diff --git a/pkg/vm/bin/kernel_service.dart b/pkg/vm/bin/kernel_service.dart
index 7e0dc6a5432..0fcaf1873e2 100644
--- a/pkg/vm/bin/kernel_service.dart
+++ b/pkg/vm/bin/kernel_service.dart
@@ -153,7 +153,7 @@ CompilerOptions setupCompilerOptions(
 }
 
 abstract class Compiler {
-  final int isolateId;
+  final int isolateGroupId;
   final FileSystem fileSystem;
   final Uri platformKernelPath;
   final bool enableAsserts;
@@ -172,7 +172,7 @@ abstract class Compiler {
 
   CompilerOptions options;
 
-  Compiler(this.isolateId, this.fileSystem, this.platformKernelPath,
+  Compiler(this.isolateGroupId, this.fileSystem, this.platformKernelPath,
       {this.enableAsserts: false,
       this.nullSafety: kNullSafetyOptionUnspecified,
       this.experimentalFlags: null,
@@ -214,7 +214,7 @@ abstract class Compiler {
 
       if (errors.isEmpty) {
         // Record dependencies only if compilation was error free.
-        _recordDependencies(isolateId, component, options.packagesFileUri);
+        _recordDependencies(isolateGroupId, component, options.packagesFileUri);
       }
 
       return compilerResult;
@@ -289,14 +289,14 @@ class IncrementalCompilerWrapper extends Compiler {
   IncrementalCompiler generator;
 
   IncrementalCompilerWrapper(
-      int isolateId, FileSystem fileSystem, Uri platformKernelPath,
+      int isolateGroupId, FileSystem fileSystem, Uri platformKernelPath,
       {bool enableAsserts: false,
       int nullSafety: kNullSafetyOptionUnspecified,
       List<String> experimentalFlags: null,
       String packageConfig: null,
       String invocationModes: '',
       String verbosityLevel: Verbosity.defaultValue})
-      : super(isolateId, fileSystem, platformKernelPath,
+      : super(isolateGroupId, fileSystem, platformKernelPath,
             enableAsserts: enableAsserts,
             nullSafety: nullSafety,
             experimentalFlags: experimentalFlags,
@@ -308,7 +308,7 @@ class IncrementalCompilerWrapper extends Compiler {
 
   factory IncrementalCompilerWrapper.forExpressionCompilationOnly(
       Component component,
-      int isolateId,
+      int isolateGroupId,
       FileSystem fileSystem,
       Uri platformKernelPath,
       {bool enableAsserts: false,
@@ -316,7 +316,7 @@ class IncrementalCompilerWrapper extends Compiler {
       String packageConfig: null,
       String invocationModes: ''}) {
     IncrementalCompilerWrapper result = IncrementalCompilerWrapper(
-        isolateId, fileSystem, platformKernelPath,
+        isolateGroupId, fileSystem, platformKernelPath,
         enableAsserts: enableAsserts,
         experimentalFlags: experimentalFlags,
         packageConfig: packageConfig,
@@ -342,9 +342,9 @@ class IncrementalCompilerWrapper extends Compiler {
   void accept() => generator.accept();
   void invalidate(Uri uri) => generator.invalidate(uri);
 
-  Future<IncrementalCompilerWrapper> clone(int isolateId) async {
+  Future<IncrementalCompilerWrapper> clone(int isolateGroupId) async {
     IncrementalCompilerWrapper clone = IncrementalCompilerWrapper(
-        isolateId, fileSystem, platformKernelPath,
+        isolateGroupId, fileSystem, platformKernelPath,
         enableAsserts: enableAsserts,
         nullSafety: nullSafety,
         experimentalFlags: experimentalFlags,
@@ -359,7 +359,7 @@ class IncrementalCompilerWrapper extends Compiler {
     // clone should be used for.
     MemoryFileSystem memoryFileSystem = (fileSystem as HybridFileSystem).memory;
 
