisOfSuccessors(node);
     visit(instruction);
   }
 
   void handleInvokeBinary(
-      HInvokeBinary node, String op, SourceInformation sourceInformation) {
+      HInvokeBinary node, String op, SourceInformation? sourceInformation) {
     use(node.left);
     js.Expression jsLeft = pop();
     use(node.right);
@@ -1534,9 +1537,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void emitIdentityComparison(
-      HIdentity instruction, SourceInformation sourceInformation,
+      HIdentity instruction, SourceInformation? sourceInformation,
       {bool inverse = false}) {
-    String op = instruction.singleComparisonOp;
+    String? op = instruction.singleComparisonOp;
     HInstruction left = instruction.left;
     HInstruction right = instruction.right;
     if (op != null) {
@@ -1636,7 +1639,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   visitGoto(HGoto node) {
-    HBasicBlock block = node.block;
+    HBasicBlock block = node.block!;
     assert(block.successors.length == 1);
     List<HBasicBlock> dominated = block.dominatedBlocks;
     // With the exception of the entry-node which dominates its successor
@@ -1657,7 +1660,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   visitLoopBranch(HLoopBranch node) {
-    assert(node.block == subGraph.end);
+    assert(node.block == subGraph!.end);
     // We are generating code for a loop condition.
     // If we are generating the subgraph as an expression, the
     // condition will be generated as the expression.
@@ -1670,9 +1673,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   visitBreak(HBreak node) {
-    assert(node.block.successors.length == 1);
+    assert(node.block!.successors.length == 1);
     if (node.label != null) {
-      LabelDefinition label = node.label;
+      LabelDefinition label = node.label!;
       if (breakAction.contains(label.target)) {
         implicitBreakWithLabel(label.target);
       } else {
@@ -1697,9 +1700,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   visitContinue(HContinue node) {
-    assert(node.block.successors.length == 1);
+    assert(node.block!.successors.length == 1);
     if (node.label != null) {
-      LabelDefinition label = node.label;
+      LabelDefinition label = node.label!;
       if (continueAction.contains(label)) {
         continueAsBreak(label);
       } else {
@@ -1742,7 +1745,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   bool tryControlFlowOperation(HIf node) {
     if (!controlFlowOperators.contains(node)) return false;
-    HPhi phi = node.joinBlock.phis.first;
+    HPhi phi = node.joinBlock!.phis.first as HPhi;
     bool atUseSite = isGenerateAtUseSite(phi);
     // Don't generate a conditional operator in this situation:
     // i = condition ? bar() : i;
@@ -1755,20 +1758,20 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       return false;
     }
     if (!atUseSite) define(phi);
-    continueSubGraph(node.joinBlock);
+    continueSubGraph(node.joinBlock!);
     return true;
   }
 
   void generateIf(HIf node, HIfBlockInformation info) {
-    HStatementInformation thenGraph = info.thenGraph;
-    HStatementInformation elseGraph = info.elseGraph;
+    HStatementInformation? thenGraph = info.thenGraph;
+    HStatementInformation? elseGraph = info.elseGraph;
     HInstruction condition = node.inputs.single;
 
     js.Expression test;
     js.Statement thenPart;
     js.Statement elsePart;
 
-    HBasicBlock thenBlock = node.block.successors[0];
+    HBasicBlock thenBlock = node.block!.successors[0];
     // If we believe we will generate S1 as empty, instead of
     //
     //     if (e) S1; else S2;
@@ -1782,7 +1785,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     if (isGenerateAtUseSite(condition) &&
         thenBlock.successors.length == 1 &&
         thenBlock.successors.single == node.joinBlock &&
-        node.joinBlock.phis.isEmpty &&
+        node.joinBlock!.phis.isEmpty &&
         thenBlock.first is HGoto) {
       generateNot(condition, condition.sourceInformation);
       test = pop();
@@ -1815,7 +1818,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         return js.ExpressionStatement(test);
       }
       test = js.Prefix('!', test);
-      var temp = thenPart;
+      js.Statement temp = thenPart;
       thenPart = elsePart;
       elsePart = temp;
     }
@@ -1836,15 +1839,15 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   visitIf(HIf node) {
     _metrics.countHIf++;
     HInstruction condition = node.inputs[0];
-    if (condition.isConstant()) _metrics.countHIfConstant++;
+    if (condition is HConstant) _metrics.countHIfConstant++;
 
     if (tryControlFlowOperation(node)) return;
 
-    HIfBlockInformation info = node.blockInformation.body;
+    HIfBlockInformation info =
+        node.blockInformation!.body as HIfBlockInformation;
 
-    if (condition.isConstant()) {
-      HConstant constant = condition;
-      if (constant.constant is TrueConstantValue) {
+    if (condition is HConstant) {
+      if (condition.constant is TrueConstantValue) {
         generateStatements(info.thenGraph);
       } else {
         generateStatements(info.elseGraph);
@@ -1853,7 +1856,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       generateIf(node, info);
     }
 
-    HBasicBlock joinBlock = node.joinBlock;
+    HBasicBlock? joinBlock = node.joinBlock;
     if (joinBlock != null && !identical(joinBlock.dominator, node.block)) {
       // The join block is dominated by a block in one of the branches.
       // The subgraph traversal never reached it, so we visit it here
@@ -1865,7 +1868,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     // branches, and is not the join block.
     // Depending on how the then/else branches terminate
     // (e.g., return/throw/break) there can be any number of these.
-    List<HBasicBlock> dominated = node.block.dominatedBlocks;
+    List<HBasicBlock> dominated = node.block!.dominatedBlocks;
     for (int i = 2; i < dominated.length; i++) {
       continueSubGraph(dominated[i]);
     }
@@ -1885,8 +1888,8 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     } else {
       _metrics.countHInterceptorGet.add();
       assert(node.inputs.length == 1);
-      _registry.registerSpecializedGetInterceptor(node.interceptedClasses);
-      js.Name name = _namer.nameForGetInterceptor(node.interceptedClasses);
+      _registry.registerSpecializedGetInterceptor(node.interceptedClasses!);
+      js.Name name = _namer.nameForGetInterceptor(node.interceptedClasses!);
       js.Expression isolate = _namer.readGlobalObjectForInterceptors();
       use(node.receiver);
       List<js.Expression> arguments = [pop()];
@@ -1902,9 +1905,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     _updateInvokeMetrics(node);
     use(node.receiver);
     js.Expression object = pop();
-    String methodName;
+    String? methodName;
     List<js.Expression> arguments = visitArguments(node.inputs);
-    MemberEntity target = node.element;
+    MemberEntity? target = node.element;
 
     // TODO(herhut): The namer should return the appropriate backend name here.
     if (target != null && !node.isInterceptedCall) {
@@ -1936,20 +1939,23 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   void visitInvokeConstructorBody(HInvokeConstructorBody node) {
+    final element = node.element as ConstructorBodyEntity;
     use(node.inputs[0]);
     js.Expression object = pop();
-    js.Name methodName = _namer.instanceMethodName(node.element);
+    js.Name methodName = _namer.instanceMethodName(element);
     List<js.Expression> arguments = visitArguments(node.inputs);
     push(js
         .propertyCall(object, methodName, arguments)
         .withSourceInformation(node.sourceInformation));
     _registry.registerStaticUse(StaticUse.constructorBodyInvoke(
-        node.element, CallStructure.unnamed(arguments.length)));
+        element, CallStructure.unnamed(arguments.length)));
   }
 
   @override
   void visitInvokeGeneratorBody(HInvokeGeneratorBody node) {
-    JGeneratorBody element = node.element;
+    // TODO(sra): Refactor HInvokeGeneratorBody so that `node.element` has this
+    // type.
+    JGeneratorBody element = node.element as JGeneratorBody;
     if (element.isInstanceMember) {
       use(node.inputs[0]);
       js.Expression object = pop();
@@ -1965,7 +1971,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
           js.Call(pop(), arguments, sourceInformation: node.sourceInformation));
     }
 
-    _registry.registerStaticUse(StaticUse.generatorBodyInvoke(node.element));
+    _registry.registerStaticUse(StaticUse.generatorBodyInvoke(element));
   }
 
