Enforce current library restrictions.

Mark all currently unimplementable types as `final`, or `sealed` for `num` and `final` for `Function`.
Mark all current classes intended as mixins as `mixin class`.

More additions and cleanup will follow,
but this change should make everything keep working as today
if we flip the switch.

TEST= No new tests, very little actual change, covered by existing tests with few changes. Will add more tests when adding more modifiers.

Change-Id: I40e724f823e7f88cdef186d2f73874df256e2f43
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/281683
Reviewed-by: Siva Annamalai <asiva@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Kallen Tu <kallentu@google.com>

CHANGELOG.md

@@ -16,6 +16,18 @@
 
 ### Libraries
 
+#### General changes
+
+- **Breaking Change**: Non-`mixin` classes in the platform libraries
+  can no longer be mixed in, unless they are explicitly marked as `mixin class`.
+  The following existing classes have been made mixin classes:
+  * `IterableMixin`
+  * `ListMixin`
+  * `SetMixin`
+  * `MapMixin`
+  * `LinkedListEntry`
+  * `StringConversionSink`
+
 #### `dart:core`
 
 - **Breaking change** [#49529][]:
@@ -48,6 +60,15 @@
     Existing bidirectional iterators can still work, they just don't have
     a shared supertype locking them to a specific name for moving backwards.
 
+- **Breaking change when migrating code to Dart 3.0**:
+  Some changes to platform libraries only affect code when it is migrated
+  to language version 3.0.
+  - The `Function` type can no longer be implemented.
+    Since Dart 2.0 writing `implements Function` has been allowed
+    for backwards compatibility, but it has not had any effect.
+    In Dart 3.0, the `Function` type is `final` and cannot be implemented
+    by class-modifier aware code.
+
 [#49529]: https://github.com/dart-lang/sdk/issues/49529
 [`List.filled`]: https://api.dart.dev/stable/2.18.6/dart-core/List/List.filled.html
 [`int.parse`]: https://api.dart.dev/stable/2.18.4/dart-core/int/parse.html
@@ -149,14 +170,14 @@ Updates the Linter to `1.34.0-dev`, which includes changes that
   - `prefer_equal_for_default_values`
   - `super_goes_last`
 - fix `unnecessary_parenthesis` false-positives with null-aware expressions.
-- fix `void_checks` to allow assignments of `Future<dynamic>?` to parameters 
+- fix `void_checks` to allow assignments of `Future<dynamic>?` to parameters
   typed `FutureOr<void>?`.
 - fix `use_build_context_synchronously` in if conditions.
 - fix a false positive for `avoid_private_typedef_functions` with generalized
   type aliases.
 - update `unnecessary_parenthesis` to detect some doubled parens.
 - update `void_checks` to allow returning `Never` as void.
-- update `no_adjacent_strings_in_list` to support set literals and for- and 
+- update `no_adjacent_strings_in_list` to support set literals and for- and
   if-elements.
 - update `avoid_types_as_parameter_names` to handle type variables.
 - update `avoid_positional_boolean_parameters` to handle typedefs.

pkg/analysis_server/test/verify_sorted_test.dart

@@ -71,7 +71,7 @@ void buildTestsForAnalysisServer() {
     'lib/src/edit/nnbd_migration/resources/resources.g.dart',
     'test/integration/support/integration_test_methods.dart',
     'test/integration/support/protocol_matchers.dart',
-    // The following are not generated, but can't be sorted because the contain
+    // The following are not generated, but can't be sorted because they contain
     // ignore comments in the directives, which sorting deletes.
     'lib/src/edit/edit_domain.dart',
     'lib/src/services/kythe/schema.dart',

pkg/analyzer/lib/src/dart/analysis/experiments.g.dart

@@ -140,7 +140,7 @@ class ExperimentalFeatures {
     isEnabledByDefault: IsEnabledByDefault.class_modifiers,
     isExpired: IsExpired.class_modifiers,
     documentation: 'Class modifiers',
-    experimentalReleaseVersion: null,
+    experimentalReleaseVersion: Version.parse('3.0.0'),
     releaseVersion: null,
   );
 
@@ -314,7 +314,7 @@ class ExperimentalFeatures {
     isEnabledByDefault: IsEnabledByDefault.records,
     isExpired: IsExpired.records,
     documentation: 'Records',
-    experimentalReleaseVersion: Version.parse('2.19.0'),
+    experimentalReleaseVersion: Version.parse('3.0.0'),
     releaseVersion: null,
   );
 
@@ -324,7 +324,7 @@ class ExperimentalFeatures {
     isEnabledByDefault: IsEnabledByDefault.sealed_class,
     isExpired: IsExpired.sealed_class,
     documentation: 'Sealed class',
-    experimentalReleaseVersion: null,
+    experimentalReleaseVersion: Version.parse('3.0.0'),
     releaseVersion: null,
   );
 

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -1746,7 +1746,10 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
         final interfaceElement = interfaceType.element;
         if (interfaceElement is ClassOrMixinElementImpl &&
             interfaceElement.isBase &&
-            interfaceElement.library != _currentLibrary) {
+            interfaceElement.library != _currentLibrary &&
+            !_mayIgnoreClassModifiers(interfaceElement.library)) {
+          // Should this be combined with _checkForImplementsClauseErrorCodes
+          // to avoid double errors if implementing `int`.
           if (interfaceElement is ClassElement) {
             errorReporter.reportErrorForNode(
                 CompileTimeErrorCode.BASE_CLASS_IMPLEMENTED_OUTSIDE_OF_LIBRARY,
@@ -2862,7 +2865,8 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
         final element = type.element;
         if (element is ClassElementImpl &&
             element.isFinal &&
-            element.library != _currentLibrary) {
+            element.library != _currentLibrary &&
+            !_mayIgnoreClassModifiers(element.library)) {
           errorReporter.reportErrorForNode(
               CompileTimeErrorCode.FINAL_CLASS_EXTENDED_OUTSIDE_OF_LIBRARY,
               superclass,
@@ -2877,7 +2881,8 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
           final element = type.element;
           if (element is MixinElementImpl &&
               element.isFinal &&
-              element.library != _currentLibrary) {
+              element.library != _currentLibrary &&
+              !_mayIgnoreClassModifiers(element.library)) {
             errorReporter.reportErrorForNode(
                 CompileTimeErrorCode.FINAL_MIXIN_MIXED_IN_OUTSIDE_OF_LIBRARY,
                 namedType,
@@ -2893,7 +2898,8 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
           final element = type.element;
           if (element is ClassOrMixinElementImpl &&
               element.isFinal &&
-              element.library != _currentLibrary) {
+              element.library != _currentLibrary &&
+              !_mayIgnoreClassModifiers(element.library)) {
             final ErrorCode errorCode;
             if (element is ClassElement) {
               errorCode = CompileTimeErrorCode
@@ -3113,7 +3119,8 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
         final superclassElement = superclassType.element;
         if (superclassElement is ClassElementImpl &&
             superclassElement.isInterface &&
-            superclassElement.library != _currentLibrary) {
+            superclassElement.library != _currentLibrary &&
+            !_mayIgnoreClassModifiers(superclassElement.library)) {
           errorReporter.reportErrorForNode(
               CompileTimeErrorCode.INTERFACE_CLASS_EXTENDED_OUTSIDE_OF_LIBRARY,
               superclass,
@@ -3128,7 +3135,8 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
           final withElement = withType.element;
           if (withElement is MixinElementImpl &&
               withElement.isInterface &&
-              withElement.library != _currentLibrary) {
+              withElement.library != _currentLibrary &&
+              !_mayIgnoreClassModifiers(withElement.library)) {
             errorReporter.reportErrorForNode(
                 CompileTimeErrorCode
                     .INTERFACE_MIXIN_MIXED_IN_OUTSIDE_OF_LIBRARY,
@@ -5485,6 +5493,26 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
     return false;
   }
 
