Add @Native(...) annotation for native class names.

tests/compiler/dart2js_native/native_property_frog_test.dart

 // Copyright (c) 2011, 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.
 
 // Properties on hidden native classes.
 
 import "package:expect/expect.dart";
+import "dart:_js_helper";
 import 'dart:_foreign_helper' show JS;
 
-class A native "A" {
+@Native("A")
+class A {
 
   // Setters and getters should be similar to these methods:
   int getX() => JS('int', '#._x', this);
   void setX(int value) { JS('void', '#._x = #', this, value); }
 
   int get X() native;
   set X(int value) native;
 
   int get Y() native;
   set Y(int value) native;
@@ skipping 25 matching lines @@
   Expect.equals(5, a.getX());
 
   a.X = 10;
   a.Y = 20;
   a.Z = 30;
 
   Expect.equals(10, a.X);
   Expect.equals(20, a.Y);
   Expect.equals(30, a.Z);
 }