   @override
@@ -1999,7 +2005,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       // [node.element] will be enqueued. We're not using the receiver
       // type because our optimizations might end up in a state where the
       // invoke dynamic knows more than the receiver.
-      ClassEntity enclosing = node.element.enclosingClass;
+      ClassEntity enclosing = node.element!.enclosingClass!;
       if (_closedWorld.classHierarchy.isInstantiated(enclosing)) {
         return _abstractValueDomain.createNonNullExact(enclosing);
       } else {
@@ -2014,7 +2020,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         return _abstractValueDomain.createNonNullSubtype(enclosing);
       }
     }
-    return mask ?? _abstractValueDomain.dynamicType;
+    return mask;
   }
 
   void registerMethodInvoke(HInvokeDynamic node) {
@@ -2023,7 +2029,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     // If we don't know what we're calling or if we are calling a getter,
     // we need to register that fact that we may be calling a closure
     // with the same arguments.
-    MemberEntity target = node.element;
+    MemberEntity? target = node.element;
     if ((target == null || target.isGetter) &&
         // TODO(johnniwinther): Remove this when kernel adds an `isFunctionCall`
         // flag to [ir.MethodInvocation]. Currently we can't tell the difference
@@ -2046,6 +2052,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
           failedAt(node, '$selector does not apply to $target'));
       assert(!selector.isGetter && !selector.isSetter,
           "Unexpected direct invocation selector: $selector.");
+      target as FunctionEntity; // TODO(sra): Make node.element have this type.
       _registry.registerStaticUse(StaticUse.directInvoke(
           target, selector.callStructure, node.typeArguments));
     } else {
@@ -2057,12 +2064,13 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void registerSetter(HInvokeDynamic node, {bool needsCheck = false}) {
-    if (node.element is FieldEntity && !needsCheck) {
+    final element = node.element;
+    if (element is FieldEntity && !needsCheck) {
       // This is a dynamic update which we have found to have a single
       // target but for some reason haven't inlined. We are _still_ accessing
       // the target dynamically but we don't need to enqueue more than target
       // for this to work.
-      _registry.registerStaticUse(StaticUse.directSet(node.element));
+      _registry.registerStaticUse(StaticUse.directSet(element));
     } else {
       Selector selector = node.selector;
       AbstractValue mask =
@@ -2073,14 +2081,14 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void registerGetter(HInvokeDynamic node) {
-    if (node.element != null &&
-        (node.element.isGetter || node.element is FieldEntity)) {
+    final element = node.element;
+    if (element != null && (element.isGetter || element is FieldEntity)) {
       // This is a dynamic read which we have found to have a single target but
       // for some reason haven't inlined. We are _still_ accessing the target
       // dynamically but we don't need to enqueue more than target for this to
       // work. The test above excludes non-getter functions since the element
       // represents two targets - a tearoff getter and the torn-off method.
-      _registry.registerStaticUse(StaticUse.directGet(node.element));
+      _registry.registerStaticUse(StaticUse.directGet(element));
     } else {
       Selector selector = node.selector;
       AbstractValue mask =
@@ -2132,7 +2140,11 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   visitInvokeStatic(HInvokeStatic node) {
-    MemberEntity element = node.element;
+    // TODO(48820): Refactor HInvokeStatic so that the element has static type
+    // FunctionEntity (`element` can be a FieldEntity in subclass HInvokeSuper,
+    // so possibly make HInvokeSuper and HInvokeStatic extend a common
+    // superclass, or have a different node for super-field accesses).
+    FunctionEntity element = node.element as FunctionEntity;
     node.instantiatedTypes?.forEach(_registry.registerInstantiation);
 
     List<js.Expression> arguments = visitArguments(node.inputs, start: 0);
@@ -2198,23 +2210,25 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   @override
   visitInvokeSuper(HInvokeSuper node) {
     MemberEntity superElement = node.element;
-    ClassEntity superClass = superElement.enclosingClass;
     Selector selector = node.selector;
     bool useAliasedSuper = canUseAliasedSuperMember(superElement, selector);
     if (selector.isGetter) {
       if (superElement is FieldEntity || superElement.isGetter) {
         _registry.registerStaticUse(StaticUse.superGet(superElement));
       } else {
-        _registry.registerStaticUse(StaticUse.superTearOff(node.element));
+        superElement as FunctionEntity; // Not a field so must be a function.
+        _registry.registerStaticUse(StaticUse.superTearOff(superElement));
       }
     } else if (selector.isSetter) {
       if (superElement is FieldEntity) {
         _registry.registerStaticUse(StaticUse.superFieldSet(superElement));
       } else {
         assert(superElement.isSetter);
+        superElement as FunctionEntity; // Not a field so must be a function.
         _registry.registerStaticUse(StaticUse.superSetterSet(superElement));
       }
     } else {
+      superElement as FunctionEntity; // Not a field so must be a function.
       if (useAliasedSuper) {
         _registry.registerStaticUse(StaticUse.superInvoke(
             superElement, CallStructure.unnamed(node.inputs.length)));
@@ -2229,7 +2243,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       js.Name fieldName = _namer.instanceFieldPropertyName(superElement);
       use(node.getDartReceiver(_closedWorld));
       js.PropertyAccess access = js.PropertyAccess(pop(), fieldName)
-          .withSourceInformation(node.sourceInformation);
+          .withSourceInformation(node.sourceInformation) as js.PropertyAccess;
       if (node.isSetter) {
         use(node.value);
         push(js.Assignment(access, pop())
@@ -2237,7 +2251,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       } else {
         push(access);
       }
-    } else {
+    } else if (superElement is FunctionEntity) {
       if (!useAliasedSuper) {
         js.Name methodName;
         if (selector.isGetter && !superElement.isGetter) {
@@ -2255,6 +2269,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
           methodName = _namer.instanceMethodName(superElement);
         }
 
+        ClassEntity superClass = superElement.enclosingClass!;
         push(js.js('#.#.call(#)', [
           _emitter.prototypeAccess(superClass),
           methodName,
@@ -2269,11 +2284,13 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         ]) // Skip receiver argument.
             .withSourceInformation(node.sourceInformation));
       }
+    } else {
+      failedAt(node, 'node.element must be FieldEntity or FunctionEntity');
     }
   }
 
   js.Expression _loadField(js.Expression receiver, FieldEntity field,
-      SourceInformation sourceInformation) {
+      SourceInformation? sourceInformation) {
     _registry.registerStaticUse(StaticUse.fieldGet(field));
     js.Name name = _namer.instanceFieldPropertyName(field);
     return js.PropertyAccess(receiver, name)
@@ -2347,7 +2364,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   visitLocalSet(HLocalSet node) {
     use(node.value);
     assignVariable(
-        variableNames.getName(node.receiver), pop(), node.sourceInformation);
+        variableNames.getName(node.receiver)!, pop(), node.sourceInformation);
   }
 
   @override
@@ -2357,7 +2374,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
     assert(_nativeData.isNativeMember(target), 'non-native target: $node');
 
-    String targetName = _nativeData.hasFixedBackendName(target)
+    String? targetName = _nativeData.hasFixedBackendName(target)
         ? _nativeData.getFixedBackendName(target)
         : target.name;
 
@@ -2387,23 +2404,23 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       }
       js.Expression expression = js.js
           .uncachedExpressionTemplate(template)
-          .instantiate(templateInputs);
+          .instantiateExpression(templateInputs);
       push(expression.withSourceInformation(node.sourceInformation));
       _registry.registerNativeMethod(target);
     }
 
     if (_nativeData.isJsInteropMember(target)) {
       if (target.isStatic || target.isTopLevel || target is ConstructorEntity) {
-        String path = _nativeData.getFixedBackendMethodPath(target);
+        String path = _nativeData.getFixedBackendMethodPath(target)!;
         js.Expression pathExpression =
-            js.js.uncachedExpressionTemplate(path).instantiate([]);
+            js.js.uncachedExpressionTemplate(path).instantiateExpression([]);
         invokeWithJavaScriptReceiver(pathExpression);
         return;
       }
     }
 
     if (_nativeData.isNativeMember(target)) {
-      _registry.registerNativeBehavior(node.nativeBehavior);
+      _registry.registerNativeBehavior(node.nativeBehavior!);
       if (target.isInstanceMember) {
         HInstruction receiver = inputs.first;
         use(receiver);
@@ -2412,8 +2429,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       }
       if (target.isStatic || target.isTopLevel) {
         var arguments = visitArguments(inputs, start: 0);
-        js.Expression targetExpression =
-            js.js.uncachedExpressionTemplate(targetName).instantiate([]);
+        js.Expression targetExpression = js.js
+            .uncachedExpressionTemplate(targetName!)
+            .instantiateExpression([]);
         js.Expression expression;
         if (target.isGetter) {
           expression = targetExpression;
@@ -2434,7 +2452,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void registerForeignTypes(HForeign node) {
-    NativeBehavior nativeBehavior = node.nativeBehavior;
+    NativeBehavior? nativeBehavior = node.nativeBehavior;
     if (nativeBehavior == null) return;
     _registry.registerNativeBehavior(nativeBehavior);
   }
@@ -2449,7 +2467,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         interpolatedExpressions.add(pop());
       }
       pushStatement(node.codeTemplate
-          .instantiate(interpolatedExpressions)
+          .instantiateStatement(interpolatedExpressions)
           .withSourceInformation(node.sourceInformation));
     } else {
       List<js.Expression> interpolatedExpressions = [];
@@ -2458,7 +2476,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         interpolatedExpressions.add(pop());
       }
       push(node.codeTemplate
-          .instantiate(interpolatedExpressions)
+          .instantiateExpression(interpolatedExpressions)
           .withSourceInformation(node.sourceInformation));
     }
 
@@ -2481,9 +2499,10 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
           .registerInstantiatedClass(node.element);
     }
     node.instantiatedTypes?.forEach(_registry.registerInstantiation);
-    if (node.callMethod != null) {
-      _registry?.registerStaticUse(StaticUse.implicitInvoke(node.callMethod));
-      _registry?.registerInstantiatedClosure(node.callMethod);
+    final callMethod = node.callMethod;
+    if (callMethod != null) {
+      _registry.registerStaticUse(StaticUse.implicitInvoke(callMethod));
+      _registry.registerInstantiatedClosure(callMethod);
     }
   }
 