-    String filename = 'full-component-$isolateId.dill';
+    String filename = 'full-component-$isolateGroupId.dill';
     Sink sink = FileSink(memoryFileSystem.entityForUri(Uri.file(filename)));
     new BinaryPrinter(sink).writeComponentFile(fullComponent);
     await sink.close();
@@ -374,7 +374,7 @@ class SingleShotCompilerWrapper extends Compiler {
   final bool requireMain;
 
   SingleShotCompilerWrapper(
-      int isolateId, FileSystem fileSystem, Uri platformKernelPath,
+      int isolateGroupId, FileSystem fileSystem, Uri platformKernelPath,
       {this.requireMain: false,
       bool enableAsserts: false,
       int nullSafety: kNullSafetyOptionUnspecified,
@@ -382,7 +382,7 @@ class SingleShotCompilerWrapper extends Compiler {
       String packageConfig: null,
       String invocationModes: '',
       String verbosityLevel: Verbosity.defaultValue})
-      : super(isolateId, fileSystem, platformKernelPath,
+      : super(isolateGroupId, fileSystem, platformKernelPath,
             enableAsserts: enableAsserts,
             nullSafety: nullSafety,
             experimentalFlags: experimentalFlags,
@@ -405,18 +405,15 @@ class SingleShotCompilerWrapper extends Compiler {
   }
 }
 
-// TODO(33428): This state is leaked on isolate shutdown.
-final Map<int, IncrementalCompilerWrapper> isolateCompilers =
-    new Map<int, IncrementalCompilerWrapper>();
-final Map<int, List<Uri>> isolateDependencies = new Map<int, List<Uri>>();
-final Map<int, _ExpressionCompilationFromDillSettings> isolateLoadNotifies =
-    new Map<int, _ExpressionCompilationFromDillSettings>();
+final Map<int, IncrementalCompilerWrapper> isolateCompilers = {};
+final Map<int, List<Uri>> isolateDependencies = {};
+final Map<int, _ExpressionCompilationFromDillSettings> isolateLoadNotifies = {};
 
-IncrementalCompilerWrapper lookupIncrementalCompiler(int isolateId) {
-  return isolateCompilers[isolateId];
+IncrementalCompilerWrapper lookupIncrementalCompiler(int isolateGroupId) {
+  return isolateCompilers[isolateGroupId];
 }
 
-Future<Compiler> lookupOrBuildNewIncrementalCompiler(int isolateId,
+Future<Compiler> lookupOrBuildNewIncrementalCompiler(int isolateGroupId,
     List sourceFiles, Uri platformKernelPath, List<int> platformKernel,
     {bool enableAsserts: false,
     int nullSafety: kNullSafetyOptionUnspecified,
@@ -426,7 +423,8 @@ Future<Compiler> lookupOrBuildNewIncrementalCompiler(int isolateId,
     String multirootScheme,
     String invocationModes: '',
     String verbosityLevel: Verbosity.defaultValue}) async {
-  IncrementalCompilerWrapper compiler = lookupIncrementalCompiler(isolateId);
+  IncrementalCompilerWrapper compiler =
+      lookupIncrementalCompiler(isolateGroupId);
   if (compiler != null) {
     updateSources(compiler, sourceFiles);
     invalidateSources(compiler, sourceFiles);
@@ -439,7 +437,7 @@ Future<Compiler> lookupOrBuildNewIncrementalCompiler(int isolateId,
         sourceFiles[1] == null) {
       // Just use first compiler that should represent main isolate as a source for cloning.
       var source = isolateCompilers.entries.first;
-      compiler = await source.value.clone(isolateId);
+      compiler = await source.value.clone(isolateGroupId);
     } else {
       FileSystem fileSystem = _buildFileSystem(
           sourceFiles, platformKernel, multirootFilepaths, multirootScheme);
@@ -449,7 +447,7 @@ Future<Compiler> lookupOrBuildNewIncrementalCompiler(int isolateId,
       // isolate needs to receive a message indicating that particular
       // isolate was shut down. Message should be handled here in this script.
       compiler = new IncrementalCompilerWrapper(
-          isolateId, fileSystem, platformKernelPath,
+          isolateGroupId, fileSystem, platformKernelPath,
           enableAsserts: enableAsserts,
           nullSafety: nullSafety,
           experimentalFlags: experimentalFlags,
@@ -457,7 +455,7 @@ Future<Compiler> lookupOrBuildNewIncrementalCompiler(int isolateId,
           invocationModes: invocationModes,
           verbosityLevel: verbosityLevel);
     }
-    isolateCompilers[isolateId] = compiler;
+    isolateCompilers[isolateGroupId] = compiler;
   }
   return compiler;
 }
@@ -494,7 +492,7 @@ void invalidateSources(IncrementalCompilerWrapper compiler, List sourceFiles) {
 // kernel_isolate.cc and Loader::SendKernelRequest in loader.cc.
 Future _processExpressionCompilationRequest(request) async {
   final SendPort port = request[1];
-  final int isolateId = request[2];
+  final int isolateGroupId = request[2];
   final dynamic dart_platform_kernel = request[3];
   final String expression = request[4];
   final List<String> definitions = request[5].cast<String>();
@@ -508,15 +506,15 @@ Future _processExpressionCompilationRequest(request) async {
   final List<String> experimentalFlags =
       request[13] != null ? request[13].cast<String>() : null;
 
