diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee06d0472d9..45c048b6b39 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -61,36 +61,13 @@
     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 that code is migrated
+  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, extended or mixed in.
+  - 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 subtyped,
-    preventing code from mistakenly assuming it works.
-  - The following declarations can only be implemented, not extended:
-    * `Comparable`
-    * `Exception`
-    * `Iterator`
-    * `Pattern`
-    * `Match`
-    * `RegExp`
-    * `RegExpMatch`
-    * `StackTrace`
-    * `StringSink`
-    None of these declarations contained any implementation to inherit,
-    and are marked as `interface` to signify that they are only intended
-    as interfaces.
-  - The following declarations can no longer be implemented or extended:
-    * `MapEntry`
-    * `OutOfMemoryError`
-    * `StackOverflowError`
-    * `Expando`
-    * `WeakReference`
-    * `Finalizer`
-    The `MapEntry` value class is restricted to enable later optimizations.
-    The remaining classes are tightly coupled to the platform and not
-    intended to be subclassed or implemented.
+    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
diff --git a/pkg/_fe_analyzer_shared/lib/src/util/link_implementation.dart b/pkg/_fe_analyzer_shared/lib/src/util/link_implementation.dart
index e5921816785..ecf1044f45f 100644
--- a/pkg/_fe_analyzer_shared/lib/src/util/link_implementation.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/util/link_implementation.dart
@@ -31,7 +31,7 @@ class LinkIterator<T> implements Iterator<T> {
 
 typedef T Transformation<S, T>(S input);
 
-class MappedLinkIterator<S, T> implements Iterator<T> {
+class MappedLinkIterator<S, T> extends Iterator<T> {
   Transformation<S, T> _transformation;
   Link<S> _link;
   T? _current;
diff --git a/pkg/compiler/lib/src/universe/class_set.dart b/pkg/compiler/lib/src/universe/class_set.dart
index ea98bdec49f..acb9ae9139e 100644
--- a/pkg/compiler/lib/src/universe/class_set.dart
+++ b/pkg/compiler/lib/src/universe/class_set.dart
@@ -967,7 +967,7 @@ class SubtypesIterable extends IterableBase<ClassEntity> {
 }
 
 /// Iterator for the subtypes in a [ClassSet].
-class SubtypesIterator implements Iterator<ClassEntity> {
+class SubtypesIterator extends Iterator<ClassEntity> {
   final SubtypesIterable iterable;
   Iterator<ClassEntity>? elements;
   Iterator<ClassHierarchyNode>? hierarchyNodes;
diff --git a/pkg/compiler/test/inference/powerset_bits3_test.dart b/pkg/compiler/test/inference/powerset_bits3_test.dart
index f0bfd2a47c5..655de6a8347 100644
--- a/pkg/compiler/test/inference/powerset_bits3_test.dart
+++ b/pkg/compiler/test/inference/powerset_bits3_test.dart
@@ -13,19 +13,19 @@ import '../helpers/element_lookup.dart';
 import 'package:compiler/src/util/memory_compiler.dart';
 
 const String CODE = """
-class A implements Comparable {
+class A extends Comparable {
   int compareTo(x) { return 0; }
 }
-class B implements Comparable {
+class B extends Comparable {
   int compareTo(x) { return 0; }
 }
-class C implements Comparable {
+class C extends Comparable {
   int compareTo(x) { return 0; }
 }
-class D implements Comparable {
+class D extends Comparable {
   int compareTo(x) { return 0; }
 }
-class E implements Comparable {
+class E extends Comparable {
   int compareTo(x) { return 0; }
 }
 