@@ -2493,7 +2512,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   js.Expression newLiteralBool(
-      bool value, SourceInformation sourceInformation) {
+      bool value, SourceInformation? sourceInformation) {
     if (_options.enableMinification) {
       // Use !0 for true, !1 for false.
       return js.Prefix("!", js.LiteralNumber(value ? "0" : "1"))
@@ -2504,7 +2523,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void generateConstant(
-      ConstantValue constant, SourceInformation sourceInformation) {
+      ConstantValue constant, SourceInformation? sourceInformation) {
     js.Expression expression = _emitter.constantReference(constant);
     if (!constant.isDummy) {
       // TODO(johnniwinther): Support source information on synthetic constants.
@@ -2543,15 +2562,11 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       ">": "<=",
       ">=": "<"
     };
-    return inverse ? inverseOperator[op] : op;
+    return inverse ? inverseOperator[op]! : op;
   }
 
-  void generateNot(HInstruction input, SourceInformation sourceInformation) {
-    bool canGenerateOptimizedComparison(HInstruction instruction) {
-      if (instruction is! HRelational) return false;
-
-      HRelational relational = instruction;
-
+  void generateNot(HInstruction input, SourceInformation? sourceInformation) {
+    bool canGenerateOptimizedComparison(HRelational relational) {
       HInstruction left = relational.left;
       HInstruction right = relational.right;
       if (left.isStringOrNull(_abstractValueDomain).isDefinitelyTrue &&
@@ -2576,9 +2591,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         emitIdentityComparison(input, sourceInformation, inverse: true);
       } else if (input is HIsLateSentinel) {
         _emitIsLateSentinel(input, sourceInformation, inverse: true);
-      } else if (canGenerateOptimizedComparison(input)) {
-        HRelational relational = input;
-        constant_system.BinaryOperation operation = relational.operation();
+      } else if (input is HRelational &&
+          canGenerateOptimizedComparison(input)) {
+        constant_system.BinaryOperation operation = input.operation();
         String op = mapRelationalOperator(operation.name, true);
         handleInvokeBinary(input, op, sourceInformation);
       } else {
@@ -2594,7 +2609,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   @override
   visitParameterValue(HParameterValue node) {
     assert(!isGenerateAtUseSite(node));
-    String name = variableNames.getName(node);
+    String name = variableNames.getName(node)!;
     parameters.add(js.Parameter(name));
     declaredLocals.add(name);
   }
@@ -2602,7 +2617,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   @override
   visitLocalValue(HLocalValue node) {
     assert(!isGenerateAtUseSite(node));
-    String name = variableNames.getName(node);
+    String name = variableNames.getName(node)!;
     collectedVariableDeclarations.add(name);
   }
 
@@ -2611,9 +2626,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     // This method is only called for phis that are generated at use
     // site. A phi can be generated at use site only if it is the
     // result of a control flow operation.
-    HBasicBlock ifBlock = node.block.dominator;
+    HBasicBlock ifBlock = node.block!.dominator!;
     assert(controlFlowOperators.contains(ifBlock.last));
-    HInstruction input = ifBlock.last.inputs[0];
+    HInstruction input = ifBlock.last!.inputs[0];
     if (input.isConstantFalse()) {
       use(node.inputs[1]);
     } else if (input.isConstantTrue()) {
@@ -2656,7 +2671,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   visitThrow(HThrow node) {
-    SourceInformation sourceInformation = node.sourceInformation;
+    SourceInformation? sourceInformation = node.sourceInformation;
     if (node.isRethrow) {
       use(node.inputs[0]);
       pushStatement(js.Throw(pop()).withSourceInformation(sourceInformation));
@@ -2720,8 +2735,8 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     // TODO(sra): Better analysis of possible NaN input.
     bool indexCanBeNaN = !_isDefinitelyNotNaN(index);
 
-    js.Expression under;
-    js.Expression over;
+    js.Expression? under;
+    js.Expression? over;
 
     if (index.isInteger(_abstractValueDomain).isPotentiallyFalse) {
       // Combined domain check and low bound check. `a >>> 0 !== a` is true for
@@ -2758,7 +2773,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     assert(over != null || under != null);
     js.Expression underOver;
     if (under == null) {
-      underOver = over;
+      underOver = over!;
     } else if (over == null) {
       underOver = under;
     } else {
@@ -2775,15 +2790,14 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
     // Generate the call to the 'throw' helper in a block in case it needs
     // multiple statements.
-    js.Statement thenBody = js.Block.empty();
+    js.Block thenBody = js.Block.empty();
     js.Block oldContainer = currentContainer;
     currentContainer = thenBody;
     _pushThrowWithHelper(_commonElements.throwIndexOutOfRangeException,
         [node.array, node.reportedIndex],
         sourceInformation: node.sourceInformation);
     currentContainer = oldContainer;
-    thenBody = unwrapStatement(thenBody);
-    pushStatement(js.If.noElse(underOver, thenBody)
+    pushStatement(js.If.noElse(underOver, unwrapStatement(thenBody))
         .withSourceInformation(node.sourceInformation));
   }
 
@@ -2803,7 +2817,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void _pushThrowWithHelper(FunctionEntity helper, List<HInstruction> inputs,
-      {SourceInformation sourceInformation}) {
+      {SourceInformation? sourceInformation}) {
     List<js.Expression> arguments = [];
     for (final input in inputs) {
       use(input);
@@ -2817,12 +2831,12 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void _pushCallStatic(FunctionEntity target, List<js.Expression> arguments,
-      SourceInformation sourceInformation) {
+      SourceInformation? sourceInformation) {
     _registry.registerStaticUse(StaticUse.staticInvoke(
         target, CallStructure.unnamed(arguments.length)));
     js.Expression jsTarget = _emitter.staticFunctionAccess(target);
-    js.Call call = js.Call(jsTarget, List.of(arguments, growable: false))
-        .withSourceInformation(sourceInformation);
+    js.Call call = js.Call(jsTarget, List.of(arguments, growable: false),
+        sourceInformation: sourceInformation);
     push(call);
   }
 
@@ -2841,17 +2855,19 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   @override
   void visitStatic(HStatic node) {
     MemberEntity element = node.element;
-    assert(element.isFunction || element is FieldEntity);
-    if (element.isFunction) {
+    if (element is FunctionEntity) {
+      // TODO(sra): Static tear-offs should be constants.
       push(_emitter
           .staticClosureAccess(element)
           .withSourceInformation(node.sourceInformation));
       _registry.registerStaticUse(StaticUse.staticTearOff(element));
-    } else {
+    } else if (element is FieldEntity) {
       push(_emitter
           .staticFieldAccess(element)
           .withSourceInformation(node.sourceInformation));
       _registry.registerStaticUse(StaticUse.staticGet(element));
+    } else {
+      failedAt(node, 'HStatic must be a FieldEntity or FunctionEntity');
     }
   }
 
@@ -2868,7 +2884,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   @override
   void visitStaticStore(HStaticStore node) {
     _registry.registerStaticUse(StaticUse.staticSet(node.element));
-    js.Node variable = _emitter.staticFieldAccess(node.element);
+    js.Expression variable = _emitter.staticFieldAccess(node.element);
     use(node.inputs[0]);
     push(js.Assignment(variable, pop())
         .withSourceInformation(node.sourceInformation));
@@ -2953,7 +2969,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void checkTypeOf(HInstruction input, String cmp, String typeName,
-      SourceInformation sourceInformation) {
+      SourceInformation? sourceInformation) {
     use(input);
     js.Expression typeOf = js.Prefix("typeof", pop());
     push(js.Binary(cmp, typeOf, js.string(typeName))
@@ -2961,12 +2977,12 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   }
 
   void checkNum(
-      HInstruction input, String cmp, SourceInformation sourceInformation) {
+      HInstruction input, String cmp, SourceInformation? sourceInformation) {
     return checkTypeOf(input, cmp, 'number', sourceInformation);
   }
 
   void checkBool(
-      HInstruction input, String cmp, SourceInformation sourceInformation) {
+      HInstruction input, String cmp, SourceInformation? sourceInformation) {
     return checkTypeOf(input, cmp, 'boolean', sourceInformation);
   }
 
@@ -2974,27 +2990,25 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   void visitPrimitiveCheck(HPrimitiveCheck node) {
     js.Expression test = _generateReceiverOrArgumentTypeTest(node);
     js.Block oldContainer = currentContainer;
-    js.Statement body = js.Block.empty();
-    currentContainer = body;
+    js.Block body = currentContainer = js.Block.empty();
+    final sourceInformation = node.sourceInformation;
     if (node.isArgumentTypeCheck) {
       use(node.checkedInput);
       _pushCallStatic(_commonElements.throwIllegalArgumentException, [pop()],
           node.sourceInformation);
-      pushStatement(
-          js.Return(pop()).withSourceInformation(node.sourceInformation));
+      pushStatement(js.Return(pop()).withSourceInformation(sourceInformation));
     } else if (node.isReceiverTypeCheck) {
       use(node.checkedInput);
       js.Name methodName =
-          _namer.invocationName(node.receiverTypeCheckSelector);
+          _namer.invocationName(node.receiverTypeCheckSelector!);
       js.Expression call = js.propertyCall(
-          pop(), methodName, []).withSourceInformation(node.sourceInformation);
-      pushStatement(
-          js.Return(call).withSourceInformation(node.sourceInformation));
+          pop(), methodName, []).withSourceInformation(sourceInformation);
+      pushStatement(js.Return(call).withSourceInformation(sourceInformation));
     }
     currentContainer = oldContainer;
-    body = unwrapStatement(body);
+    final then = unwrapStatement(body);
     pushStatement(
-        js.If.noElse(test, body).withSourceInformation(node.sourceInformation));
+        js.If.noElse(test, then).withSourceInformation(sourceInformation));
   }
 
   js.Expression _generateReceiverOrArgumentTypeTest(HPrimitiveCheck node) {
@@ -3027,7 +3041,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     _registry.registerStaticUse(staticUse);
     use(node.checkedInput);
     List<js.Expression> arguments = [pop()];
-    push(js.Call(_emitter.staticFunctionAccess(staticUse.element), arguments)
+    push(js.Call(
+            _emitter.staticFunctionAccess(staticUse.element as FunctionEntity),
+            arguments)
         .withSourceInformation(node.sourceInformation));
   }
 
