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

tests/compiler/dart2js_native/native_class_avoids_hidden_name_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.
 
+import "dart:_js_helper";
 import "package:expect/expect.dart";
 
 // Test that hidden native class names are not used by generated code.
 
-class AA native "BB" {
+@Native("BB")
+class AA {
   get name => 'AA';
   static AA create() => makeA();
 }
 
-class BB native "CC" {
+@Native("CC")
+class BB {
   get name => 'BB';
   static BB create() => makeB();
 }
 
 class CC {  // Ordinary class with name clashing with native class.
   get name => 'CC';
   static CC create() => new CC();
 }
 
 makeA() native;
@@ skipping 22 matching lines @@
 
   var things = [AA.create(), BB.create(), CC.create()];
   var a = things[inscrutable(0)];
   var b = things[inscrutable(1)];
   var c = things[inscrutable(2)];
 
   Expect.equals('AA', a.name);
   Expect.equals('BB', b.name);
   Expect.equals('CC', c.name);
 }