+  /// Checks whether a `final`, `base` or `interface` modifier can be ignored.
+  ///
+  /// Checks whether a subclass in the current library
+  /// can ignore a class modifier of a declaration in [superLibrary].
+  ///
+  /// Only true if the supertype library is a platform library, and
+  /// either the current library is also a platform library,
+  /// or the current library has a language version which predates
+  /// class modifiers
+  bool _mayIgnoreClassModifiers(LibraryElement superLibrary) {
+    // Only modifiers in platform libraries can be ignored.
+    if (!superLibrary.isInSdk) return false;
+
+    // Other platform libraries can ignore modifiers.
+    if (_currentLibrary.isInSdk) return true;
+
+    // Libraries predating class modifiers can ignore platform modifiers.
+    return !_currentLibrary.featureSet.isEnabled(Feature.class_modifiers);
+  }
+
   /// Return the name of the [parameter], or `null` if the parameter does not
   /// have a name.
   Token? _parameterName(FormalParameter parameter) {

pkg/analyzer/lib/src/generated/source.dart

@@ -66,7 +66,7 @@ class DartUriResolver extends UriResolver {
   ///
   /// @param uri the URI being tested
   /// @return `true` if the given URI is a `dart:` URI
-  static bool isDartUri(Uri uri) => DART_SCHEME == uri.scheme;
+  static bool isDartUri(Uri uri) => uri.isScheme(DART_SCHEME);
 }
 
 /// An implementation of an non-existing [Source].

pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart

@@ -771,6 +771,8 @@ final Map<ExperimentalFlag, bool> defaultExperimentalFlags = {
 const AllowedExperimentalFlags defaultAllowedExperimentalFlags =
     const AllowedExperimentalFlags(sdkDefaultExperiments: {
   ExperimentalFlag.records,
+  ExperimentalFlag.classModifiers,
+  ExperimentalFlag.sealedClass,
 }, sdkLibraryExperiments: {}, packageExperiments: {
   "async": {
     ExperimentalFlag.nonNullable,

pkg/front_end/lib/src/fasta/source/source_loader.dart

@@ -2159,6 +2159,56 @@ severity: $severity
         typeBuilder.libraryBuilder.library.languageVersion >=
         ExperimentalFlag.sealedClass.experimentEnabledVersion;
 
+    /// Set when we know whether this library can ignore class modifiers.
+    ///
+    /// The same decision applies to all declarations in the library,
+    /// so the value only needs to be computed once.
+    bool? isExempt;
+
+    /// Whether the [cls] declaration can ignore (some) class modifiers.
+    ///
+    /// Checks whether the [cls] can ignore modifiers
+    /// from the [supertypeDeclaration].
+    /// This is only possible if the supertype declaration comes
+    /// from a platform library (`dart:` URI scheme),
+    /// and then only if the library is another platform library which is
+    /// exempt from restrictions on extending otherwise sealed platform types,
+    /// or if the library is a pre-class-modifiers-feature language version
+    /// library.
+    bool mayIgnoreClassModifiers(ClassBuilder supertypeDeclaration) {
+      // Only use this to ignore `final`, `base`, and `interface`.
+      // Nobody can ignore `abstract`, `sealed` or `mixin`.
+
+      // We already know the library cannot ignore modifiers.
+      if (isExempt == false) return false;
+
+      // Exception only applies to platform libraries.
+      final LibraryBuilder superLibrary = supertypeDeclaration.libraryBuilder;
+      if (!superLibrary.importUri.isScheme("dart")) return false;
+
+      // Remaining tests depend on the source library only,
+      // and the result can be cached.
+      if (isExempt == true) return true;
+
+      final LibraryBuilder subLibrary = cls.libraryBuilder;
+
+      // Some platform libraries may implement types like `int`,
+      // even if they are final.
+      if (subLibrary.mayImplementRestrictedTypes) {
+        isExempt = true;
+        return true;
+      }
+      // "Legacy" libraries may ignore `final`, `base` and `interface`
+      // from platform libraries. (But still cannot implement `int`.)
+      if (subLibrary.library.languageVersion <
+          ExperimentalFlag.classModifiers.experimentEnabledVersion) {
+        isExempt = true;
+        return true;
+      }
+      isExempt = false;
+      return false;
+    }
+
     TypeDeclarationBuilder? unaliasDeclaration(TypeBuilder typeBuilder) {
       TypeDeclarationBuilder? typeDeclarationBuilder = typeBuilder.declaration;
       if (typeDeclarationBuilder is TypeAliasBuilder) {
@@ -2181,7 +2231,9 @@ severity: $severity
           cls.libraryBuilder.origin !=
               supertypeDeclaration.libraryBuilder.origin) {
         if (isClassModifiersEnabled(supertypeDeclaration)) {
-          if (supertypeDeclaration.isInterface && !cls.isMixinDeclaration) {
+          if (supertypeDeclaration.isInterface &&
+              !cls.isMixinDeclaration &&
+              !mayIgnoreClassModifiers(supertypeDeclaration)) {
             cls.addProblem(
                 templateInterfaceClassExtendedOutsideOfLibrary
                     .withArguments(supertypeDeclaration.fullNameForErrors),
@@ -2190,7 +2242,8 @@ severity: $severity
           } else if (supertypeDeclaration.isFinal &&
               // TODO(kallentu): Error case where the class has a singular on
               // clause. Change with the new spec changes.
-              !cls.isMixinDeclaration) {
+              !cls.isMixinDeclaration &&
+              !mayIgnoreClassModifiers(supertypeDeclaration)) {
             cls.addProblem(
                 templateFinalClassExtendedOutsideOfLibrary
                     .withArguments(supertypeDeclaration.fullNameForErrors),
@@ -2235,13 +2288,15 @@ severity: $severity
 