-  IncrementalCompilerWrapper compiler = isolateCompilers[isolateId];
+  IncrementalCompilerWrapper compiler = isolateCompilers[isolateGroupId];
 
   _ExpressionCompilationFromDillSettings isolateLoadDillData =
-      isolateLoadNotifies[isolateId];
+      isolateLoadNotifies[isolateGroupId];
   if (isolateLoadDillData != null) {
     // Check if we can reuse the compiler.
     if (isolateLoadDillData.blobLoadCount != blobLoadCount ||
         isolateLoadDillData.prevDillCount != dillData.length) {
-      compiler = isolateCompilers[isolateId] = null;
+      compiler = isolateCompilers[isolateGroupId] = null;
     }
   }
 
@@ -525,7 +523,7 @@ Future _processExpressionCompilationRequest(request) async {
       if (verbose) {
         print("DFE: Initializing compiler from ${dillData.length} dill files");
       }
-      isolateLoadNotifies[isolateId] =
+      isolateLoadNotifies[isolateGroupId] =
           new _ExpressionCompilationFromDillSettings(
               blobLoadCount, dillData.length);
 
@@ -585,11 +583,11 @@ Future _processExpressionCompilationRequest(request) async {
       // isolate was shut down. Message should be handled here in this script.
       try {
         compiler = new IncrementalCompilerWrapper.forExpressionCompilationOnly(
-            component, isolateId, fileSystem, null,
+            component, isolateGroupId, fileSystem, null,
             enableAsserts: enableAsserts,
             experimentalFlags: experimentalFlags,
             packageConfig: dotPackagesFile);
-        isolateCompilers[isolateId] = compiler;
+        isolateCompilers[isolateGroupId] = compiler;
         await compiler.compile(
             component.mainMethod?.enclosingLibrary?.importUri ??
                 component.libraries.last.importUri);
@@ -640,8 +638,8 @@ Future _processExpressionCompilationRequest(request) async {
 }
 
 void _recordDependencies(
-    int isolateId, Component component, Uri packageConfig) {
-  final dependencies = isolateDependencies[isolateId] ??= <Uri>[];
+    int isolateGroupId, Component component, Uri packageConfig) {
+  final dependencies = isolateDependencies[isolateGroupId] ??= <Uri>[];
 
   if (component != null) {
     for (var lib in component.libraries) {
@@ -673,8 +671,9 @@ List<int> _serializeDependencies(List<Uri> uris) {
   return utf8.encode(uris.map(_escapeDependency).join(" "));
 }
 
-Future _processListDependenciesRequest(SendPort port, int isolateId) async {
-  final List<Uri> dependencies = isolateDependencies[isolateId] ?? <Uri>[];
+Future _processListDependenciesRequest(
+    SendPort port, int isolateGroupId) async {
+  final List<Uri> dependencies = isolateDependencies[isolateGroupId] ?? <Uri>[];
 