@@ -3177,7 +3193,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     js.Expression typeof(String type) =>
         js.Binary(relation, js.Prefix('typeof', value), js.string(type));
 
-    js.Expression isTest(MemberEntity helper) {
+    js.Expression isTest(FunctionEntity helper) {
       _registry.registerStaticUse(
           StaticUse.staticInvoke(helper, CallStructure.ONE_ARG));
       js.Expression test =
@@ -3185,13 +3201,12 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       return handleNegative(test);
     }
 
-    js.Expression test;
+    late js.Expression test;
     switch (node.specialization) {
       case IsTestSpecialization.isNull:
       case IsTestSpecialization.notNull:
         // These cases should be lowered using [HIdentity] during optimization.
         failedAt(node, 'Missing lowering');
-        break;
 
       case IsTestSpecialization.string:
         test = typeof("string");
@@ -3220,7 +3235,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
             (dartType is LegacyType &&
                 !dartType.baseType.isObject &&
                 dartType.baseType is! NeverType));
-        InterfaceType type = dartType.withoutNullability;
+        InterfaceType type = dartType.withoutNullability as InterfaceType;
         _registry.registerTypeUse(TypeUse.constructorReference(type));
         test = handleNegative(js.js('# instanceof #',
             [value, _emitter.constructorAccess(type.element)]));
@@ -3247,7 +3262,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   @override
   visitAsCheckSimple(HAsCheckSimple node) {
     use(node.checkedInput);
-    MemberEntity method = node.method;
+    FunctionEntity method = node.method;
     _registry.registerStaticUse(
         StaticUse.staticInvoke(method, CallStructure.ONE_ARG));
     js.Expression methodAccess = _emitter.staticFunctionAccess(method);
@@ -3307,7 +3322,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       // in [receiverMask].
       // TODO(sra): Store the context class on the HInstanceEnvironment. This
       // would allow the subtype classes to be iterated.
-      ClassEntity receiverClass =
+      ClassEntity? receiverClass =
           _abstractValueDomain.getExactClass(receiverMask);
       if (receiverClass != null) {
         if (_closedWorld.rtiNeed.classNeedsTypeArguments(receiverClass)) {
@@ -3343,9 +3358,9 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     TypeRecipe typeExpression = node.typeExpression;
     if (envStructure is FullTypeEnvironmentStructure &&
         typeExpression is TypeExpressionRecipe) {
-      if (typeExpression.type is TypeVariableType) {
-        TypeVariableType type = typeExpression.type;
-        int index = indexTypeVariable(
+      final type = typeExpression.type;
+      if (type is TypeVariableType) {
+        int? index = indexTypeVariable(
             _closedWorld, _rtiSubstitutions, envStructure, type);
         if (index != null) {
           assert(index >= 1);
@@ -3382,7 +3397,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       _registry.registerTypeUse(TypeUse.namedTypeVariableNewRti(typeVariable));
     }
 
-    MemberEntity method = _commonElements.rtiEvalMethod;
+    final method = _commonElements.rtiEvalMethod;
     Selector selector = Selector.fromElement(method);
     js.Name methodLiteral = _namer.invocationName(selector);
     push(js.js('#.#(#)', [
@@ -3404,7 +3419,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     use(node.inputs[1]);
     js.Expression extensions = pop();
 
-    MemberEntity method = _commonElements.rtiBindMethod;
+    final method = _commonElements.rtiBindMethod;
     Selector selector = Selector.fromElement(method);
     js.Name methodLiteral = _namer.invocationName(selector);
     push(js.js('#.#(#)', [
@@ -3417,7 +3432,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         StaticUse.directInvoke(method, selector.callStructure, null));
   }
 
-  _emitIsLateSentinel(HInstruction input, SourceInformation sourceInformation,
+  _emitIsLateSentinel(HInstruction input, SourceInformation? sourceInformation,
       {inverse = false}) {
     use(input);
     js.Expression value = pop();
@@ -3429,7 +3444,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   @override
   visitIsLateSentinel(HIsLateSentinel node) {
-    _metrics.countHIsLateSentinel;
+    _metrics.countHIsLateSentinel++;
     _emitIsLateSentinel(node.inputs.single, node.sourceInformation);
   }
 }
diff --git a/pkg/compiler/lib/src/ssa/codegen_helpers.dart b/pkg/compiler/lib/src/ssa/codegen_helpers.dart
index 93d9c743252..3edf57170cf 100644
--- a/pkg/compiler/lib/src/ssa/codegen_helpers.dart
+++ b/pkg/compiler/lib/src/ssa/codegen_helpers.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.10
-
 import '../constants/values.dart';
 import '../elements/entities.dart';
 import '../inferrer/abstract_value_domain.dart';
@@ -30,11 +28,11 @@ bool canUseAliasedSuperMember(MemberEntity member, Selector selector) {
 ///
 /// - Combine read/modify/write sequences into HReadModifyWrite instructions to
 ///   simplify codegen of expressions like `a.x += y`.
-class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
+class SsaInstructionSelection extends HBaseVisitor<HInstruction?>
     with CodegenPhase {
   final JClosedWorld _closedWorld;
   final CompilerOptions _options;
-  HGraph graph;
+  late final HGraph graph;
 
   SsaInstructionSelection(this._options, this._closedWorld);
 
@@ -49,10 +47,10 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
 
   @override
   void visitBasicBlock(HBasicBlock block) {
-    HInstruction instruction = block.first;
+    HInstruction? instruction = block.first;
     while (instruction != null) {
-      HInstruction next = instruction.next;
-      HInstruction replacement = instruction.accept(this);
+      HInstruction? next = instruction.next;
+      HInstruction? replacement = instruction.accept(this);
       if (replacement != instruction && replacement != null) {
         block.rewrite(instruction, replacement);
 
@@ -84,7 +82,7 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
   @override
   HInstruction visitNullCheck(HNullCheck node) {
     // If we remove this NullCheck, does the program behave the same?
-    HInstruction faultingInstruction = _followingSameFaultInstruction(node);
+    HInstruction? faultingInstruction = _followingSameFaultInstruction(node);
     if (faultingInstruction != null) {
       // Force [faultingInstruction] to appear in same source location as
       // [node]. This avoids the source-mapped stack trace containing an
@@ -103,13 +101,13 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
   /// Searches the instructions following [nullCheck] to see if the first
   /// instruction with an effect or exception will fault on a `null` input just
   /// like the [nullCheck].
-  HInstruction _followingSameFaultInstruction(HNullCheck nullCheck) {
-    HInstruction current = nullCheck.next;
+  HInstruction? _followingSameFaultInstruction(HNullCheck nullCheck) {
+    HInstruction? current = nullCheck.next;
     do {
       // The instructionType of [nullCheck] is not nullable (since it is the
       // (not) null check!) This means that if we do need to check the type, we
       // should test against nullCheck.checkedInput, not the direct input.
-      if (current.getDartReceiver(_closedWorld) == nullCheck) {
+      if (current!.getDartReceiver(_closedWorld) == nullCheck) {
         if (current is HFieldGet) return current;
         if (current is HFieldSet) return current;
         if (current is HGetLength) return current;
@@ -148,13 +146,13 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
         return null;
       }
 
-      HInstruction next = current.next;
+      HInstruction? next = current.next;
       if (next == null) {
         // We do not merge blocks in our SSA graph, so if this block just jumps
         // to a single successor, visit the successor, avoiding back-edges.
-        HBasicBlock successor;
+        HBasicBlock? successor;
         if (current is HGoto) {
-          successor = current.block.successors.single;
+          successor = current.block!.successors.single;
         } else if (current is HIf) {
           // We also leave HIf nodes in place when one branch is dead.
           HInstruction condition = current.inputs.first;
@@ -164,7 +162,7 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
                 : current.elseBlock;
           }
         }
-        if (successor != null && successor.id > current.block.id) {
+        if (successor != null && successor.id > current.block!.id) {
           next = successor.first;
         }
       }
@@ -182,7 +180,7 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
   /// Returns the single JavaScript comparison (`==` or `===`) if that
   /// implements `identical(left, right)`, or returns `null` if the more complex
   /// ternary `left == null ? right == null : left === right` is required.
-  String simpleOp(HInstruction left, HInstruction right) {
+  String? simpleOp(HInstruction left, HInstruction right) {
     AbstractValue leftType = left.instructionType;
     AbstractValue rightType = right.instructionType;
     if (_abstractValueDomain.isNull(leftType).isDefinitelyFalse) {
@@ -225,12 +223,12 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
       .isPotentiallyTrue;
 