           if (cls.libraryBuilder.origin !=
               mixedInTypeDeclaration.libraryBuilder.origin) {
-            if (mixedInTypeDeclaration.isInterface) {
+            if (mixedInTypeDeclaration.isInterface &&
+                !mayIgnoreClassModifiers(mixedInTypeDeclaration)) {
               cls.addProblem(
                   templateInterfaceMixinMixedInOutsideOfLibrary
                       .withArguments(mixedInTypeDeclaration.fullNameForErrors),
                   mixedInTypeBuilder.charOffset ?? TreeNode.noOffset,
                   noLength);
-            } else if (mixedInTypeDeclaration.isFinal) {
+            } else if (mixedInTypeDeclaration.isFinal &&
+                !mayIgnoreClassModifiers(mixedInTypeDeclaration)) {
               cls.addProblem(
                   templateFinalMixinMixedInOutsideOfLibrary
                       .withArguments(mixedInTypeDeclaration.fullNameForErrors),
@@ -2276,7 +2331,8 @@ severity: $severity
           if (isClassModifiersEnabled(interfaceDeclaration)) {
             // Report an error for a class implementing a base class outside of
             // its library.
-            if (interfaceDeclaration.isBase) {
+            if (interfaceDeclaration.isBase &&
+                !mayIgnoreClassModifiers(interfaceDeclaration)) {
               if (interfaceDeclaration.isMixinDeclaration) {
                 cls.addProblem(
                     templateBaseMixinImplementedOutsideOfLibrary
@@ -2293,7 +2349,8 @@ severity: $severity
             } else if (interfaceDeclaration.isFinal &&
                 // TODO(kallentu): Error case where the class has multiple on
                 // clauses. Change with the new spec changes.
-                !cls.cls.isAnonymousMixin) {
+                !cls.cls.isAnonymousMixin &&
+                !mayIgnoreClassModifiers(interfaceDeclaration)) {
               if (interfaceDeclaration.isMixinDeclaration) {
                 cls.addProblem(
                     templateFinalMixinImplementedOutsideOfLibrary

pkg/front_end/test/spell_checking_list_common.txt

@@ -2601,6 +2601,7 @@ restored
 restores
 restrict
 restricted
+restrictions
 restrictive
 result
 resulted

pkg/front_end/testcases/records/type_record_as_supertype.dart.strong.expect

@@ -38,9 +38,9 @@ library /*isNonNullableByDefault*/;
 // pkg/front_end/testcases/records/type_record_as_supertype.dart:22:16: Error: 'RR' is restricted and can't be extended or implemented.
 // abstract class C2 with RR {} // Error.
 //                ^
-// sdk/lib/core/record.dart:11:16: Context: This is the type denoted by the type alias.
-// abstract class Record {}
-//                ^
+// sdk/lib/core/record.dart:11:22: Context: This is the type denoted by the type alias.
+// abstract final class Record {}
+//                      ^
 //
 import self as self;
 import "dart:core" as core;

pkg/front_end/testcases/records/type_record_as_supertype.dart.strong.transformed.expect

@@ -38,9 +38,9 @@ library /*isNonNullableByDefault*/;
 // pkg/front_end/testcases/records/type_record_as_supertype.dart:22:16: Error: 'RR' is restricted and can't be extended or implemented.
 // abstract class C2 with RR {} // Error.
 //                ^
-// sdk/lib/core/record.dart:11:16: Context: This is the type denoted by the type alias.
-// abstract class Record {}
-//                ^
+// sdk/lib/core/record.dart:11:22: Context: This is the type denoted by the type alias.
+// abstract final class Record {}
+//                      ^
 //
 import self as self;
 import "dart:core" as core;

pkg/front_end/testcases/records/type_record_as_supertype.dart.weak.expect

@@ -38,9 +38,9 @@ library /*isNonNullableByDefault*/;
 // pkg/front_end/testcases/records/type_record_as_supertype.dart:22:16: Error: 'RR' is restricted and can't be extended or implemented.
 // abstract class C2 with RR {} // Error.
 //                ^
-// sdk/lib/core/record.dart:11:16: Context: This is the type denoted by the type alias.
-// abstract class Record {}
-//                ^
+// sdk/lib/core/record.dart:11:22: Context: This is the type denoted by the type alias.
+// abstract final class Record {}
+//                      ^
 //
 import self as self;
 import "dart:core" as core;

pkg/front_end/testcases/records/type_record_as_supertype.dart.weak.modular.expect

@@ -38,9 +38,9 @@ library /*isNonNullableByDefault*/;
 // pkg/front_end/testcases/records/type_record_as_supertype.dart:22:16: Error: 'RR' is restricted and can't be extended or implemented.
 // abstract class C2 with RR {} // Error.
 //                ^
-// sdk/lib/core/record.dart:11:16: Context: This is the type denoted by the type alias.
-// abstract class Record {}
-//                ^
+// sdk/lib/core/record.dart:11:22: Context: This is the type denoted by the type alias.
+// abstract final class Record {}
+//                      ^
 //
 import self as self;
 import "dart:core" as core;

pkg/front_end/testcases/records/type_record_as_supertype.dart.weak.outline.expect

@@ -38,9 +38,9 @@ library /*isNonNullableByDefault*/;
 // pkg/front_end/testcases/records/type_record_as_supertype.dart:22:16: Error: 'RR' is restricted and can't be extended or implemented.
 // abstract class C2 with RR {} // Error.
 //                ^
-// sdk/lib/core/record.dart:11:16: Context: This is the type denoted by the type alias.
-// abstract class Record {}
-//                ^
+// sdk/lib/core/record.dart:11:22: Context: This is the type denoted by the type alias.
+// abstract final class Record {}
+//                      ^
 //
 import self as self;
 import "dart:core" as core;

pkg/front_end/testcases/records/type_record_as_supertype.dart.weak.transformed.expect

@@ -38,9 +38,9 @@ library /*isNonNullableByDefault*/;
 // pkg/front_end/testcases/records/type_record_as_supertype.dart:22:16: Error: 'RR' is restricted and can't be extended or implemented.
 // abstract class C2 with RR {} // Error.
 //                ^
-// sdk/lib/core/record.dart:11:16: Context: This is the type denoted by the type alias.
-// abstract class Record {}
-//                ^
+// sdk/lib/core/record.dart:11:22: Context: This is the type denoted by the type alias.
+// abstract final class Record {}
+//                      ^
 //
 import self as self;
 import "dart:core" as core;

runtime/vm/json_test.cc

@@ -182,7 +182,7 @@ ISOLATE_UNIT_TEST_CASE(JSON_JSONStream_DartObject) {
       "Class\",\"fixedId\":true,\"id\":\"\",\"name\":\"Null\",\"location\":{"
       "\"type\":\"SourceLocation\",\"script\":{\"type\":\"@Script\","
       "\"fixedId\":true,\"id\":\"\",\"uri\":\"dart:core\\/null.dart\",\"_"
-      "kind\":\"kernel\"},\"tokenPos\":925,\"endTokenPos\":1165,\"line\":23,"
+      "kind\":\"kernel\"},\"tokenPos\":925,\"endTokenPos\":1171,\"line\":23,"
       "\"column\":1},\"library\":{"
       "\"type\":\"@Library\",\"fixedId\":true,\"id\":\"\",\"name\":\"dart."
       "core\",\"uri\":\"dart:core\"}},\"kind\":\"Null\",\"fixedId\":true,"
@@ -191,7 +191,7 @@ ISOLATE_UNIT_TEST_CASE(JSON_JSONStream_DartObject) {
       "\"fixedId\":true,\"id\":\"\",\"name\":\"Null\",\"location\":{\"type\":"
       "\"SourceLocation\",\"script\":{\"type\":\"@Script\",\"fixedId\":true,"
       "\"id\":\"\",\"uri\":\"dart:core\\/null.dart\",\"_kind\":\"kernel\"},"
-      "\"tokenPos\":925,\"endTokenPos\":1165,\"line\":23,\"column\":1},"
+      "\"tokenPos\":925,\"endTokenPos\":1171,\"line\":23,\"column\":1},"
       "\"library\":{\"type\":\"@"
       "Library\",\"fixedId\":true,\"id\":\"\",\"name\":\"dart.core\",\"uri\":"
       "\"dart:core\"}},\"kind\":\"Null\",\"fixedId\":true,\"id\":\"\","