   CompilationResult result;
   try {
@@ -687,10 +686,10 @@ Future _processListDependenciesRequest(SendPort port, int isolateId) async {
 }
 
 Future _processIsolateShutdownNotification(request) async {
-  final int isolateId = request[1];
-  isolateCompilers.remove(isolateId);
-  isolateDependencies.remove(isolateId);
-  isolateLoadNotifies.remove(isolateId);
+  final int isolateGroupId = request[1];
+  isolateCompilers.remove(isolateGroupId);
+  isolateDependencies.remove(isolateGroupId);
+  isolateLoadNotifies.remove(isolateGroupId);
 }
 
 Future _processLoadRequest(request) async {
@@ -733,10 +732,10 @@ Future _processLoadRequest(request) async {
   }
 
   final SendPort port = request[1];
-  final int isolateId = request[7];
+  final int isolateGroupId = request[7];
 
   if (tag == kListDependenciesTag) {
-    await _processListDependenciesRequest(port, isolateId);
+    await _processListDependenciesRequest(port, isolateGroupId);
     return;
   }
 
@@ -777,7 +776,7 @@ Future _processLoadRequest(request) async {
   if (tag == kUpdateSourcesTag) {
     assert(incremental,
         "Incremental compiler required for use of 'kUpdateSourcesTag'");
-    compiler = lookupIncrementalCompiler(isolateId);
+    compiler = lookupIncrementalCompiler(isolateGroupId);
     if (compiler == null) {
       port.send(new CompilationResult.errors(
               ["No incremental compiler available for this isolate."], null)
@@ -790,7 +789,7 @@ Future _processLoadRequest(request) async {
   } else if (tag == kAcceptTag) {
     assert(
         incremental, "Incremental compiler required for use of 'kAcceptTag'");
-    compiler = lookupIncrementalCompiler(isolateId);
+    compiler = lookupIncrementalCompiler(isolateGroupId);
     // There are unit tests that invoke the IncrementalCompiler directly and
     // request a reload, meaning that we won't have a compiler for this isolate.
     if (compiler != null) {
@@ -841,7 +840,7 @@ Future _processLoadRequest(request) async {
   // watch the performance though.
   if (incremental) {
     compiler = await lookupOrBuildNewIncrementalCompiler(
-        isolateId, sourceFiles, platformKernelPath, platformKernel,
+        isolateGroupId, sourceFiles, platformKernelPath, platformKernel,
         enableAsserts: enableAsserts,
         nullSafety: nullSafety,
         experimentalFlags: experimentalFlags,
@@ -854,7 +853,7 @@ Future _processLoadRequest(request) async {
     FileSystem fileSystem = _buildFileSystem(
         sourceFiles, platformKernel, multirootFilepaths, multirootScheme);
     compiler = new SingleShotCompilerWrapper(
-        isolateId, fileSystem, platformKernelPath,
+        isolateGroupId, fileSystem, platformKernelPath,
         requireMain: false,
         enableAsserts: enableAsserts,
         nullSafety: nullSafety,
@@ -1007,7 +1006,7 @@ Future trainInternal(String scriptUri, String platformKernelPath) async {
     false /* incremental */,
     false /* snapshot */,
     kNullSafetyOptionUnspecified /* null safety */,
-    1 /* isolateId chosen randomly */,
+    1 /* isolateGroupId chosen randomly */,
     [] /* source files */,
     false /* enable asserts */,
     null /* experimental_flags */,
diff --git a/runtime/lib/isolate.cc b/runtime/lib/isolate.cc
index d2de37cc4e7..a532bb4a46f 100644
--- a/runtime/lib/isolate.cc
+++ b/runtime/lib/isolate.cc
@@ -886,10 +886,10 @@ static const char* CanonicalizeUri(Thread* thread,
                                    char** error) {
   const char* result = NULL;
   Zone* zone = thread->zone();
-  Isolate* isolate = thread->isolate();
-  if (isolate->HasTagHandler()) {
+  auto isolate_group = thread->isolate_group();
+  if (isolate_group->HasTagHandler()) {
     const Object& obj = Object::Handle(
-        isolate->CallTagHandler(Dart_kCanonicalizeUrl, library, uri));
+        isolate_group->CallTagHandler(Dart_kCanonicalizeUrl, library, uri));
     if (obj.IsString()) {
       result = String2UTF8(String::Cast(obj));
     } else if (obj.IsError()) {
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index a245378e639..2cc879a0f6c 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -662,7 +662,7 @@ static void ThrowLanguageError(const char* message) {
 DEFINE_NATIVE_ENTRY(IsolateMirror_loadUri, 0, 1) {
   GET_NON_NULL_NATIVE_ARGUMENT(String, uri, arguments->NativeArgAt(0));
 
