Delete dart2js specific private library code

tool/input_sdk/lib/core/annotations.dart

 // 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.
 
 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();
+class SupportJsExtensionMethods {
+  const SupportJsExtensionMethods();
 }
 
 /**
  * The annotation `@Deprecated('expires when')` marks a feature as deprecated.
  *
  * The annotation `@deprecated` is a shorthand for deprecating until
  * an unspecified "next release".
  *
  * The intent of the `@Deprecated` annotation is to inform users of a feature
  * that they should change their code, even if it is currently still working
@@ skipping 118 matching lines @@
  *
  * Tools that understand `@proxy` should tell the user if a class using `@proxy`
  * does not override the `noSuchMethod` declared on [Object].
  *
  * The intent of the `@proxy` notation is to create objects that implement a
  * type (or multiple types) that are not known at compile time. If the types
  * are known at compile time, a class can be written that implements these
  * types.
  */
 const Object proxy = const _Proxy();