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

tests/compiler/dart2js_native/native_null_closure_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.
 
 // Test that exception unwrapping handle cases like ({foo:null}).foo().
 
-import 'dart:_js_helper';
+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; };
 A.prototype.check = function(f) { return this.f === f; };
 A.prototype.invoke = function() { return this.f(); };
 makeA = function(){return new A;};
 """;
 
 main() {
   setup();
   A a = makeA();
   a.setClosure(null);
   Expect.isTrue(a.check(null));
   bool caughtException = false;
   try {
     a.invoke();
   } on JsNoSuchMethodError catch (e) {
     print(e);
     caughtException = true;
   }
   Expect.isTrue(caughtException);
 }