fixes #40, extension methods for primitive types

test/generated_sdk/lib/_internal/compiler/js_lib/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 _js_helper;
 
 /// Tells the optimizing compiler that the annotated method has no
 /// side-effects.
 /// Requires @NoInline() to function correctly.
 class NoSideEffects {
@@ skipping 16 matching lines @@
 class IrRepresentation {
   final bool value;
   const IrRepresentation(this.value);
 }
 
 /// Marks a class as native and defines its JavaScript name(s).
 class Native {
   final String name;
   const Native(this.name);
 }
+
+// TODO(jmesserly): move these somewhere else, e.g. package:js or 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();
+}