sdk/lib/_http/http_impl.dart

@@ -289,7 +289,7 @@ class _HttpProfileData {
 int _nextServiceId = 1;
 
 // TODO(ajohnsen): Use other way of getting a unique id.
-abstract class _ServiceObject {
+mixin _ServiceObject {
   int __serviceId = 0;
   int get _serviceId {
     if (__serviceId == 0) __serviceId = _nextServiceId++;

sdk/lib/_internal/allowed_experiments.json

@@ -2,7 +2,9 @@
   "version": 1,
   "experimentSets": {
     "sdkExperiments": [
-      "records"
+      "records",
+      "class-modifiers",
+      "sealed-class"
     ],
     "nullSafety": [
       "non-nullable"

sdk/lib/_internal/js_dev_runtime/patch/typed_data_patch.dart

@@ -333,8 +333,7 @@ class _UnmodifiableByteDataView implements ByteData, UnmodifiableByteDataView {
   }
 }
 
-abstract class _UnmodifiableListMixin<N, L extends List<N>,
-    TD extends TypedData> {
+mixin _UnmodifiableListMixin<N, L extends List<N>, TD extends TypedData> {
   L get _list;
   TD get _data => (_list as TD);
 

sdk/lib/_internal/js_runtime/lib/typed_data_patch.dart

@@ -333,8 +333,7 @@ class _UnmodifiableByteDataView implements ByteData, UnmodifiableByteDataView {
   }
 }
 
-abstract class _UnmodifiableListMixin<N, L extends List<N>,
-    TD extends TypedData> {
+mixin _UnmodifiableListMixin<N, L extends List<N>, TD extends TypedData> {
   L get _list;
   TD get _data => (_list as TD);
 

sdk/lib/_internal/vm/lib/typed_data_patch.dart

@@ -829,7 +829,7 @@ mixin _TypedDoubleListMixin<SpawnedType extends List<double>>
   }
 }
 
-abstract class _Float32x4ListMixin implements List<Float32x4> {
+mixin _Float32x4ListMixin implements List<Float32x4> {
   int get elementSizeInBytes;
   int get offsetInBytes;
   _ByteBuffer get buffer;
@@ -1187,7 +1187,7 @@ abstract class _Float32x4ListMixin implements List<Float32x4> {
   }
 }
 
-abstract class _Int32x4ListMixin implements List<Int32x4> {
+mixin _Int32x4ListMixin implements List<Int32x4> {
   int get elementSizeInBytes;
   int get offsetInBytes;
   _ByteBuffer get buffer;
@@ -1544,7 +1544,7 @@ abstract class _Int32x4ListMixin implements List<Int32x4> {
   }
 }
 
-abstract class _Float64x2ListMixin implements List<Float64x2> {
+mixin _Float64x2ListMixin implements List<Float64x2> {
   int get elementSizeInBytes;
   int get offsetInBytes;
   _ByteBuffer get buffer;

sdk/lib/async/future.dart

@@ -36,10 +36,10 @@ part of dart.async;
 /// `Future<Object>`.
 @pragma("vm:entry-point")
 abstract class FutureOr<T> {
-  // Private generative constructor, so that it is not subclassable, mixable, or
-  // instantiable.
+  // Private generative constructor, so that it is not subclassable, mixable,
+  // or instantiable.
   FutureOr._() {
-    throw new UnsupportedError("FutureOr can't be instantiated");
+    throw new UnsupportedError("FutureOr cannot be instantiated");
   }
 }
 

sdk/lib/async/stream_controller.dart

@@ -768,7 +768,7 @@ abstract class _StreamController<T> implements _StreamControllerBase<T> {
   }
 }
 
-abstract class _SyncStreamControllerDispatch<T>
+mixin _SyncStreamControllerDispatch<T>
     implements _StreamController<T>, SynchronousStreamController<T> {
   void _sendData(T data) {
     _subscription._add(data);
@@ -783,8 +783,7 @@ abstract class _SyncStreamControllerDispatch<T>
   }
 }
 
-abstract class _AsyncStreamControllerDispatch<T>
-    implements _StreamController<T> {
+mixin _AsyncStreamControllerDispatch<T> implements _StreamController<T> {
   void _sendData(T data) {
     _subscription._addPending(_DelayedData<T>(data));
   }

sdk/lib/collection/iterable.dart

@@ -7,7 +7,7 @@ part of dart.collection;
 /// This [Iterable] mixin implements all [Iterable] members except `iterator`.
 ///
 /// All other methods are implemented in terms of `iterator`.
-abstract class IterableMixin<E> implements Iterable<E> {
+abstract mixin class IterableMixin<E> implements Iterable<E> {
   // This class has methods copied verbatim into:
   // - IterableBase
   // - SetMixin

sdk/lib/collection/linked_list.dart

@@ -270,7 +270,7 @@ class _LinkedListIterator<E extends LinkedListEntry<E>> implements Iterator<E> {
 /// linked list, and otherwise the `list` property is `null`.
 ///
 /// When created, an entry is not in any linked list.
-abstract class LinkedListEntry<E extends LinkedListEntry<E>> {
+abstract base mixin class LinkedListEntry<E extends LinkedListEntry<E>> {
   LinkedList<E>? _list;
   E? _next;
   E? _previous;

sdk/lib/collection/list.dart

@@ -69,7 +69,7 @@ abstract class ListBase<E> extends Object with ListMixin<E> {
 /// To avoid this, override 'add' and 'addAll' to also forward directly
 /// to the growable list, or, if possible, use `DelegatingList` from
 /// "package:collection/collection.dart" instead of a `ListMixin`.
-abstract class ListMixin<E> implements List<E> {
+abstract mixin class ListMixin<E> implements List<E> {
   // Iterable interface.
   // TODO(lrn): When we get composable mixins, reuse IterableMixin instead
   // of redeclaring everything.

