Extension method support to move us closer to a valid List implementation.

tool/input_sdk/lib/core/annotations.dart

Index: tool/input_sdk/lib/core/annotations.dart
diff --git a/tool/input_sdk/lib/core/annotations.dart b/tool/input_sdk/lib/core/annotations.dart
index 5ceed34fd63bb8b0f3123bb06c892afa9cbe3d5e..536eddff94fa255f4751c854df03fd03c79980b6 100644
--- a/tool/input_sdk/lib/core/annotations.dart
+++ b/tool/input_sdk/lib/core/annotations.dart
@@ -4,6 +4,31 @@
 
 part of dart.core;
 
+// TODO(jacobr): move these annotations to a different package. E.g. dart:js
+
+class JsName {
+  /// The JavaScript name.
+  /// Used for classes and libraries.
+  /// Note that this could be an expression, e.g. `lib.TypeName` in JS, but it
+  /// should be kept simple, as it will be generated directly into the code.
+  final String name;
+  const JsName({this.name});
+}
+
+class JsPeerInterface {
+  /// The JavaScript type that we should match the API of.
+  /// Used for classes where Dart subclasses should be callable from JavaScript
+  /// matching the JavaScript calling conventions.
+  final String name;
+  const JsPeerInterface({this.name});
+}
+
+/// A Dart interface may only be implemented by a native JavaScript object
+/// if it is marked with this annotation.
+class SupportJsExtensionMethod {
+  const SupportJsExtensionMethod();
+}
+
 /**
  * The annotation `@Deprecated('expires when')` marks a feature as deprecated.
  *