diff --git a/pkg/front_end/testcases/incremental/issue_32366.yaml b/pkg/front_end/testcases/incremental/issue_32366.yaml
index 4108b175fb0..3fc46c3feb0 100644
--- a/pkg/front_end/testcases/incremental/issue_32366.yaml
+++ b/pkg/front_end/testcases/incremental/issue_32366.yaml
@@ -8,7 +8,7 @@ worlds:
   - entry: main.dart
     sources:
       main.dart: |
-        abstract class AIterator implements Iterator {
+        abstract class AIterator extends Iterator {
         }
         class Foo {
           final a;
@@ -25,7 +25,7 @@ worlds:
     errors: true
     sources:
       main.dart: |
-        abstract class BIterator implements Iterator {
+        abstract class BIterator extends Iterator {
         }
         class Foo {
           final a kjsdf ksjdf ;
diff --git a/pkg/front_end/testcases/incremental/issue_32366.yaml.world.1.expect b/pkg/front_end/testcases/incremental/issue_32366.yaml.world.1.expect
index 3ac341d4355..bc95a7c5fcf 100644
--- a/pkg/front_end/testcases/incremental/issue_32366.yaml.world.1.expect
+++ b/pkg/front_end/testcases/incremental/issue_32366.yaml.world.1.expect
@@ -1,9 +1,9 @@
 main = main::main;
 library from "org-dartlang-test:///main.dart" as main {
 
-  abstract class AIterator extends dart.core::Object implements dart.core::Iterator<dynamic> {
+  abstract class AIterator extends dart.core::Iterator<dynamic> {
     synthetic constructor •() → main::AIterator
-      : super dart.core::Object::•()
+      : super dart.core::Iterator::•()
       ;
   }
   class Foo extends dart.core::Object {
diff --git a/pkg/front_end/testcases/incremental/issue_32366.yaml.world.2.expect b/pkg/front_end/testcases/incremental/issue_32366.yaml.world.2.expect
index b9a726ac08a..84dceba1628 100644
--- a/pkg/front_end/testcases/incremental/issue_32366.yaml.world.2.expect
+++ b/pkg/front_end/testcases/incremental/issue_32366.yaml.world.2.expect
@@ -30,9 +30,9 @@ library from "org-dartlang-test:///main.dart" as main {
 //            ^
 //
 
-  abstract class BIterator extends dart.core::Object implements dart.core::Iterator<dynamic> {
+  abstract class BIterator extends dart.core::Iterator<dynamic> {
     synthetic constructor •() → main::BIterator
-      : super dart.core::Object::•()
+      : super dart.core::Iterator::•()
       ;
   }
   class Foo extends dart.core::Object {
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart b/pkg/front_end/testcases/inference/try_catch_promotion.dart
index abdc8f392c4..17f1e21d1ce 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart
@@ -11,7 +11,7 @@ class C {}
 
 class D extends C {}
 
-class E implements StackTrace {}
+class E extends StackTrace {}
 
 void test(void f()) {
   try {
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.expect
index cba89691040..c5c1336ed8c 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.expect
@@ -29,11 +29,12 @@ class D extends self::C {
     : super self::C::•()
     ;
 }
-class E extends core::Object implements core::StackTrace {
+class E extends core::StackTrace {
   synthetic constructor •() → self::E*
-    : super core::Object::•()
+    : super core::StackTrace::•()
     ;
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature method toString() → core::String*; -> core::StackTrace::toString
   abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
   abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
   abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
@@ -41,7 +42,6 @@ class E extends core::Object implements core::StackTrace {
   abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
   abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
   abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
-  abstract member-signature method toString() → core::String*; -> core::Object::toString
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
 static method test(() →* void f) → void {
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.transformed.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.transformed.expect
index cba89691040..c5c1336ed8c 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.strong.transformed.expect
@@ -29,11 +29,12 @@ class D extends self::C {
     : super self::C::•()
     ;
 }
-class E extends core::Object implements core::StackTrace {
+class E extends core::StackTrace {
   synthetic constructor •() → self::E*
-    : super core::Object::•()
+    : super core::StackTrace::•()
     ;
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature method toString() → core::String*; -> core::StackTrace::toString
   abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
   abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
   abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
@@ -41,7 +42,6 @@ class E extends core::Object implements core::StackTrace {
   abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
   abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
   abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
-  abstract member-signature method toString() → core::String*; -> core::Object::toString
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
 static method test(() →* void f) → void {
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline.expect
index df1115e37a9..654304607db 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline.expect
@@ -5,7 +5,7 @@ class C {}
 