sdk/lib/collection/maps.dart

@@ -108,7 +108,7 @@ abstract class MapBase<K, V> extends MapMixin<K, V> {
 ///
 /// A more efficient implementation is usually possible by overriding
 /// some of the other members as well.
-abstract class MapMixin<K, V> implements Map<K, V> {
+abstract mixin class MapMixin<K, V> implements Map<K, V> {
   Iterable<K> get keys;
   V? operator [](Object? key);
   operator []=(K key, V value);
@@ -263,7 +263,7 @@ class _MapBaseValueIterator<K, V> implements Iterator<V> {
 
 /// Mixin that overrides mutating map operations with implementations that
 /// throw.
-abstract class _UnmodifiableMapMixin<K, V> implements Map<K, V> {
+mixin _UnmodifiableMapMixin<K, V> implements Map<K, V> {
   /// This operation is not supported by an unmodifiable map.
   void operator []=(K key, V value) {
     throw UnsupportedError("Cannot modify unmodifiable map");

sdk/lib/collection/set.dart

@@ -20,7 +20,7 @@ part of dart.collection;
 /// Implementations of `Set` using this mixin should consider also implementing
 /// `clear` in constant time. The default implementation works by removing every
 /// element.
-abstract class SetMixin<E> implements Set<E> {
+abstract mixin class SetMixin<E> implements Set<E> {
   // This class reimplements all of [IterableMixin].
   // If/when Dart mixins get more powerful, we should just create a single
   // Mixin class from IterableMixin and the new methods of this class.
@@ -341,7 +341,7 @@ abstract class _SetBase<E> with SetMixin<E> {
   Set<E> toSet() => _newSet()..addAll(this);
 }
 
-abstract class _UnmodifiableSetMixin<E> implements Set<E> {
+mixin _UnmodifiableSetMixin<E> implements Set<E> {
   static Never _throwUnmodifiable() {
     throw UnsupportedError("Cannot change an unmodifiable set");
   }

sdk/lib/convert/json.dart

@@ -882,7 +882,7 @@ abstract class _JsonStringifier {
 /// [Map] objects using the specified indent value.
 ///
 /// Subclasses should implement [writeIndentation].
-abstract class _JsonPrettyPrintMixin implements _JsonStringifier {
+mixin _JsonPrettyPrintMixin implements _JsonStringifier {
   int _indentLevel = 0;
 
   /// Add [indentLevel] indentations to the JSON output.

sdk/lib/convert/string_conversion.dart

@@ -155,7 +155,7 @@ abstract class StringConversionSinkBase extends StringConversionSinkMixin {}
 
 /// This class provides a mixin for converters that need to accept String
 /// inputs.
-abstract class StringConversionSinkMixin implements StringConversionSink {
+abstract mixin class StringConversionSinkMixin implements StringConversionSink {
   void addSlice(String str, int start, int end, bool isLast);
   void close();
 

sdk/lib/core/bool.dart

@@ -10,7 +10,7 @@ part of dart.core;
 /// It is a compile-time error for a class to attempt to extend or implement
 /// bool.
 @pragma("vm:entry-point")
-class bool {
+final class bool {
   /// Returns the boolean value of the environment declaration [name].
   ///
   /// The boolean value of the declaration is `true` if the declared value is

sdk/lib/core/double.dart

@@ -21,7 +21,7 @@ part of dart.core;
 /// * [num] the super class for [double].
 /// * [Numbers](https://dart.dev/guides/language/numbers) in
 /// [A tour of the Dart language](https://dart.dev/guides/language/language-tour).
-abstract class double extends num {
+abstract final class double extends num {
   static const double nan = 0.0 / 0.0;
   static const double infinity = 1.0 / 0.0;
   static const double negativeInfinity = -infinity;

sdk/lib/core/enum.dart

@@ -11,7 +11,7 @@ part of dart.core;
 /// Non-platform classes cannot implement, extend or
 /// mix in this class.
 @Since("2.14")
-abstract class Enum {
+abstract final class Enum {
   /// A numeric identifier for the enumerated value.
   ///
   /// The values of a single enumeration are numbered

sdk/lib/core/function.dart

@@ -101,7 +101,7 @@ part of dart.core;
 /// Function? maybeFun = Random().nextBool() ? fun : null;
 /// print(maybeFun?.call(1)); // Prints "1" or "null".
 /// ```
-abstract class Function {
+abstract final class Function {
   /// Dynamically call [function] with the specified arguments.
   ///
   /// Acts the same as dynamically calling [function] with

sdk/lib/core/int.dart

@@ -25,7 +25,7 @@ part of dart.core;
 /// * [num] the super class for [int].
 /// * [Numbers](https://dart.dev/guides/language/numbers) in
 /// [A tour of the Dart language](https://dart.dev/guides/language/language-tour).
-abstract class int extends num {
+abstract final class int extends num {
   /// Returns the integer value of the given environment declaration [name].
   ///
   /// The result is the same as would be returned by:

sdk/lib/core/null.dart

@@ -21,7 +21,7 @@ part of dart.core;
 /// [...? e6] // spreads e6 into the list literal, unless e6 is null.
 /// ```
 @pragma("vm:entry-point")
-class Null {
+final class Null {
   factory Null._uninstantiable() {
     throw UnsupportedError('class Null cannot be instantiated');
   }

sdk/lib/core/num.dart

@@ -15,7 +15,7 @@ part of dart.core;
 /// * [Numbers](https://dart.dev/guides/language/numbers) in
 /// [A tour of the Dart language](https://dart.dev/guides/language/language-tour).
 
-abstract class num implements Comparable<num> {
+sealed class num implements Comparable<num> {
   /// Test whether this value is numerically equal to `other`.
   ///
   /// If both operands are [double]s, they are equal if they have the same

sdk/lib/core/record.dart

@@ -8,4 +8,4 @@ part of dart.core;
 ///
 /// The run-time type of a record object is a record type, and as such, a
 /// subtype of [Record].
-abstract class Record {}
+abstract final class Record {}

sdk/lib/core/string.dart

@@ -105,7 +105,7 @@ part of dart.core;
 /// * [RegExp] to work with regular expressions.
 /// * [Strings and regular expressions](https://dart.dev/guides/libraries/library-tour#strings-and-regular-expressions)
 @pragma('vm:entry-point')
-abstract class String implements Comparable<String>, Pattern {
+abstract final class String implements Comparable<String>, Pattern {
   /// Allocates a new string containing the specified [charCodes].
   ///
   /// The [charCodes] can be both UTF-16 code units and runes.

sdk/lib/html/dart2js/html_dart2js.dart

@@ -5913,7 +5913,7 @@ class _CssStyleDeclarationSet extends Object with CssStyleDeclarationBase {
   // items in the MEMBERS set if you want that functionality.
 }
 
-abstract class CssStyleDeclarationBase {
+abstract mixin class CssStyleDeclarationBase {
   String getPropertyValue(String propertyName);
   void setProperty(String propertyName, String? value, [String? priority]);
 
@@ -37899,7 +37899,7 @@ class _Html5NodeValidator implements NodeValidator {
 // 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.
 