-  if (!isolate->HasTagHandler()) {
+  if (!isolate->group()->HasTagHandler()) {
     ThrowLanguageError("no library handler registered");
   }
 
@@ -675,7 +675,7 @@ DEFINE_NATIVE_ENTRY(IsolateMirror_loadUri, 0, 1) {
   } else {
     isolate->BlockClassFinalization();
     const Object& result = Object::Handle(
-        zone, isolate->CallTagHandler(
+        zone, isolate->group()->CallTagHandler(
                   Dart_kCanonicalizeUrl,
                   Library::Handle(
                       zone, isolate->group()->object_store()->root_library()),
@@ -703,7 +703,7 @@ DEFINE_NATIVE_ENTRY(IsolateMirror_loadUri, 0, 1) {
   // Request the embedder to load the library.
   isolate->BlockClassFinalization();
   Object& result = Object::Handle(
-      zone, isolate->CallTagHandler(
+      zone, isolate->group()->CallTagHandler(
                 Dart_kImportTag,
                 Library::Handle(
                     zone, isolate->group()->object_store()->root_library()),
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 97dab6cd016..f371e1b0421 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1895,9 +1895,9 @@ Thread* Isolate::mutator_thread() const {
   return mutator_thread_;
 }
 
-ObjectPtr Isolate::CallTagHandler(Dart_LibraryTag tag,
-                                  const Object& arg1,
-                                  const Object& arg2) {
+ObjectPtr IsolateGroup::CallTagHandler(Dart_LibraryTag tag,
+                                       const Object& arg1,
+                                       const Object& arg2) {
   Thread* thread = Thread::Current();
   Api::Scope api_scope(thread);
   Dart_Handle api_arg1 = Api::NewHandle(thread, arg1.ptr());
@@ -1906,7 +1906,7 @@ ObjectPtr Isolate::CallTagHandler(Dart_LibraryTag tag,
   {
     TransitionVMToNative transition(thread);
     ASSERT(HasTagHandler());
-    api_result = group()->library_tag_handler()(tag, api_arg1, api_arg2);
+    api_result = library_tag_handler()(tag, api_arg1, api_arg2);
   }
   return Api::UnwrapHandle(api_result);
 }
@@ -2473,7 +2473,6 @@ void Isolate::Shutdown() {
     StackZone zone(thread);
     HandleScope handle_scope(thread);
     ServiceIsolate::SendIsolateShutdownMessage();
-    KernelIsolate::NotifyAboutIsolateShutdown(this);
 #if !defined(PRODUCT)
     debugger()->Shutdown();
 #endif
@@ -2545,6 +2544,8 @@ void Isolate::LowLevelCleanup(Isolate* isolate) {
   const bool shutdown_group =
       isolate_group->UnregisterIsolateDecrementCount(isolate);
   if (shutdown_group) {
+    KernelIsolate::NotifyAboutIsolateGroupShutdown(isolate_group);
+
 #if !defined(DART_PRECOMPILED_RUNTIME)
     if (!is_vm_isolate) {
       Thread::EnterIsolateGroupAsHelper(isolate_group, Thread::kUnknownTask,
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index aa8575f0dbe..96f69080e9d 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -600,6 +600,10 @@ class IsolateGroup : public IntrusiveDListEntry<IsolateGroup> {
   void IncreaseMutatorCount(Isolate* mutator);
   void DecreaseMutatorCount(Isolate* mutator);
 