 class D extends C {}
 
-class E implements StackTrace {}
+class E extends StackTrace {}
 
 void test(void f()) {}
 main() {}
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline_modelled.expect
index 2db370d3b1e..ab3f6f6f4de 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline_modelled.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.textual_outline_modelled.expect
@@ -5,7 +5,7 @@ class C {}
 
 class D extends C {}
 
-class E implements StackTrace {}
+class E extends StackTrace {}
 
 main() {}
 void test(void f()) {}
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.expect
index f035cfe145e..bf9fab61431 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.expect
@@ -22,11 +22,12 @@ class D extends self::C {
     : super self::C::•()
     ;
 }
-class E extends core::Object implements core::StackTrace {
+class E extends core::StackTrace {
   synthetic constructor •() → self::E*
-    : super core::Object::•()
+    : super core::StackTrace::•()
     ;
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature method toString() → core::String*; -> core::StackTrace::toString
   abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
   abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
   abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
@@ -34,7 +35,6 @@ class E extends core::Object implements core::StackTrace {
   abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
   abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
   abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
-  abstract member-signature method toString() → core::String*; -> core::Object::toString
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
 static method test(() →* void f) → void {
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.modular.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.modular.expect
index f035cfe145e..bf9fab61431 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.modular.expect
@@ -22,11 +22,12 @@ class D extends self::C {
     : super self::C::•()
     ;
 }
-class E extends core::Object implements core::StackTrace {
+class E extends core::StackTrace {
   synthetic constructor •() → self::E*
-    : super core::Object::•()
+    : super core::StackTrace::•()
     ;
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature method toString() → core::String*; -> core::StackTrace::toString
   abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
   abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
   abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
@@ -34,7 +35,6 @@ class E extends core::Object implements core::StackTrace {
   abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
   abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
   abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
-  abstract member-signature method toString() → core::String*; -> core::Object::toString
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
 static method test(() →* void f) → void {
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.outline.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.outline.expect
index fb937f03a7c..0bb2c870c34 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.outline.expect
@@ -20,10 +20,11 @@ class D extends self::C {
   synthetic constructor •() → self::D*
     ;
 }
-class E extends core::Object implements core::StackTrace {
+class E extends core::StackTrace {
   synthetic constructor •() → self::E*
     ;
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature method toString() → core::String*; -> core::StackTrace::toString
   abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
   abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
   abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
@@ -31,7 +32,6 @@ class E extends core::Object implements core::StackTrace {
   abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
   abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
   abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
-  abstract member-signature method toString() → core::String*; -> core::Object::toString
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
 static method test(() →* void f) → void
diff --git a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.transformed.expect b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.transformed.expect
index f035cfe145e..bf9fab61431 100644
--- a/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/inference/try_catch_promotion.dart.weak.transformed.expect
@@ -22,11 +22,12 @@ class D extends self::C {
     : super self::C::•()
     ;
 }
-class E extends core::Object implements core::StackTrace {
+class E extends core::StackTrace {
   synthetic constructor •() → self::E*
-    : super core::Object::•()
+    : super core::StackTrace::•()
     ;
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature method toString() → core::String*; -> core::StackTrace::toString
   abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
   abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
   abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
@@ -34,7 +35,6 @@ class E extends core::Object implements core::StackTrace {
   abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
   abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
   abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
-  abstract member-signature method toString() → core::String*; -> core::Object::toString
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
 static method test(() →* void f) → void {
diff --git a/runtime/tests/concurrency/stress_test_list.json b/runtime/tests/concurrency/stress_test_list.json
index 670923a026e..2d8369d7ae4 100644
--- a/runtime/tests/concurrency/stress_test_list.json
+++ b/runtime/tests/concurrency/stress_test_list.json
@@ -2176,6 +2176,7 @@
     "../../../tests/language/regress/r24720_test.dart",
     "../../../tests/language/regress/regress10204_test.dart",
     "../../../tests/language/regress/regress10321_test.dart",
+    "../../../tests/language/regress/regress10561_test.dart",
     "../../../tests/language/regress/regress10581_test.dart",
     "../../../tests/language/regress/regress10721_test.dart",
     "../../../tests/language/regress/regress10747_test.dart",
@@ -5499,6 +5500,7 @@
     "../../../tests/language_2/regress/r24720_test.dart",
     "../../../tests/language_2/regress/regress10204_test.dart",
     "../../../tests/language_2/regress/regress10321_test.dart",
+    "../../../tests/language_2/regress/regress10561_test.dart",
     "../../../tests/language_2/regress/regress10581_test.dart",
     "../../../tests/language_2/regress/regress10721_test.dart",
     "../../../tests/language_2/regress/regress10747_test.dart",
diff --git a/runtime/tests/vm/dart/regress_48323_1_test.dart b/runtime/tests/vm/dart/regress_48323_1_test.dart
index 65d3c201112..d9c7595ac84 100644
--- a/runtime/tests/vm/dart/regress_48323_1_test.dart
+++ b/runtime/tests/vm/dart/regress_48323_1_test.dart
@@ -6,7 +6,7 @@
 // Verifies that class finalization doesn't crash when seeing
 // superclass with type arguments which were not finalized yet.
 