-abstract class ImmutableListMixin<E> implements List<E> {
+abstract mixin class ImmutableListMixin<E> implements List<E> {
   // From Iterable<$E>:
   Iterator<E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't

sdk/lib/internal/list.dart

@@ -9,7 +9,7 @@ part of dart._internal;
  *
  * Intended to mix-in on top of [ListMixin] for fixed-length lists.
  */
-abstract class FixedLengthListMixin<E> {
+mixin FixedLengthListMixin<E> {
   /** This operation is not supported by a fixed length list. */
   set length(int newLength) {
     throw new UnsupportedError(
@@ -84,7 +84,7 @@ abstract class FixedLengthListMixin<E> {
  * This mixin is intended to be mixed in on top of [ListMixin] on
  * unmodifiable lists.
  */
-abstract class UnmodifiableListMixin<E> implements List<E> {
+mixin UnmodifiableListMixin<E> implements List<E> {
   /** This operation is not supported by an unmodifiable list. */
   void operator []=(int index, E value) {
     throw new UnsupportedError("Cannot modify an unmodifiable list");

sdk/lib/io/service_object.dart

@@ -7,7 +7,7 @@ part of dart.io;
 int _nextServiceId = 1;
 
 // TODO(ajohnsen): Use other way of getting a unique id.
-abstract class _ServiceObject {
+mixin _ServiceObject {
   int __serviceId = 0;
   int get _serviceId {
     if (__serviceId == 0) __serviceId = _nextServiceId++;

sdk/lib/typed_data/typed_data.dart

@@ -27,7 +27,7 @@ part "unmodifiable_typed_data.dart";
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// ByteBuffer.
-abstract class ByteBuffer {
+abstract final class ByteBuffer {
   /// Returns the length of this byte buffer, in bytes.
   int get lengthInBytes;
 
@@ -346,7 +346,7 @@ abstract class ByteBuffer {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// TypedData.
-abstract class TypedData {
+abstract final class TypedData {
   /// Returns the number of bytes in the representation of each element in this
   /// list.
   int get elementSizeInBytes;
@@ -361,7 +361,7 @@ abstract class TypedData {
   ByteBuffer get buffer;
 }
 
-abstract class _TypedIntList extends TypedData {
+abstract final class _TypedIntList extends TypedData {
   /// Returns the concatenation of this list and [other].
   ///
   /// If other is also a typed-data integer list, the returned list will
@@ -371,7 +371,7 @@ abstract class _TypedIntList extends TypedData {
   List<int> operator +(List<int> other);
 }
 
-abstract class _TypedFloatList extends TypedData {
+abstract final class _TypedFloatList extends TypedData {
   /// Returns the concatenation of this list and [other].
   ///
   /// If other is also a typed-data floating point number list,
@@ -386,7 +386,7 @@ abstract class _TypedFloatList extends TypedData {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Endian.
-class Endian {
+final class Endian {
   final bool _littleEndian;
   const Endian._(this._littleEndian);
 
@@ -433,7 +433,7 @@ class Endian {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// ByteData.
-abstract class ByteData implements TypedData {
+abstract final class ByteData implements TypedData {
   /// Creates a [ByteData] of the specified length (in elements), all of
   /// whose bytes are initially zero.
   @pragma("vm:entry-point")
@@ -719,7 +719,7 @@ abstract class ByteData implements TypedData {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Int8List.
-abstract class Int8List implements List<int>, _TypedIntList {
+abstract final class Int8List implements List<int>, _TypedIntList {
   /// Creates an [Int8List] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -832,7 +832,7 @@ abstract class Int8List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Uint8List.
-abstract class Uint8List implements List<int>, _TypedIntList {
+abstract final class Uint8List implements List<int>, _TypedIntList {
   /// Creates a [Uint8List] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -952,7 +952,7 @@ abstract class Uint8List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Uint8ClampedList.
-abstract class Uint8ClampedList implements List<int>, _TypedIntList {
+abstract final class Uint8ClampedList implements List<int>, _TypedIntList {
   /// Creates a [Uint8ClampedList] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -1068,7 +1068,7 @@ abstract class Uint8ClampedList implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Int16List.
-abstract class Int16List implements List<int>, _TypedIntList {
+abstract final class Int16List implements List<int>, _TypedIntList {
   /// Creates an [Int16List] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -1193,7 +1193,7 @@ abstract class Int16List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Uint16List.
-abstract class Uint16List implements List<int>, _TypedIntList {
+abstract final class Uint16List implements List<int>, _TypedIntList {
   /// Creates a [Uint16List] of the specified length (in elements), all
   /// of whose elements are initially zero.
   ///
@@ -1319,7 +1319,7 @@ abstract class Uint16List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Int32List.
-abstract class Int32List implements List<int>, _TypedIntList {
+abstract final class Int32List implements List<int>, _TypedIntList {
   /// Creates an [Int32List] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -1444,7 +1444,7 @@ abstract class Int32List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Uint32List.
-abstract class Uint32List implements List<int>, _TypedIntList {
+abstract final class Uint32List implements List<int>, _TypedIntList {
   /// Creates a [Uint32List] of the specified length (in elements), all
   /// of whose elements are initially zero.
   ///
@@ -1570,7 +1570,7 @@ abstract class Uint32List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Int64List.
-abstract class Int64List implements List<int>, _TypedIntList {
+abstract final class Int64List implements List<int>, _TypedIntList {
   /// Creates an [Int64List] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -1695,7 +1695,7 @@ abstract class Int64List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Uint64List.
-abstract class Uint64List implements List<int>, _TypedIntList {
+abstract final class Uint64List implements List<int>, _TypedIntList {
   /// Creates a [Uint64List] of the specified length (in elements), all
   /// of whose elements are initially zero.
   ///
@@ -1822,7 +1822,7 @@ abstract class Uint64List implements List<int>, _TypedIntList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Float32List.
-abstract class Float32List implements List<double>, _TypedFloatList {
+abstract final class Float32List implements List<double>, _TypedFloatList {
   /// Creates a [Float32List] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -1944,7 +1944,7 @@ abstract class Float32List implements List<double>, _TypedFloatList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Float64List.
-abstract class Float64List implements List<double>, _TypedFloatList {
+abstract final class Float64List implements List<double>, _TypedFloatList {
   /// Creates a [Float64List] of the specified length (in elements), all of
   /// whose elements are initially zero.
   ///
@@ -2062,7 +2062,7 @@ abstract class Float64List implements List<double>, _TypedFloatList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Float32x4List.
-abstract class Float32x4List implements List<Float32x4>, TypedData {
+abstract final class Float32x4List implements List<Float32x4>, TypedData {
   /// Creates a [Float32x4List] of the specified length (in elements),
   /// all of whose elements are initially zero.
   ///
@@ -2183,7 +2183,7 @@ abstract class Float32x4List implements List<Float32x4>, TypedData {
 ///
 /// For long lists, this implementation will be considerably more
 /// space- and time-efficient than the default [List] implementation.
-abstract class Int32x4List implements List<Int32x4>, TypedData {
+abstract final class Int32x4List implements List<Int32x4>, TypedData {
   /// Creates a [Int32x4List] of the specified length (in elements),
   /// all of whose elements are initially zero.
   ///
@@ -2307,7 +2307,7 @@ abstract class Int32x4List implements List<Int32x4>, TypedData {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Float64x2List.
-abstract class Float64x2List implements List<Float64x2>, TypedData {
+abstract final class Float64x2List implements List<Float64x2>, TypedData {
   /// Creates a [Float64x2List] of the specified length (in elements),
   /// all of whose elements have all lanes set to zero.
   ///
@@ -2430,7 +2430,7 @@ abstract class Float64x2List implements List<Float64x2>, TypedData {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Float32x4.
-abstract class Float32x4 {
+abstract final class Float32x4 {
   external factory Float32x4(double x, double y, double z, double w);
   external factory Float32x4.splat(double v);
   external factory Float32x4.zero();
@@ -2800,7 +2800,7 @@ abstract class Float32x4 {
 ///
 /// Int32x4 stores 4 32-bit bit-masks in "lanes".
 /// The lanes are "x", "y", "z", and "w" respectively.
-abstract class Int32x4 {
+abstract final class Int32x4 {
   external factory Int32x4(int x, int y, int z, int w);
   external factory Int32x4.bool(bool x, bool y, bool z, bool w);
   external factory Int32x4.fromFloat32x4Bits(Float32x4 x);
@@ -3154,7 +3154,7 @@ abstract class Int32x4 {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// Float64x2.
-abstract class Float64x2 {
+abstract final class Float64x2 {
   external factory Float64x2(double x, double y);
   external factory Float64x2.splat(double v);
   external factory Float64x2.zero();

