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

test/dart_codegen/expect/core/annotations.dart

 part of dart.core;
+ class JsName {final String name;
+ const JsName({
+  this.name}
+);
+}
+ class JsPeerInterface {final String name;
+ const JsPeerInterface({
+this.name}
+);
+}
+ class SupportJsExtensionMethod {const SupportJsExtensionMethod();
+}
  class Deprecated {final String expires;
  const Deprecated(String expires) : this.expires = expires;
  String toString() => "Deprecated feature. Will be removed $expires";
 }
  class _Override {const _Override();
 }
  const Deprecated deprecated = const Deprecated("next release");
  const Object override = const _Override();
  class _Proxy {const _Proxy();
 }
  const Object proxy = const _Proxy();