+  bool HasTagHandler() const { return library_tag_handler() != nullptr; }
+  ObjectPtr CallTagHandler(Dart_LibraryTag tag,
+                           const Object& arg1,
+                           const Object& arg2);
   Dart_LibraryTagHandler library_tag_handler() const {
     return library_tag_handler_;
   }
@@ -724,7 +728,7 @@ class IsolateGroup : public IntrusiveDListEntry<IsolateGroup> {
 #endif
   }
 
-  uint64_t id() { return id_; }
+  uint64_t id() const { return id_; }
 
   static void Init();
   static void Cleanup();
@@ -1102,12 +1106,6 @@ class Isolate : public BaseIsolate, public IntrusiveDListEntry<Isolate> {
     environment_callback_ = value;
   }
 
-  bool HasTagHandler() const {
-    return group()->library_tag_handler() != nullptr;
-  }
-  ObjectPtr CallTagHandler(Dart_LibraryTag tag,
-                           const Object& arg1,
-                           const Object& arg2);
   bool HasDeferredLoadHandler() const {
     return group()->deferred_load_handler() != nullptr;
   }
diff --git a/runtime/vm/kernel_binary.cc b/runtime/vm/kernel_binary.cc
index 00ceb7dd61d..9e6e614aaca 100644
--- a/runtime/vm/kernel_binary.cc
+++ b/runtime/vm/kernel_binary.cc
@@ -194,18 +194,18 @@ std::unique_ptr<Program> Program::ReadFrom(Reader* reader, const char** error) {
 std::unique_ptr<Program> Program::ReadFromFile(
     const char* script_uri, const char** error /* = nullptr */) {
   Thread* thread = Thread::Current();
-  Isolate* isolate = thread->isolate();
+  auto isolate_group = thread->isolate_group();
   if (script_uri == NULL) {
     return nullptr;
   }
-  if (!isolate->HasTagHandler()) {
+  if (!isolate_group->HasTagHandler()) {
     return nullptr;
   }
   std::unique_ptr<kernel::Program> kernel_program;
 
   const String& uri = String::Handle(String::New(script_uri));
-  const Object& ret = Object::Handle(
-      isolate->CallTagHandler(Dart_kKernelTag, Object::null_object(), uri));
+  const Object& ret = Object::Handle(isolate_group->CallTagHandler(
+      Dart_kKernelTag, Object::null_object(), uri));
   if (ret.IsExternalTypedData()) {
     const auto& typed_data = ExternalTypedData::Handle(
         thread->zone(), ExternalTypedData::RawCast(ret.ptr()));
diff --git a/runtime/vm/kernel_isolate.cc b/runtime/vm/kernel_isolate.cc
index 168cb765d0c..9eab9787f6d 100644
--- a/runtime/vm/kernel_isolate.cc
+++ b/runtime/vm/kernel_isolate.cc
@@ -559,13 +559,12 @@ class KernelCompilationRequest : public ValueObject {
     is_static_object.type = Dart_CObject_kBool;
     is_static_object.value.as_bool = is_static;
 
-    auto isolate = thread->isolate();
     auto isolate_group = thread->isolate_group();
     auto source = isolate_group->source();
 
     Dart_CObject isolate_id;
     isolate_id.type = Dart_CObject_kInt64;
-    isolate_id.value.as_int64 = static_cast<int64_t>(isolate->main_port());
+    isolate_id.value.as_int64 = static_cast<int64_t>(isolate_group->id());
 
     intptr_t num_dills = 0;
     if (source->kernel_buffer != nullptr) {
@@ -756,16 +755,16 @@ class KernelCompilationRequest : public ValueObject {
     // compilation logic out of CreateIsolateAndSetupHelper and into
     // IsolateSetupHelper in main.cc.
     auto thread = Thread::Current();
-    auto isolate = thread != nullptr ? thread->isolate() : nullptr;
     auto isolate_group = thread != nullptr ? thread->isolate_group() : nullptr;
 