-abstract class GraphNode implements Comparable<dynamic> {
+abstract class GraphNode extends Comparable<dynamic> {
   int compareTo(dynamic other) => 0;
 }
 
diff --git a/samples/ffi/sqlite/lib/src/collections/closable_iterator.dart b/samples/ffi/sqlite/lib/src/collections/closable_iterator.dart
index 49abbd2131a..a86a58b2ad5 100644
--- a/samples/ffi/sqlite/lib/src/collections/closable_iterator.dart
+++ b/samples/ffi/sqlite/lib/src/collections/closable_iterator.dart
@@ -7,7 +7,7 @@
 /// [ClosableIterator]s often use resources which should be freed after use.
 /// The consumer of the iterator can either manually [close] the iterator, or
 /// consume all elements on which the iterator will automatically be closed.
-abstract class ClosableIterator<T> implements Iterator<T> {
+abstract class ClosableIterator<T> extends Iterator<T> {
   /// Close this iterator.
   void close();
 
diff --git a/sdk/lib/_internal/wasm/lib/js_helper.dart b/sdk/lib/_internal/wasm/lib/js_helper.dart
index 6111d646650..295904cc7e5 100644
--- a/sdk/lib/_internal/wasm/lib/js_helper.dart
+++ b/sdk/lib/_internal/wasm/lib/js_helper.dart
@@ -100,7 +100,7 @@ extension JSObjectExtension on JSObject {
   external void operator []=(String key, Object? value);
 }
 
-class JSArrayIteratorAdapter<T> implements Iterator<T> {
+class JSArrayIteratorAdapter<T> extends Iterator<T> {
   final JSArray array;
   int index = -1;
 
diff --git a/sdk/lib/collection/splay_tree.dart b/sdk/lib/collection/splay_tree.dart
index 55b6402f492..1ab21dbc4d3 100644
--- a/sdk/lib/collection/splay_tree.dart
+++ b/sdk/lib/collection/splay_tree.dart
@@ -26,8 +26,8 @@ class _SplayTreeSetNode<K> extends _SplayTreeNode<K, _SplayTreeSetNode<K>> {
 ///
 /// A [_SplayTreeNode] that also contains a value,
 /// and which implements [MapEntry].
-class _SplayTreeMapNode<K, V>
-    extends _SplayTreeNode<K, _SplayTreeMapNode<K, V>> {
+class _SplayTreeMapNode<K, V> extends _SplayTreeNode<K, _SplayTreeMapNode<K, V>>
+    implements MapEntry<K, V> {
   final V value;
   _SplayTreeMapNode(K key, this.value) : super(key);
 