   @override
-  HInstruction visitFieldSet(HFieldSet setter) {
+  HInstruction? visitFieldSet(HFieldSet setter) {
     // Pattern match
     //     t1 = x.f; t2 = t1 + 1; x.f = t2; use(t2)   -->  ++x.f
     //     t1 = x.f; t2 = t1 op y; x.f = t2; use(t2)  -->  x.f op= y
     //     t1 = x.f; t2 = t1 + 1; x.f = t2; use(t1)   -->  x.f++
-    HBasicBlock block = setter.block;
+    HBasicBlock block = setter.block!;
     HInstruction op = setter.value;
     HInstruction receiver = setter.receiver;
 
@@ -249,12 +247,12 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
       return false;
     }
 
-    HInstruction noMatchingRead() {
+    HInstruction? noMatchingRead() {
       // If we have other HFieldSet optimizations, they go here.
       return null;
     }
 
-    HInstruction replaceOp(HInstruction replacement, HInstruction getter) {
+    HInstruction? replaceOp(HInstruction replacement, HInstruction getter) {
       block.addBefore(setter, replacement);
       block.remove(setter);
       block.rewrite(op, replacement);
@@ -263,10 +261,8 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
       return null;
     }
 
-    HInstruction plusOrMinus(String assignOp, String incrementOp) {
-      HInvokeBinary binary = op;
-      HInstruction left = binary.left;
-      HInstruction right = binary.right;
+    HInstruction? plusOrMinus(String assignOp, String incrementOp,
+        HInstruction left, HInstruction right) {
       if (isMatchingRead(left)) {
         if (left.usedBy.length == 1) {
           if (right is HConstant && right.constant.isOne) {
@@ -294,7 +290,7 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
       return noMatchingRead();
     }
 
-    HInstruction simple(
+    HInstruction? simple(
         String assignOp, HInstruction left, HInstruction right) {
       if (isMatchingRead(left)) {
         if (left.usedBy.length == 1) {
@@ -306,31 +302,26 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
       return noMatchingRead();
     }
 
-    HInstruction simpleBinary(String assignOp) {
-      HInvokeBinary binary = op;
-      return simple(assignOp, binary.left, binary.right);
-    }
-
-    HInstruction bitop(String assignOp) {
+    HInstruction? bitop(String assignOp, HInvokeBinary binary) {
       // HBitAnd, HBitOr etc. are more difficult because HBitAnd(a.x, y)
       // sometimes needs to be forced to unsigned: a.x = (a.x & y) >>> 0.
       if (op.isUInt31(_abstractValueDomain).isDefinitelyTrue) {
-        return simpleBinary(assignOp);
+        return simple(assignOp, binary.left, binary.right);
       }
       return noMatchingRead();
     }
 
-    if (op is HAdd) return plusOrMinus('+', '++');
-    if (op is HSubtract) return plusOrMinus('-', '--');
+    if (op is HAdd) return plusOrMinus('+', '++', op.left, op.right);
+    if (op is HSubtract) return plusOrMinus('-', '--', op.left, op.right);
 
     if (op is HStringConcat) return simple('+', op.left, op.right);
 
-    if (op is HMultiply) return simpleBinary('*');
-    if (op is HDivide) return simpleBinary('/');
+    if (op is HMultiply) return simple('*', op.left, op.right);
+    if (op is HDivide) return simple('/', op.left, op.right);
 
-    if (op is HBitAnd) return bitop('&');
-    if (op is HBitOr) return bitop('|');
-    if (op is HBitXor) return bitop('^');
+    if (op is HBitAnd) return bitop('&', op);
+    if (op is HBitOr) return bitop('|', op);
+    if (op is HBitXor) return bitop('^', op);
 
     return noMatchingRead();
   }
@@ -349,8 +340,8 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
             !_intercepted(operand1.instructionType)) {
           if (test.usedBy.length == 1 && condition.usedBy.length == 1) {
             node.changeUse(condition, operand1);
-            condition.block.remove(condition);
-            test.block.remove(test);
+            condition.block!.remove(condition);
+            test.block!.remove(test);
           }
         }
       }
@@ -362,9 +353,9 @@ class SsaInstructionSelection extends HBaseVisitor<HInstruction /*?*/ >
       if (operand2.isNull(_abstractValueDomain).isDefinitelyTrue &&
           !_intercepted(operand1.instructionType)) {
         var not = HNot(operand1, _abstractValueDomain.boolType);
-        node.block.addBefore(node, not);
+        node.block!.addBefore(node, not);
         node.changeUse(condition, not);
-        condition.block.remove(condition);
+        condition.block!.remove(condition);
       }
     }
     return node;
@@ -383,9 +374,9 @@ class SsaTypeKnownRemover extends HBaseVisitor<void> with CodegenPhase {
 
   @override
   void visitBasicBlock(HBasicBlock block) {
-    HInstruction instruction = block.last;
+    HInstruction? instruction = block.last;
     while (instruction != null) {
-      HInstruction previous = instruction.previous;
+      HInstruction? previous = instruction.previous;
       instruction.accept(this);
       instruction = previous;
     }
@@ -393,8 +384,8 @@ class SsaTypeKnownRemover extends HBaseVisitor<void> with CodegenPhase {
 
   @override
   void visitTypeKnown(HTypeKnown instruction) {
-    instruction.block.rewrite(instruction, instruction.checkedInput);
-    instruction.block.remove(instruction);
+    instruction.block!.rewrite(instruction, instruction.checkedInput);
+    instruction.block!.remove(instruction);
   }
 
   @override
@@ -419,9 +410,9 @@ class SsaTrustedPrimitiveCheckRemover extends HBaseVisitor<void>
 
   @override
   void visitBasicBlock(HBasicBlock block) {
-    HInstruction instruction = block.first;
+    HInstruction? instruction = block.first;
     while (instruction != null) {
-      HInstruction next = instruction.next;
+      HInstruction? next = instruction.next;
       instruction.accept(this);
       instruction = next;
     }
@@ -429,14 +420,14 @@ class SsaTrustedPrimitiveCheckRemover extends HBaseVisitor<void>
 
   @override
   void visitPrimitiveCheck(HPrimitiveCheck instruction) {
-    instruction.block.rewrite(instruction, instruction.checkedInput);
-    instruction.block.remove(instruction);
+    instruction.block!.rewrite(instruction, instruction.checkedInput);
+    instruction.block!.remove(instruction);
   }
 
   @override
   void visitBoolConversion(HBoolConversion instruction) {
-    instruction.block.rewrite(instruction, instruction.checkedInput);
-    instruction.block.remove(instruction);
+    instruction.block!.rewrite(instruction, instruction.checkedInput);
+    instruction.block!.remove(instruction);
   }
 }
 
@@ -453,9 +444,9 @@ class SsaTrustedLateCheckRemover extends HBaseVisitor<void> with CodegenPhase {
 
   @override
   void visitBasicBlock(HBasicBlock block) {
-    HInstruction instruction = block.first;
+    HInstruction? instruction = block.first;
     while (instruction != null) {
-      HInstruction next = instruction.next;
+      HInstruction? next = instruction.next;
       instruction.accept(this);
       instruction = next;
     }
@@ -465,14 +456,14 @@ class SsaTrustedLateCheckRemover extends HBaseVisitor<void> with CodegenPhase {
   void visitLateCheck(HLateCheck instruction) {
     if (!instruction.isTrusted) return;
     final inputs = instruction.inputs.toList();
-    instruction.block.rewrite(instruction, instruction.checkedInput);
-    instruction.block.remove(instruction);
+    instruction.block!.rewrite(instruction, instruction.checkedInput);
+    instruction.block!.remove(instruction);
     // TODO(sra): There might be a unused name.
 
     // Remove pure unused inputs.
     for (HInstruction input in inputs) {
       if (input.usedBy.isNotEmpty) continue;
-      HBasicBlock block = input.block;
+      HBasicBlock? block = input.block;
       if (block == null) continue; // Already removed.
       if (input.isPure(_abstractValueDomain)) {
         // Special cases that are removed properly by other phases.
@@ -497,7 +488,7 @@ class SsaTrustedLateCheckRemover extends HBaseVisitor<void> with CodegenPhase {
 ///     b.y = v;
 /// -->
 ///     b.y = a.x = v;
-class SsaAssignmentChaining extends HBaseVisitor<HInstruction /*?*/ >
+class SsaAssignmentChaining extends HBaseVisitor<HInstruction?>
     with CodegenPhase {
   final JClosedWorld _closedWorld;
 
@@ -508,35 +499,34 @@ class SsaAssignmentChaining extends HBaseVisitor<HInstruction /*?*/ >
 
   @override
   void visitGraph(HGraph graph) {
-    //this.graph = graph;
     visitDominatorTree(graph);
   }
 
   @override
   void visitBasicBlock(HBasicBlock block) {
-    HInstruction instruction = block.first;
+    HInstruction? instruction = block.first;
     while (instruction != null) {
-      instruction = instruction.accept(this);
+      instruction = instruction.accept<HInstruction?>(this);
     }
   }
 