sdk/lib/typed_data/unmodifiable_typed_data.dart

@@ -8,7 +8,7 @@ part of dart.typed_data;
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableByteBufferView.
-abstract class UnmodifiableByteBufferView implements ByteBuffer {
+abstract final class UnmodifiableByteBufferView implements ByteBuffer {
   external factory UnmodifiableByteBufferView(ByteBuffer data);
 }
 
@@ -16,7 +16,7 @@ abstract class UnmodifiableByteBufferView implements ByteBuffer {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableByteDataView.
-abstract class UnmodifiableByteDataView implements ByteData {
+abstract final class UnmodifiableByteDataView implements ByteData {
   external factory UnmodifiableByteDataView(ByteData data);
 }
 
@@ -24,7 +24,7 @@ abstract class UnmodifiableByteDataView implements ByteData {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableUint8ListView.
-abstract class UnmodifiableUint8ListView implements Uint8List {
+abstract final class UnmodifiableUint8ListView implements Uint8List {
   external factory UnmodifiableUint8ListView(Uint8List list);
 }
 
@@ -32,7 +32,7 @@ abstract class UnmodifiableUint8ListView implements Uint8List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableInt8ListView.
-abstract class UnmodifiableInt8ListView implements Int8List {
+abstract final class UnmodifiableInt8ListView implements Int8List {
   external factory UnmodifiableInt8ListView(Int8List list);
 }
 
@@ -40,7 +40,7 @@ abstract class UnmodifiableInt8ListView implements Int8List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableUint8ClampedListView.
-abstract class UnmodifiableUint8ClampedListView implements Uint8ClampedList {
+abstract final class UnmodifiableUint8ClampedListView implements Uint8ClampedList {
   external factory UnmodifiableUint8ClampedListView(Uint8ClampedList list);
 }
 
@@ -48,7 +48,7 @@ abstract class UnmodifiableUint8ClampedListView implements Uint8ClampedList {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableUint16ListView.
-abstract class UnmodifiableUint16ListView implements Uint16List {
+abstract final class UnmodifiableUint16ListView implements Uint16List {
   external factory UnmodifiableUint16ListView(Uint16List list);
 }
 
@@ -56,7 +56,7 @@ abstract class UnmodifiableUint16ListView implements Uint16List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableInt16ListView.
-abstract class UnmodifiableInt16ListView implements Int16List {
+abstract final class UnmodifiableInt16ListView implements Int16List {
   external factory UnmodifiableInt16ListView(Int16List list);
 }
 
@@ -64,7 +64,7 @@ abstract class UnmodifiableInt16ListView implements Int16List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableUint32ListView.
-abstract class UnmodifiableUint32ListView implements Uint32List {
+abstract final class UnmodifiableUint32ListView implements Uint32List {
   external factory UnmodifiableUint32ListView(Uint32List list);
 }
 
@@ -72,7 +72,7 @@ abstract class UnmodifiableUint32ListView implements Uint32List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableInt32ListView.
-abstract class UnmodifiableInt32ListView implements Int32List {
+abstract final class UnmodifiableInt32ListView implements Int32List {
   external factory UnmodifiableInt32ListView(Int32List list);
 }
 
@@ -80,7 +80,7 @@ abstract class UnmodifiableInt32ListView implements Int32List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableUint64ListView.
-abstract class UnmodifiableUint64ListView implements Uint64List {
+abstract final class UnmodifiableUint64ListView implements Uint64List {
   external factory UnmodifiableUint64ListView(Uint64List list);
 }
 
@@ -88,7 +88,7 @@ abstract class UnmodifiableUint64ListView implements Uint64List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableInt64ListView.
-abstract class UnmodifiableInt64ListView implements Int64List {
+abstract final class UnmodifiableInt64ListView implements Int64List {
   external factory UnmodifiableInt64ListView(Int64List list);
 }
 
@@ -96,7 +96,7 @@ abstract class UnmodifiableInt64ListView implements Int64List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableInt32x4ListView.
-abstract class UnmodifiableInt32x4ListView implements Int32x4List {
+abstract final class UnmodifiableInt32x4ListView implements Int32x4List {
   external factory UnmodifiableInt32x4ListView(Int32x4List list);
 }
 
@@ -104,7 +104,7 @@ abstract class UnmodifiableInt32x4ListView implements Int32x4List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableFloat32x4ListView.
-abstract class UnmodifiableFloat32x4ListView implements Float32x4List {
+abstract final class UnmodifiableFloat32x4ListView implements Float32x4List {
   external factory UnmodifiableFloat32x4ListView(Float32x4List list);
 }
 
@@ -112,7 +112,7 @@ abstract class UnmodifiableFloat32x4ListView implements Float32x4List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableFloat64x2ListView.
-abstract class UnmodifiableFloat64x2ListView implements Float64x2List {
+abstract final class UnmodifiableFloat64x2ListView implements Float64x2List {
   external factory UnmodifiableFloat64x2ListView(Float64x2List list);
 }
 
@@ -120,7 +120,7 @@ abstract class UnmodifiableFloat64x2ListView implements Float64x2List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableFloat32ListView.
-abstract class UnmodifiableFloat32ListView implements Float32List {
+abstract final class UnmodifiableFloat32ListView implements Float32List {
   external factory UnmodifiableFloat32ListView(Float32List list);
 }
 
@@ -128,6 +128,6 @@ abstract class UnmodifiableFloat32ListView implements Float32List {
 ///
 /// It is a compile-time error for a class to attempt to extend or implement
 /// UnmodifiableFloat64ListView.
-abstract class UnmodifiableFloat64ListView implements Float64List {
+abstract final class UnmodifiableFloat64ListView implements Float64List {
   external factory UnmodifiableFloat64ListView(Float64List list);
 }

tests/language/nonfunction_type_aliases/generic_usage_function_error_test.dart

@@ -2,7 +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.
 