@@ -35,6 +35,8 @@ class _SplayTreeMapNode<K, V>
       _SplayTreeMapNode<K, V>(key, value)
         .._left = _left
         .._right = _right;
+
+  String toString() => "MapEntry($key: $value)";
 }
 
 /// A splay tree is a self-balancing binary search tree.
@@ -834,8 +836,7 @@ class _SplayTreeValueIterator<K, V>
 class _SplayTreeMapEntryIterator<K, V>
     extends _SplayTreeIterator<K, _SplayTreeMapNode<K, V>, MapEntry<K, V>> {
   _SplayTreeMapEntryIterator(SplayTreeMap<K, V> tree) : super(tree);
-  MapEntry<K, V> _getValue(_SplayTreeMapNode<K, V> node) =>
-      MapEntry<K, V>(node.key, node.value);
+  MapEntry<K, V> _getValue(_SplayTreeMapNode<K, V> node) => node;
 
   // Replaces the value of the current node.
   void _replaceValue(V value) {
diff --git a/sdk/lib/convert/string_conversion.dart b/sdk/lib/convert/string_conversion.dart
index b0dec898db6..5a310937f20 100644
--- a/sdk/lib/convert/string_conversion.dart
+++ b/sdk/lib/convert/string_conversion.dart
@@ -53,7 +53,7 @@ abstract class StringConversionSink extends ChunkedConversionSink<String> {
 
 /// A [ClosableStringSink] extends the [StringSink] interface by adding a
 /// `close` method.
-abstract class ClosableStringSink implements StringSink {
+abstract class ClosableStringSink extends StringSink {
   /// Creates a new instance combining a [StringSink] [sink] and a callback
   /// [onClose] which is invoked when the returned instance is closed.
   factory ClosableStringSink.fromStringSink(StringSink sink, void onClose()) =
diff --git a/sdk/lib/core/annotations.dart b/sdk/lib/core/annotations.dart
index e437d90bd15..8183375e0f3 100644
--- a/sdk/lib/core/annotations.dart
+++ b/sdk/lib/core/annotations.dart
@@ -147,7 +147,7 @@ const Object override = _Override();
 /// function `foo` is annotated with a pragma 'other-pragma'
 /// specific to OtherTool.
 @pragma('vm:entry-point')
-final class pragma {
+class pragma {
   /// The name of the hint.
   ///
   /// A string that is recognized by one or more tools, or such a string prefixed
diff --git a/sdk/lib/core/bigint.dart b/sdk/lib/core/bigint.dart
index 28da09dbff4..e5a871c750d 100644
--- a/sdk/lib/core/bigint.dart
+++ b/sdk/lib/core/bigint.dart
@@ -41,7 +41,7 @@ part of dart.core;
 /// * [num]: The super class for [int] and [double].
 /// * [Numbers](https://dart.dev/guides/language/numbers) in
 /// [A tour of the Dart language](https://dart.dev/guides/language/language-tour).
-abstract final class BigInt implements Comparable<BigInt> {
+abstract class BigInt implements Comparable<BigInt> {
   /// A big integer with the numerical value 0.
   external static BigInt get zero;
 
diff --git a/sdk/lib/core/comparable.dart b/sdk/lib/core/comparable.dart
index 3d0bd51be8f..67df844a0c9 100644
--- a/sdk/lib/core/comparable.dart
+++ b/sdk/lib/core/comparable.dart
@@ -66,7 +66,7 @@ typedef Comparator<T> = int Function(T a, T b);
 /// The [DateTime] class has no comparison operators, instead it has the more
 /// precisely named [DateTime.isBefore] and [DateTime.isAfter], which both
 /// agree with [DateTime.compareTo].
-abstract interface class Comparable<T> {
+abstract class Comparable<T> {
   /// Compares this object to another object.
   ///
   /// Returns a value like a [Comparator] when comparing `this` to [other].
diff --git a/sdk/lib/core/errors.dart b/sdk/lib/core/errors.dart
index 368891cae77..d54dcd0400c 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -596,7 +596,7 @@ class ConcurrentModificationError extends Error {
 }
 