   /// Returns the next instruction.
   @override
-  HInstruction visitInstruction(HInstruction node) {
+  HInstruction? visitInstruction(HInstruction node) {
     return node.next;
   }
 
   @override
-  HInstruction visitFieldSet(HFieldSet setter) {
+  HInstruction? visitFieldSet(HFieldSet setter) {
     return tryChainAssignment(setter, setter.value);
   }
 
   @override
-  HInstruction visitStaticStore(HStaticStore store) {
+  HInstruction? visitStaticStore(HStaticStore store) {
     return tryChainAssignment(store, store.inputs.single);
   }
 
-  HInstruction tryChainAssignment(HInstruction setter, HInstruction value) {
+  HInstruction? tryChainAssignment(HInstruction setter, HInstruction value) {
     // Try to use result of field or static assignment
     //
     //     t1 = v;  x.f = t1;  ... t1 ...
@@ -551,7 +541,7 @@ class SsaAssignmentChaining extends HBaseVisitor<HInstruction /*?*/ >
     // the number of references to [value].
     HInstruction chain = setter;
     setter.instructionType = value.instructionType;
-    for (HInstruction current = setter.next;;) {
+    for (HInstruction? current = setter.next;;) {
       if (current is HFieldSet) {
         HFieldSet nextSetter = current;
         if (nextSetter.value == value && nextSetter.receiver != value) {
@@ -579,7 +569,7 @@ class SsaAssignmentChaining extends HBaseVisitor<HInstruction /*?*/ >
       break;
     }
 
-    final HInstruction next = chain.next;
+    final HInstruction? next = chain.next;
 
     if (value.usedBy.length <= 1) return next; // setter is only remaining use.
 
@@ -597,7 +587,7 @@ class SsaAssignmentChaining extends HBaseVisitor<HInstruction /*?*/ >
         // assignment.
         // TODO(sra): Better analysis to permit phis that are part of a
         // forwards-only tree.
-        if (use.block.id < chain.block.id) return next;
+        if (use.block!.id < chain.block!.id) return next;
         if (use.usedBy.any((node) => node is HPhi)) return next;
 
         // A forward phi often has a new name. We want to avoid [value] having a
@@ -670,12 +660,12 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
 
   /// List of [HInstruction] that the instruction merger expects in
   /// order when visiting the inputs of an instruction.
-  List<HInstruction> expectedInputs;
+  List<HInstruction>? expectedInputs;
 
   /// Set of pure [HInstruction] that the instruction merger expects to
   /// find. The order of pure instructions do not matter, as they will
   /// not be affected by side effects.
-  Set<HInstruction> pureInputs;
+  Set<HInstruction>? pureInputs;
   Set<HInstruction> generateAtUseSite;
 
   void markAsGenerateAtUseSite(HInstruction instruction) {
@@ -708,7 +698,7 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
             // Move it closer to [user], so that instructions in
             // between do not prevent making it generate at use site.
             input.moveBefore(user);
-            pureInputs.add(input);
+            pureInputs!.add(input);
             // Previous computations done on [input] are now invalid
             // because we moved [input] to another place. So all
             // non code motion invariant instructions need
@@ -723,7 +713,7 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
             input.accept(this);
           }
         } else {
-          expectedInputs.add(input);
+          expectedInputs!.add(input);
         }
       }
     }
@@ -889,8 +879,8 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
     // Return true if it is found, or false if not.
     bool findInInputsAndPopNonMatching(HInstruction instruction) {
       assert(!isEffectivelyPure(instruction));
-      while (!expectedInputs.isEmpty) {
-        HInstruction nextInput = expectedInputs.removeLast();
+      while (!expectedInputs!.isEmpty) {
+        HInstruction nextInput = expectedInputs!.removeLast();
         assert(!generateAtUseSite.contains(nextInput));
         assert(nextInput.usedBy.length == 1);
         if (identical(nextInput, instruction)) {
@@ -900,8 +890,8 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
       return false;
     }
 
-    block.last.accept(this);
-    for (HInstruction instruction = block.last.previous;
+    block.last!.accept(this);
+    for (HInstruction? instruction = block.last!.previous;
         instruction != null;
         instruction = instruction.previous) {
       if (generateAtUseSite.contains(instruction)) {
@@ -912,7 +902,7 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
         continue;
       }
       if (isEffectivelyPure(instruction)) {
-        if (pureInputs.contains(instruction)) {
+        if (pureInputs!.contains(instruction)) {
           tryGenerateAtUseSite(instruction);
         } else {
           // If the input is not in the [pureInputs] set, it has not
@@ -961,6 +951,7 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
           //   f(bar(), t3);
           //   use(t3);
           //
+          final expectedInputs = this.expectedInputs!;
           int oldLength = expectedInputs.length;
           instruction.accept(this);
           if (oldLength != 0 && oldLength != expectedInputs.length) {
@@ -978,7 +969,7 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
           // expected input.
           tryGenerateAtUseSite(instruction);
         } else {
-          assert(expectedInputs.isEmpty);
+          assert(expectedInputs!.isEmpty);
         }
         instruction.accept(this);
       }
@@ -1000,7 +991,7 @@ class SsaInstructionMerger extends HBaseVisitor<void> with CodegenPhase {
 ///  using these operators instead of nested ifs and boolean variables.
 class SsaConditionMerger extends HGraphVisitor with CodegenPhase {
   Set<HInstruction> generateAtUseSite;
-  Set<HInstruction> controlFlowOperators;
+  Set<HIf> controlFlowOperators;
 
   void markAsGenerateAtUseSite(HInstruction instruction) {
     assert(!instruction.isJsStatement());
@@ -1027,16 +1018,16 @@ class SsaConditionMerger extends HGraphVisitor with CodegenPhase {
     // before the control flow instruction, or the last instruction,
     // then we will have to emit a statement for that last instruction.
     if (instruction != block.last &&
-        !identical(instruction, block.last.previous)) return true;
+        !identical(instruction, block.last!.previous)) return true;
 
     // If one of the instructions in the block until [instruction] is
     // not generated at use site, then we will have to emit a
     // statement for it.
     // TODO(ngeoffray): we could generate a comma separated
     // list of expressions.
-    for (HInstruction temp = block.first;
+    for (HInstruction? temp = block.first;
         !identical(temp, instruction);
-        temp = temp.next) {
+        temp = temp!.next) {
       if (!generateAtUseSite.contains(temp)) return true;
     }
 
@@ -1059,8 +1050,8 @@ class SsaConditionMerger extends HGraphVisitor with CodegenPhase {
   @override
   void visitBasicBlock(HBasicBlock block) {
     if (block.last is! HIf) return;
-    HIf startIf = block.last;
-    HBasicBlock end = startIf.joinBlock;
+    HIf startIf = block.last as HIf;
+    HBasicBlock? end = startIf.joinBlock;
 
     // We check that the structure is the following:
     //         If
@@ -1096,7 +1087,7 @@ class SsaConditionMerger extends HGraphVisitor with CodegenPhase {
     HBasicBlock elseBlock = startIf.elseBlock;
 
     if (!identical(end.predecessors[1], elseBlock)) return;
-    HPhi phi = end.phis.first;
+    HPhi phi = end.phis.first as HPhi;
     // This useless phi should have been removed.  Do not generate-at-use if
     // there is no use. See #48383.
     if (phi.usedBy.isEmpty) return;
@@ -1118,16 +1109,16 @@ class SsaConditionMerger extends HGraphVisitor with CodegenPhase {
     // have any statement and its join block is [end], we can emit a
     // sequence of control flow operation.
     if (controlFlowOperators.contains(thenBlock.last)) {
-      HIf otherIf = thenBlock.last;
+      HIf otherIf = thenBlock.last as HIf;
       if (!identical(otherIf.joinBlock, end)) {
         // This could be a join block that just feeds into our join block.
-        HBasicBlock otherJoin = otherIf.joinBlock;
+        HBasicBlock otherJoin = otherIf.joinBlock!;
         if (otherJoin.first != otherJoin.last) return;
         if (otherJoin.successors.length != 1) return;
         if (otherJoin.successors[0] != end) return;
         if (otherJoin.phis.isEmpty) return;
         if (!identical(otherJoin.phis.first, otherJoin.phis.last)) return;
-        HPhi otherPhi = otherJoin.phis.first;
+        HPhi otherPhi = otherJoin.phis.first as HPhi;
         if (thenInput != otherPhi) return;
         if (elseInput != otherPhi.inputs[1]) return;
       }
@@ -1143,9 +1134,9 @@ class SsaConditionMerger extends HGraphVisitor with CodegenPhase {
     controlFlowOperators.add(startIf);
 
     // Find the next non-HGoto instruction following the phi.
-    HInstruction nextInstruction = phi.block.first;
+    HInstruction? nextInstruction = phi.block!.first;
     while (nextInstruction is HGoto) {
-      nextInstruction = nextInstruction.block.successors[0].first;
+      nextInstruction = nextInstruction.block!.successors[0].first;
     }
 
     // If the operation is only used by the first instruction
@@ -1190,9 +1181,9 @@ class SsaShareRegionConstants extends HBaseVisitor<void> with CodegenPhase {
 
