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

tests/compiler/dart2js_native/native_class_with_dart_methods_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";
 
 // Additional Dart code may be 'placed on' hidden native classes.
 
-class A native "A" {
+@Native("A")
+class A {
 
   var _field;
 
   int get X => _field;
   void set X(int x) { _field = x; }
 
   int method(int z) => _field + z;
 }
 
 A makeA() native { return new A(); }
 
 void setup() native """
 function A() {}
 makeA = function(){return new A;};
 """;
 
 
 main() {
   setup();
 
   var a = makeA();
 
   a.X = 100;
   Expect.equals(100, a.X);
   Expect.equals(150, a.method(50));
 }