 /// Error that the platform can use in case of memory shortage.
-final class OutOfMemoryError implements Error {
+class OutOfMemoryError implements Error {
   @pragma("vm:entry-point")
   const OutOfMemoryError();
   String toString() => "Out of Memory";
@@ -605,7 +605,7 @@ final class OutOfMemoryError implements Error {
 }
 
 /// Error that the platform can use in case of stack overflow.
-final class StackOverflowError implements Error {
+class StackOverflowError implements Error {
   @pragma("vm:entry-point")
   const StackOverflowError();
   String toString() => "Stack Overflow";
diff --git a/sdk/lib/core/exceptions.dart b/sdk/lib/core/exceptions.dart
index c7a064d8ba1..c4c441bc1d8 100644
--- a/sdk/lib/core/exceptions.dart
+++ b/sdk/lib/core/exceptions.dart
@@ -20,7 +20,7 @@ part of dart.core;
 /// For failures that are not intended to be caught, use [Error]
 /// and its subclasses.
 @pragma('flutter:keep-to-string-in-subtypes')
-abstract interface class Exception {
+abstract class Exception {
   factory Exception([var message]) => _Exception(message);
 }
 
diff --git a/sdk/lib/core/iterator.dart b/sdk/lib/core/iterator.dart
index 915e6220bb4..a33e2733113 100644
--- a/sdk/lib/core/iterator.dart
+++ b/sdk/lib/core/iterator.dart
@@ -31,7 +31,7 @@ part of dart.core;
 /// **See also:**
 /// [Iteration](https://dart.dev/guides/libraries/library-tour#iteration)
 /// in the [library tour](https://dart.dev/guides/libraries/library-tour)
-abstract interface class Iterator<E> {
+abstract class Iterator<E> {
   /// Advances the iterator to the next element of the iteration.
   ///
   /// Should be called before reading [current].
diff --git a/sdk/lib/core/map.dart b/sdk/lib/core/map.dart
index aa8646719af..ea609e03905 100644
--- a/sdk/lib/core/map.dart
+++ b/sdk/lib/core/map.dart
@@ -468,7 +468,9 @@ abstract class Map<K, V> {
 /// the `MapEntry` class will be changed to such a type,
 /// and will likely no longer be able to be implemented or extended
 /// by classes.
-final class MapEntry<K, V> {
+// TODO(lrn): Make this class `final` when class modifiers are introduced.
+// Change to an `inline class` when those are available.
+class MapEntry<K, V> {
   /// The key of the entry.
   ///
   /// ```dart
diff --git a/sdk/lib/core/pattern.dart b/sdk/lib/core/pattern.dart
index 9f6e34debdc..fbf70ea1b4c 100644
--- a/sdk/lib/core/pattern.dart
+++ b/sdk/lib/core/pattern.dart
@@ -5,7 +5,7 @@
 part of dart.core;
 
 /// An interface for basic searches within strings.
-abstract interface class Pattern {
+abstract class Pattern {
   /// Matches this pattern against the string repeatedly.
   ///
   /// If [start] is provided, matching will start at that index.
@@ -82,7 +82,7 @@ abstract interface class Pattern {
 /// that were part of the matching. These are called _groups_ in the `Match`
 /// object. Some patterns may never have any groups, and their matches always
 /// have zero [groupCount].
-abstract interface class Match {
+abstract class Match {
   /// The index in the string where the match starts.
   int get start;
 