   @override
   void visitBasicBlock(HBasicBlock block) {
-    HInstruction instruction = block.first;
+    HInstruction? instruction = block.first;
     while (instruction != null) {
-      HInstruction next = instruction.next;
+      HInstruction? next = instruction.next;
       instruction.accept(this);
       instruction = next;
     }
@@ -1214,7 +1205,7 @@ class SsaShareRegionConstants extends HBaseVisitor<void> with CodegenPhase {
     // entry, not the use of `this`.
     reference.sourceInformation = node.sourceInformation;
     reference.sourceElement = _ExpressionName(name);
-    node.block.addAfter(node, reference);
+    node.block!.addAfter(node, reference);
     for (HInstruction user in users) {
       if (cacheable(user)) {
         user.changeUse(node, reference);
diff --git a/pkg/compiler/lib/src/ssa/nodes.dart b/pkg/compiler/lib/src/ssa/nodes.dart
index 79b2ca099ef..c60d9d124c2 100644
--- a/pkg/compiler/lib/src/ssa/nodes.dart
+++ b/pkg/compiler/lib/src/ssa/nodes.dart
@@ -768,6 +768,9 @@ class HBasicBlock extends HInstructionList {
   static const int STATUS_CLOSED = 2;
   int status = STATUS_NEW;
 
+  // TODO(48820): Can we make the Phi list better typed? As it stands, the
+  // first/last fields and the next/previous fields of the HPhi nodes are all
+  // typed as HInstruction, requiring downcasts to HPhi/HPhi?
   HInstructionList phis = HInstructionList();
 
   HLoopInformation? loopInformation = null;
@@ -3417,7 +3420,7 @@ class HLazyStatic extends HInstruction {
 }
 
 class HStaticStore extends HInstruction {
-  MemberEntity element;
+  FieldEntity element;
   HStaticStore(AbstractValueDomain domain, this.element, HInstruction value)
       : super([value], domain.emptyType) {
     sideEffects.clearAllSideEffects();
@@ -4036,7 +4039,6 @@ abstract class HStatementInformationVisitor {
 }
 
 abstract class HExpressionInformationVisitor {
-  bool visitAndOrInfo(HAndOrBlockInformation info);
   bool visitSubExpressionInfo(HSubExpressionBlockInformation info);
 }
 
@@ -4184,28 +4186,6 @@ class HIfBlockInformation implements HStatementInformation {
       visitor.visitIfInfo(this);
 }
 
-class HAndOrBlockInformation implements HExpressionInformation {
-  final bool isAnd;
-  final HExpressionInformation left;
-  final HExpressionInformation right;
-  HAndOrBlockInformation(this.isAnd, this.left, this.right);
-
-  @override
-  HBasicBlock get start => left.start;
-  @override
-  HBasicBlock get end => right.end;
-
-  // We don't currently use HAndOrBlockInformation.
-  @override
-  HInstruction? get conditionExpression {
-    return null;
-  }
-
-  @override
-  bool accept(HExpressionInformationVisitor visitor) =>
-      visitor.visitAndOrInfo(this);
-}
-
 class HTryBlockInformation implements HStatementInformation {
   final HStatementInformation? body;
   final HLocalValue? catchVariable;
@@ -4494,7 +4474,7 @@ class HAsCheckSimple extends HCheck {
   final DartType dartType;
   final AbstractValueWithPrecision checkedType;
   final bool isTypeError;
-  final MemberEntity method;
+  final FunctionEntity method;
 
   HAsCheckSimple(HInstruction checked, this.dartType, this.checkedType,
       this.isTypeError, this.method, AbstractValue type)
diff --git a/pkg/compiler/lib/src/ssa/variable_allocator.dart b/pkg/compiler/lib/src/ssa/variable_allocator.dart
index acfeb33f5ef..413f223bb20 100644
--- a/pkg/compiler/lib/src/ssa/variable_allocator.dart
+++ b/pkg/compiler/lib/src/ssa/variable_allocator.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.10
-
 import '../common.dart';
 import '../js_backend/namer.dart' show ModularNamer;
 import 'codegen.dart' show CodegenPhase;
@@ -28,7 +26,9 @@ class LiveInterval {
   /// The id where the instruction is defined.
   int start;
   final List<LiveRange> ranges;
-  LiveInterval() : ranges = [];
+  LiveInterval()
+      : start = -1,
+        ranges = [];
 
   // We want [HCheck] instructions to have the same name as the
   // instruction it checks, so both instructions should share the same
@@ -73,7 +73,7 @@ class LiveInterval {
 class LiveEnvironment {
   /// The instruction id where the basic block starts. See
   /// [SsaLiveIntervalBuilder.instructionId].
-  int startId;
+  int startId = -1;
 
   /// The instruction id where the basic block ends.
   final int endId;
@@ -101,7 +101,7 @@ class LiveEnvironment {
   void remove(HInstruction instruction, int id) {
     LiveInterval interval =
         liveIntervals.putIfAbsent(instruction, () => LiveInterval());
-    int lastId = liveInstructions[instruction];
+    int? lastId = liveInstructions[instruction];
     // If [lastId] is null, then this instruction is not being used.
     interval.add(LiveRange(id, lastId ?? id));
     // The instruction is defined at [id].
@@ -159,7 +159,7 @@ class LiveEnvironment {
 /// instruction, and computes the liveIns of each basic block.
 class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
   final Set<HInstruction> generateAtUseSite;
-  final Set<HInstruction> controlFlowOperators;
+  final Set<HIf> controlFlowOperators;
 
   /// A counter to assign start and end ids to live ranges. The initial
   /// value is not relevant. Note that instructionId goes downward to ease
@@ -181,14 +181,14 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
 
   SsaLiveIntervalBuilder(this.generateAtUseSite, this.controlFlowOperators) {
     for (HIf ifNode in controlFlowOperators) {
-      _phiToCondition[ifNode.joinBlock.phis.first] = ifNode.condition;
+      _phiToCondition[ifNode.joinBlock!.phis.first!] = ifNode.condition;
     }
   }
 
   @override
   void visitGraph(HGraph graph) {
     visitPostDominatorTree(graph);
-    if (!liveInstructions[graph.entry].isEmpty) {
+    if (!liveInstructions[graph.entry]!.isEmpty) {
       failedAt(CURRENT_ELEMENT_SPANNABLE, 'LiveIntervalBuilder.');
     }
   }
@@ -196,7 +196,7 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
   void markInputsAsLiveInEnvironment(
       HInstruction instruction, LiveEnvironment environment) {
     if (instruction is HPhi) {
-      HInstruction condition = _phiToCondition[instruction];
+      HInstruction? condition = _phiToCondition[instruction];
       if (condition != null) {
         markAsLiveInEnvironment(condition, environment);
       }
@@ -223,9 +223,8 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
   // When looking for the checkedInstructionOrNonGenerateAtUseSite of t3 we must
   // return t2.
   HInstruction checkedInstructionOrNonGenerateAtUseSite(HCheck check) {
-    dynamic checked = check.checkedInput;
+    HInstruction checked = check.checkedInput;
     while (checked is HCheck) {
-      // ignore: avoid_dynamic_calls
       HInstruction next = checked.checkedInput;
       if (generateAtUseSite.contains(next)) break;
       checked = next;
@@ -265,7 +264,7 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
         // Unconditionally force the live ranges of the HCheck to
         // be the live ranges of the instruction it is checking.
         liveIntervals[instruction] =
-            LiveInterval.forCheck(instructionId, liveIntervals[checked]);
+            LiveInterval.forCheck(instructionId, liveIntervals[checked]!);
       }
     }
   }
@@ -278,7 +277,7 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
     // the inputs of the phis of the successor that flow from this block.
     for (int i = 0; i < block.successors.length; i++) {
       HBasicBlock successor = block.successors[i];
-      LiveEnvironment successorEnv = liveInstructions[successor];
+      LiveEnvironment? successorEnv = liveInstructions[successor];
       if (successorEnv != null) {
         environment.mergeWith(successorEnv);
       } else {
@@ -286,14 +285,14 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
       }
 
       int index = successor.predecessors.indexOf(block);
-      for (HPhi phi = successor.phis.first; phi != null; phi = phi.next) {
+      for (var phi = successor.phis.first; phi != null; phi = phi.next) {
         markAsLiveInEnvironment(phi.inputs[index], environment);
       }
     }
 
     // Iterate over all instructions to remove an instruction from the
     // environment and add its inputs.
-    HInstruction instruction = block.last;
+    HInstruction? instruction = block.last;
     while (instruction != null) {
       if (!generateAtUseSite.contains(instruction)) {
         removeFromEnvironment(instruction, environment);
@@ -305,7 +304,7 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
 
     // We just remove the phis from the environment. The inputs of the
     // phis will be put in the environment of the predecessors.
-    for (HPhi phi = block.phis.first; phi != null; phi = phi.next) {
+    for (var phi = block.phis.first; phi != null; phi = phi.next) {
       if (!generateAtUseSite.contains(phi)) {
         environment.remove(phi, instructionId);
       }
@@ -325,8 +324,8 @@ class SsaLiveIntervalBuilder extends HBaseVisitor<void> with CodegenPhase {
   }
 
   void updateLoopMarker(HBasicBlock header) {
-    LiveEnvironment env = liveInstructions[header];
-    int lastId = env.loopMarkers[header];
+    LiveEnvironment env = liveInstructions[header]!;
+    int lastId = env.loopMarkers[header]!;
     // Update all instructions that are liveIns in [header] to have a
     // range that covers the loop.
     env.liveInstructions.forEach((HInstruction instruction, int id) {
@@ -411,11 +410,11 @@ class VariableNames {
 
   int get numberOfVariables => allUsedNames.length;
 