     if (incremental_compile) {
-      ASSERT(isolate != NULL);
+      ASSERT(isolate_group != nullptr);
     }
     Dart_CObject isolate_id;
     isolate_id.type = Dart_CObject_kInt64;
-    isolate_id.value.as_int64 =
-        isolate != nullptr ? static_cast<int64_t>(isolate->main_port()) : 0;
+    isolate_id.value.as_int64 = isolate_group != nullptr
+                                    ? static_cast<int64_t>(isolate_group->id())
+                                    : 0;
 
     Dart_CObject message;
     message.type = Dart_CObject_kArray;
@@ -1155,7 +1154,8 @@ Dart_KernelCompilationResult KernelIsolate::UpdateInMemorySources(
       Dart_KernelCompilationVerbosityLevel_Error);
 }
 
-void KernelIsolate::NotifyAboutIsolateShutdown(const Isolate* isolate) {
+void KernelIsolate::NotifyAboutIsolateGroupShutdown(
+    const IsolateGroup* isolate_group) {
   if (!KernelIsolate::IsRunning()) {
     return;
   }
@@ -1170,8 +1170,7 @@ void KernelIsolate::NotifyAboutIsolateShutdown(const Isolate* isolate) {
 
   Dart_CObject isolate_id;
   isolate_id.type = Dart_CObject_kInt64;
-  isolate_id.value.as_int64 =
-      isolate != NULL ? static_cast<int64_t>(isolate->main_port()) : 0;
+  isolate_id.value.as_int64 = static_cast<int64_t>(isolate_group->id());
 
   Dart_CObject message;
   message.type = Dart_CObject_kArray;
diff --git a/runtime/vm/kernel_isolate.h b/runtime/vm/kernel_isolate.h
index 438e8837834..2ba4f2c2df4 100644
--- a/runtime/vm/kernel_isolate.h
+++ b/runtime/vm/kernel_isolate.h
@@ -79,7 +79,8 @@ class KernelIsolate : public AllStatic {
 
   static Dart_KernelCompilationResult ListDependencies();
 
-  static void NotifyAboutIsolateShutdown(const Isolate* isolate);
+  static void NotifyAboutIsolateGroupShutdown(
+      const IsolateGroup* isolate_group);
 
   static void AddExperimentalFlag(const char* value);
   static bool GetExperimentalFlag(ExperimentalFeature feature);
@@ -115,7 +116,8 @@ class KernelIsolate : public AllStatic {
   static bool IsRunning() { return false; }
   static void Shutdown() {}
   static bool IsKernelIsolate(const Isolate* isolate) { return false; }
-  static void NotifyAboutIsolateShutdown(const Isolate* isolate) {}
+  static void NotifyAboutIsolateGroupShutdown(
+      const IsolateGroup* isolate_group) {}
   static bool GetExperimentalFlag(const char* value) { return false; }
 
  protected:
diff --git a/runtime/vm/kernel_loader.cc b/runtime/vm/kernel_loader.cc
index 977686f6e5f..7dc46ad7a11 100644
--- a/runtime/vm/kernel_loader.cc
+++ b/runtime/vm/kernel_loader.cc
@@ -704,13 +704,13 @@ void KernelLoader::LoadNativeExtensionLibraries() {
 void KernelLoader::LoadNativeExtension(const Library& library,
                                        const String& uri_path) {
 #if !defined(DART_PRECOMPILER)
-  if (!I->HasTagHandler()) {
+  if (!IG->HasTagHandler()) {
     H.ReportError("no library handler registered.");
   }
 
   I->BlockClassFinalization();
   const auto& result = Object::Handle(
-      Z, I->CallTagHandler(Dart_kImportExtensionTag, library, uri_path));
+      Z, IG->CallTagHandler(Dart_kImportExtensionTag, library, uri_path));
   I->UnblockClassFinalization();
 
   if (result.IsError()) {