diff --git a/sdk/lib/core/regexp.dart b/sdk/lib/core/regexp.dart
index 921af06189c..9ca82395f8e 100644
--- a/sdk/lib/core/regexp.dart
+++ b/sdk/lib/core/regexp.dart
@@ -213,7 +213,7 @@ part of dart.core;
 /// when the regexp does not find a match.
 /// Several guides to [improving the performance of regular expressions](https://www.google.com/search?q=performance+of+regular+expressions)
 /// exist on the internet. Use these as inspirations, too.
-abstract interface class RegExp implements Pattern {
+abstract class RegExp implements Pattern {
   /// Constructs a regular expression.
   ///
   /// Throws a [FormatException] if [source] does not follow valid regular
@@ -469,7 +469,7 @@ abstract interface class RegExp implements Pattern {
 /// }
 /// ```
 @Since("2.3")
-abstract interface class RegExpMatch implements Match {
+abstract class RegExpMatch implements Match {
   /// The string captured by the named capture group [name].
   ///
   /// Returns the substring of the input that the
diff --git a/sdk/lib/core/stacktrace.dart b/sdk/lib/core/stacktrace.dart
index 24c8ea23abe..dbba199885d 100644
--- a/sdk/lib/core/stacktrace.dart
+++ b/sdk/lib/core/stacktrace.dart
@@ -11,7 +11,7 @@ part of dart.core;
 ///
 /// These objects are created by the runtime, it is not possible to create
 /// them programmatically.
-abstract interface class StackTrace {
+abstract class StackTrace {
   /// A stack trace object with no information.
   ///
   /// This stack trace is used as the default in situations where
diff --git a/sdk/lib/core/string_sink.dart b/sdk/lib/core/string_sink.dart
index 35013a47002..565ce563255 100644
--- a/sdk/lib/core/string_sink.dart
+++ b/sdk/lib/core/string_sink.dart
@@ -4,7 +4,7 @@
 
 part of dart.core;
 
-abstract interface class StringSink {
+abstract class StringSink {
   /// Writes the string representation of [object].
   ///
   /// Converts [object] to a string using `object.toString()`.
diff --git a/sdk/lib/core/weak.dart b/sdk/lib/core/weak.dart
index 0c74cd92820..0ae32c7013d 100644
--- a/sdk/lib/core/weak.dart
+++ b/sdk/lib/core/weak.dart
@@ -30,7 +30,7 @@ part of dart.core;
 /// There is no restriction on other classes, even for compile time constant
 /// objects. Be careful if adding expando properties to compile time constants,
 /// since they will stay alive forever.
-final class Expando<T extends Object> {
+class Expando<T extends Object> {
   /// The name of the this [Expando] as passed to the constructor.
   ///
   /// If no name was passed to the constructor, the value is the `null` value.
@@ -123,7 +123,7 @@ final class Expando<T extends Object> {
 /// }
 /// ```
 @Since("2.17")
-abstract final class WeakReference<T extends Object> {
+abstract class WeakReference<T extends Object> {
   /// Creates a [WeakReference] pointing to the given [target].
   ///
   /// The [target] must be an object supported as an [Expando] key,
@@ -220,7 +220,7 @@ abstract final class WeakReference<T extends Object> {
 /// function rather than a Dart function, use `dart:ffi`'s [NativeFinalizer]
 /// instead.
 @Since("2.17")
-abstract final class Finalizer<T> {
+abstract class Finalizer<T> {
   /// Creates a finalizer with the given finalization callback.
   ///
   /// The [callback] is bound to the current zone
diff --git a/sdk/lib/internal/iterable.dart b/sdk/lib/internal/iterable.dart
index 15f9f6cfa62..4dbd20a9039 100644
--- a/sdk/lib/internal/iterable.dart
+++ b/sdk/lib/internal/iterable.dart
@@ -381,7 +381,7 @@ class EfficientLengthMappedIterable<S, T> extends MappedIterable<S, T>
       : super._(iterable, function);
 }
 
-class MappedIterator<S, T> implements Iterator<T> {
+class MappedIterator<S, T> extends Iterator<T> {
   T? _current;
   final Iterator<S> _iterator;
   final _Transformation<S, T> _f;
@@ -430,7 +430,7 @@ class WhereIterable<E> extends Iterable<E> {
       MappedIterable<E, T>._(this, toElement);
 }
 