-  String getName(HInstruction instruction) {
+  String? getName(HInstruction? instruction) {
     return ownName[instruction];
   }
 
-  CopyHandler getCopyHandler(HBasicBlock block) {
+  CopyHandler? getCopyHandler(HBasicBlock block) {
     return copyHandlers[block];
   }
 
@@ -423,7 +422,7 @@ class VariableNames {
     allUsedNames.add(name);
   }
 
-  bool hasName(HInstruction instruction) => ownName.containsKey(instruction);
+  bool hasName(HInstruction? instruction) => ownName.containsKey(instruction);
 
   void addCopy(HBasicBlock block, HInstruction source, HPhi destination) {
     CopyHandler handler = copyHandlers.putIfAbsent(block, () => CopyHandler());
@@ -453,7 +452,7 @@ class VariableNamer {
     // All liveIns instructions must have a name at this point, so we
     // add them to the list of used names.
     environment.liveInstructions.forEach((HInstruction instruction, int index) {
-      String name = names.getName(instruction);
+      String? name = names.getName(instruction);
       if (name != null) {
         usedNames.add(name);
         names.addNameUsed(name);
@@ -480,7 +479,7 @@ class VariableNamer {
     return name;
   }
 
-  HPhi firstPhiUserWithElement(HInstruction instruction) {
+  HPhi? firstPhiUserWithElement(HInstruction instruction) {
     for (HInstruction user in instruction.usedBy) {
       if (user is HPhi && user.sourceElement != null) {
         return user;
@@ -490,7 +489,7 @@ class VariableNamer {
   }
 
   String allocateName(HInstruction instruction) {
-    String name;
+    String? name;
     if (instruction is HCheck) {
       // Special case this instruction to use the name of its
       // input if it has one.
@@ -503,8 +502,8 @@ class VariableNamer {
     }
 
     if (instruction.sourceElement != null) {
-      if (instruction.sourceElement.name != null) {
-        name = allocateWithHint(instruction.sourceElement.name);
+      if (instruction.sourceElement!.name != null) {
+        name = allocateWithHint(instruction.sourceElement!.name!);
       } else {
         // Source element is synthesized and has no name.
         name = allocateTemporary();
@@ -513,9 +512,10 @@ class VariableNamer {
       // We could not find an element for the instruction. If the
       // instruction is used by a phi, try to use the name of the phi.
       // Otherwise, just allocate a temporary name.
-      HPhi phi = firstPhiUserWithElement(instruction);
-      if (phi != null && phi.sourceElement.name != null) {
-        name = allocateWithHint(phi.sourceElement.name);
+      HPhi? phi = firstPhiUserWithElement(instruction);
+      final phiName = phi?.sourceElement?.name;
+      if (phiName != null) {
+        name = allocateWithHint(phiName);
       } else {
         name = allocateTemporary();
       }
@@ -532,7 +532,7 @@ class VariableNamer {
 
   /// Frees [instruction]'s name so it can be used for other instructions.
   void freeName(HInstruction instruction) {
-    String ownName = names.ownName[instruction];
+    String? ownName = names.ownName[instruction];
     if (ownName != null) {
       // We check if we have already looked for temporary names
       // because if we haven't, chances are the temporary we allocate
@@ -576,7 +576,7 @@ class SsaVariableAllocator extends HBaseVisitor<void> with CodegenPhase {
   @override
   void visitBasicBlock(HBasicBlock block) {
     VariableNamer variableNamer =
-        VariableNamer(liveInstructions[block], names, _namer);
+        VariableNamer(liveInstructions[block]!, names, _namer);
 
     block.forEachPhi((HPhi phi) {
       handlePhi(phi, variableNamer);
@@ -599,8 +599,8 @@ class SsaVariableAllocator extends HBaseVisitor<void> with CodegenPhase {
 
   /// Returns whether [instruction] dies at the instruction [at].
   bool diesAt(HInstruction instruction, HInstruction at) {
-    LiveInterval atInterval = liveIntervals[at];
-    LiveInterval instructionInterval = liveIntervals[instruction];
+    LiveInterval atInterval = liveIntervals[at]!;
+    LiveInterval instructionInterval = liveIntervals[instruction]!;
     int start = atInterval.start;
     return instructionInterval.diesAt(start);
   }
@@ -642,7 +642,7 @@ class SsaVariableAllocator extends HBaseVisitor<void> with CodegenPhase {
 
     for (int i = 0; i < phi.inputs.length; i++) {
       HInstruction input = phi.inputs[i];
-      HBasicBlock predecessor = phi.block.predecessors[i];
+      HBasicBlock predecessor = phi.block!.predecessors[i];
       // A [HTypeKnown] instruction never has a name, but its checked
       // input might, therefore we need to do a copy instead of an
       // assignment.
diff --git a/pkg/compiler/lib/src/universe/use.dart b/pkg/compiler/lib/src/universe/use.dart
index feb5aa88ad6..a1cc26071c2 100644
--- a/pkg/compiler/lib/src/universe/use.dart
+++ b/pkg/compiler/lib/src/universe/use.dart
@@ -496,7 +496,7 @@ class StaticUse {
 
   /// Direct invocation of a method [element] with the given [callStructure].
   factory StaticUse.directInvoke(FunctionEntity element,
-      CallStructure callStructure, List<DartType> typeArguments) {
+      CallStructure callStructure, List<DartType>? typeArguments) {
     assert(
         element.isInstanceMember,
         failedAt(element,
diff --git a/pkg/js_ast/lib/src/nodes.dart b/pkg/js_ast/lib/src/nodes.dart
index fb2c4809d6a..264dfb8bf09 100644
--- a/pkg/js_ast/lib/src/nodes.dart
+++ b/pkg/js_ast/lib/src/nodes.dart
@@ -805,7 +805,11 @@ class For extends Loop {
   final Expression? condition;
   final Expression? update;
 
-  For(this.init, this.condition, this.update, Statement body) : super(body);
+  For(this.init, this.condition, this.update, Statement body,
+      {JavaScriptNodeSourceInformation? sourceInformation})
+      : super(body) {
+    _sourceInformation = sourceInformation;
+  }
 
   @override
   T accept<T>(NodeVisitor<T> visitor) => visitor.visitFor(this);
@@ -840,7 +844,11 @@ class ForIn extends Loop {
   final Expression leftHandSide;
   final Expression object;
 
-  ForIn(this.leftHandSide, this.object, Statement body) : super(body);
+  ForIn(this.leftHandSide, this.object, Statement body,
+      {JavaScriptNodeSourceInformation? sourceInformation})
+      : super(body) {
+    _sourceInformation = sourceInformation;
+  }
 
   @override
   T accept<T>(NodeVisitor<T> visitor) => visitor.visitForIn(this);
@@ -870,7 +878,11 @@ class ForIn extends Loop {
 class While extends Loop {
   final Expression condition;
 
-  While(this.condition, Statement body) : super(body);
+  While(this.condition, Statement body,
+      {JavaScriptNodeSourceInformation? sourceInformation})
+      : super(body) {
+    _sourceInformation = sourceInformation;
+  }
 
   @override
   T accept<T>(NodeVisitor<T> visitor) => visitor.visitWhile(this);
@@ -898,7 +910,11 @@ class While extends Loop {
 class Do extends Loop {
   final Expression condition;
 
-  Do(Statement body, this.condition) : super(body);
+  Do(Statement body, this.condition,
+      {JavaScriptNodeSourceInformation? sourceInformation})
+      : super(body) {
+    _sourceInformation = sourceInformation;
+  }
 
   @override
   T accept<T>(NodeVisitor<T> visitor) => visitor.visitDo(this);
@@ -1509,7 +1525,10 @@ class VariableInitialization extends Expression {
   // The initializing value can be missing, e.g. for `a` in `var a, b=1;`.
   final Expression? value;
 
-  VariableInitialization(this.declaration, this.value);
+  VariableInitialization(this.declaration, this.value,
+      {JavaScriptNodeSourceInformation? sourceInformation}) {
+    _sourceInformation = sourceInformation;
+  }
 
   @override
   int get precedenceLevel => ASSIGNMENT;
diff --git a/pkg/js_ast/lib/src/template.dart b/pkg/js_ast/lib/src/template.dart
index 208c575436d..0da617b36fb 100644
--- a/pkg/js_ast/lib/src/template.dart
+++ b/pkg/js_ast/lib/src/template.dart
@@ -134,6 +134,16 @@ class Template {
     }
     throw ArgumentError.value(arguments, 'arguments', 'Must be a List or Map');
   }
+
+  // TODO(sra): We should rather make the return type of `instantiate` be what
+  // we need, either by making Template be generic or have Expression and
+  // Statement subclasses that override `instantiate`. Checking is likely still
+  // required since the argument can be the result (e.g. "#").
+  Expression instantiateExpression(Object arguments) =>
+      instantiate(arguments) as Expression;
+
+  Statement instantiateStatement(Object arguments) =>
+      instantiate(arguments) as Statement;
 }
 
 /// An Instantiator is a Function that generates a JS AST tree or List of