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

tests/compiler/dart2js_native/native_field_invocation2_test.dart

 // Copyright (c) 2014, 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";
 
-class NNative native "NNative" {
+@Native("NNative")
+class NNative {
   var status;
 
   var f;
 
   ClickCounter() {
     f = wrap(g);
   }
 
   g(val) => "### $val ###";
 }
@@ skipping 23 matching lines @@
   };
 }
 
 main() {
   setup();
   // Make sure the ClickCounter class goes through interceptors.
   Expect.equals("### !!! 42 !!! ###", nativeId(new ClickCounter()).f(42));
   // We are interested in the direct call, where the type is known.
   Expect.equals("### !!! 499 !!! ###", new ClickCounter().f(499));
 }