-class WhereIterator<E> implements Iterator<E> {
+class WhereIterator<E> extends Iterator<E> {
   final Iterator<E> _iterator;
   final _ElementPredicate<E> _f;
 
@@ -522,7 +522,7 @@ class EfficientLengthTakeIterable<E> extends TakeIterable<E>
   }
 }
 
-class TakeIterator<E> implements Iterator<E> {
+class TakeIterator<E> extends Iterator<E> {
   final Iterator<E> _iterator;
   int _remaining;
 
@@ -560,7 +560,7 @@ class TakeWhileIterable<E> extends Iterable<E> {
   }
 }
 
-class TakeWhileIterator<E> implements Iterator<E> {
+class TakeWhileIterator<E> extends Iterator<E> {
   final Iterator<E> _iterator;
   final _ElementPredicate<E> _f;
   bool _isFinished = false;
@@ -631,7 +631,7 @@ int _checkCount(int count) {
   return count;
 }
 
-class SkipIterator<E> implements Iterator<E> {
+class SkipIterator<E> extends Iterator<E> {
   final Iterator<E> _iterator;
   int _skipCount;
 
@@ -659,7 +659,7 @@ class SkipWhileIterable<E> extends Iterable<E> {
   }
 }
 
-class SkipWhileIterator<E> implements Iterator<E> {
+class SkipWhileIterator<E> extends Iterator<E> {
   final Iterator<E> _iterator;
   final _ElementPredicate<E> _f;
   bool _hasSkipped = false;
diff --git a/tests/language/regress/regress10561_test.dart b/tests/language/regress/regress10561_test.dart
new file mode 100644
index 00000000000..16e0cca67f5
--- /dev/null
+++ b/tests/language/regress/regress10561_test.dart
@@ -0,0 +1,16 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// Regression test for dart2js that used to miscompile classes
+// extending HashMap, because HashMap is patched.
+
+import "package:expect/expect.dart";
+
+import 'dart:collection';
+
+class Foo extends Expando {}
+
+main() {
+  Expect.isNull(new Foo()[new Object()]);
+}
diff --git a/tests/language/regress/regress9949_test.dart b/tests/language/regress/regress9949_test.dart
index 3bd0f9a79f6..38ff6edb07e 100644
--- a/tests/language/regress/regress9949_test.dart
+++ b/tests/language/regress/regress9949_test.dart
@@ -8,11 +8,11 @@
 import "package:expect/expect.dart";
 import 'dart:collection';
 
-class Crash extends StringBuffer {
+class Crash extends Expando<String> {
   Crash() : super();
 }
 
 void main() {
   Crash expando = new Crash();
-  Expect.isTrue(expando is StringBuffer);
+  Expect.isTrue(expando is Expando);
 }
diff --git a/tests/language_2/regress/regress10561_test.dart b/tests/language_2/regress/regress10561_test.dart
new file mode 100644
index 00000000000..0961a2e7f7e
--- /dev/null
+++ b/tests/language_2/regress/regress10561_test.dart
@@ -0,0 +1,18 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// 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 = 2.9
+
+// Regression test for dart2js that used to miscompile classes
+// extending HashMap, because HashMap is patched.
+
+import "package:expect/expect.dart";
+
+import 'dart:collection';
+
+class Foo extends Expando {}
+
+main() {
+  Expect.isNull(new Foo()[new Object()]);
+}
diff --git a/tests/language_2/regress/regress9949_test.dart b/tests/language_2/regress/regress9949_test.dart
index b3d50c45ca6..0054039ee3e 100644
--- a/tests/language_2/regress/regress9949_test.dart
+++ b/tests/language_2/regress/regress9949_test.dart
@@ -10,11 +10,11 @@
 import "package:expect/expect.dart";
 import 'dart:collection';
 
-class Crash extends StringBuffer {
+class Crash extends Expando<String> {
   Crash() : super();
 }
 
 void main() {
   Crash expando = new Crash();
-  Expect.isTrue(expando is StringBuffer);
+  Expect.isTrue(expando is Expando);
 }