-
 import 'dart:async';
 
 // Introduce an aliased type.
@@ -14,12 +13,32 @@ typedef T<X> = Function;
 abstract class C {
   final T<Null> v7;
 
-  C(): v7 = T();
+  C() : v7 = T();
   //        ^
   // [analyzer] unspecified
   // [cfe] unspecified
 }
 
+class D1<X> extends T<X> {}
+//                  ^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+abstract class D2 extends C with T<int> {}
+//                               ^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+abstract class D3<X, Y> implements T<T> {}
+//                                 ^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+abstract class D4 = C with T<void>;
+//                         ^^^^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
 X foo<X>(X x) => x;
 
 main() {

tests/language/nonfunction_type_aliases/generic_usage_function_test.dart

@@ -2,7 +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.
 
-
 import 'dart:async';
 
 // Introduce an aliased type.
@@ -27,7 +26,7 @@ abstract class C {
   List<T<T>> v6 = [];
   final T<Null> v7;
 
-  C(): v7 = print;
+  C() : v7 = print;
   C.name1(this.v5, this.v7);
   factory C.name2(T<C> arg1, T<Null> arg2) = C1.name1;
 
@@ -43,11 +42,6 @@ class C1 implements C {
   noSuchMethod(Invocation invocation) => throw 0;
 }
 
-class D1<X> extends T<X> {}
-abstract class D2 extends C with T<int> {}
-abstract class D3<X, Y> implements T<T> {}
-abstract class D4 = C with T<void>;
-
 extension E on T<dynamic> {
   T<dynamic> foo(T<dynamic> t) => t;
 }

tests/language/nonfunction_type_aliases/mixed/generic_usage_function_error_test.dart

@@ -22,6 +22,17 @@ abstract class C {
   // [cfe] unspecified
 }
 
+abstract class D2 extends C with T {}
+//                               ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+abstract class D4 = C with T;
+//                         ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+
 X foo<X>(X x) => x;
 
 main() {

tests/language/nonfunction_type_aliases/mixed/generic_usage_function_test.dart

@@ -4,7 +4,6 @@
 // @dart = 2.9
 // Requirements=nnbd-weak
 
-
 // Test that a generic type alias `T` denoting `Function`
 // can give rise to the expected erros.
 
@@ -47,12 +46,8 @@ class C1 implements C {
 
 class D1<X> extends T<X> {}
 
-abstract class D2 extends C with T<int> {}
-
 abstract class D3<X, Y> implements T<T> {}
 
-abstract class D4 = C with T<void>;
-
 extension E on T<dynamic> {
   T<dynamic> foo(T<dynamic> t) => t;
 }

tests/language/nonfunction_type_aliases/mixed/usage_function_test.dart

@@ -4,7 +4,6 @@
 // @dart = 2.9
 // Requirements=nnbd-weak
 
-
 // Test that a type alias `T` denoting `Function` can be used.
 
 import 'usage_function_lib.dart';
@@ -45,12 +44,8 @@ class C1 implements C {
 
 class D1 extends T {}
 
-abstract class D2 extends C with T {}
-
 abstract class D3 implements T {}
 
-abstract class D4 = C with T;
-
 extension E on T {
   T foo(T t) => t;
 }

tests/language/nonfunction_type_aliases/usage_function_error_test.dart

@@ -2,7 +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.
 
-
 // Introduce an aliased type.
 
 typedef T = Function;
@@ -12,12 +11,32 @@ typedef T = Function;
 abstract class C {
   final T v12;
 
-  C(): v12 = T();
+  C() : v12 = T();
   //         ^
   // [analyzer] unspecified
   // [cfe] unspecified
 }
 
+class D1<X> extends T {}
+//                  ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+abstract class D2 extends C with T {}
+//                               ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+abstract class D3<X, Y> implements T {}
+//                                 ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+abstract class D4 = C with T;
+//                         ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
 X foo<X>(X x) => x;
 
 main() {

tests/language/nonfunction_type_aliases/usage_function_test.dart

@@ -2,7 +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.
 
-
 // Introduce an aliased type.
 
 typedef T = Function;
@@ -25,7 +24,7 @@ abstract class C {
   List<T> v11 = [];
   final T v12;
 
-  C(): v12 = (() {});
+  C() : v12 = (() {});
   C.name1(this.v10, this.v12);
   factory C.name2(T arg1, T arg2) = C1.name1;
 
@@ -41,14 +40,6 @@ class C1 implements C {
   noSuchMethod(Invocation invocation) => throw 0;
 }
 
-// Awaiting updates in front end to handle crash caused by null from
-// `ClassHierarchyBuilder.getKernelTypeAsInstanceOf`. So for now the
-// following are multi-test cases, so that the rest can be tested.
-class D1 extends T {} //# 01: ok
-abstract class D2 extends C with T {} //# 02: ok
-abstract class D3 implements T {} //# 03: ok
-abstract class D4 = C with T; //# 04: ok
-
 extension E on T {
   T foo(T t) => t;
 }

tools/dom/scripts/css_code_generator.py

@@ -277,7 +277,7 @@ class _CssStyleDeclarationSet extends Object with CssStyleDeclarationBase {
   // items in the MEMBERS set if you want that functionality.
 }
 
-abstract class CssStyleDeclarationBase {
+abstract mixin class CssStyleDeclarationBase {
   String getPropertyValue(String propertyName);
   void setProperty(String propertyName, String$NULLABLE value,
       [String$NULLABLE priority]);

tools/dom/src/ImmutableListMixin.dart

@@ -4,7 +4,7 @@
 
 part of dart.dom.html;
 
-abstract class ImmutableListMixin<E> implements List<E> {
+abstract mixin class ImmutableListMixin<E> implements List<E> {
   // From Iterable<$E>:
   Iterator<E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't

tools/dom/templates/html/impl/impl_CSSStyleDeclaration.darttemplate

@@ -1855,7 +1855,7 @@ class _CssStyleDeclarationSet extends Object with CssStyleDeclarationBase {
   // items in the MEMBERS set if you want that functionality.
 }
 
-abstract class CssStyleDeclarationBase {
+abstract mixin class CssStyleDeclarationBase {
   String getPropertyValue(String propertyName);
   void setProperty(String propertyName, String$NULLABLE value,
       [String$NULLABLE priority]);

tools/experimental_features.yaml

@@ -142,9 +142,11 @@ features:
 
   sealed-class:
     help: "Sealed class"
+    experimentalReleaseVersion: "3.0.0"
 
   class-modifiers:
     help: "Class modifiers"
+    experimentalReleaseVersion: "3.0.0"
 
 # Experiment flag only used for testing.
   test-experiment: