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

tests/compiler/dart2js_native/native_null_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 for values of some basic types.
 
 
-class A native "A" {
+@Native("A")
+class A {
   returnNull() native;
   returnUndefined() native;
   returnEmptyString() native;
   returnZero() native;
 }
 
 A makeA() native;
 
 void setup() native """
 function A() {}
@@ skipping 11 matching lines @@
   Expect.equals(null, a.returnNull());
   Expect.equals(null, a.returnUndefined());
 
   Expect.equals('', a.returnEmptyString());
   Expect.isTrue(a.returnEmptyString().isEmpty);
   Expect.isTrue(a.returnEmptyString() is String);
 
   Expect.isTrue(a.returnZero() is int);
   Expect.equals(0, a.returnZero());
 }