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

tests/compiler/dart2js_native/native_closure_identity_frog_test.dart

 // Copyright (c) 2012, 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";
 
 typedef void MyFunctionType();
 
-class A native "A" {
+@Native("A")
+class A {
   setClosure(MyFunctionType f) native;
   check(MyFunctionType f) native;
   invoke() native;
 }
 
 makeA() native { return new A(); }
 
 void setup() native """
 function A() {}
 A.prototype.setClosure = function(f) { this.f = f; };
@@ skipping 43 matching lines @@
 
   checkUntyped(makeA(), b.instanceMethod);
   checkTyped(makeA(), b.instanceMethod);
 
   checkUntyped(makeA(), closureStatement);
   checkTyped(makeA(), closureStatement);
 
   checkUntyped(makeA(), closureExpression);
   checkTyped(makeA(), closureExpression);
 }