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

tests/compiler/dart2js_native/native_wrapping_function3_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 "package:expect/expect.dart";
+import "dart:_js_helper";
 
 typedef void Callback0();
 typedef void Callback1(arg1);
 typedef void Callback2(arg1, arg2);
 
-class A native "A" {
+@Native("A")
+class A {
   foo1(Callback1 closure, [arg1 = 0]) native;
   foo2(Callback2 closure, [arg1 = 0, arg2 = 1]) native;
 }
 
 makeA() native;
 
 void setup() native """
 function A() {}
 A.prototype.foo1 = function(closure, arg1) { return closure(arg1); };
 A.prototype.foo2 = function(closure, arg1, arg2) {
@@ skipping 12 matching lines @@
   Expect.equals(44, a.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
   Expect.equals(22, a.foo2((arg1, arg2) => arg1 + arg2, 21));
 
   A aa = a;
   Expect.equals(43, aa.foo1((arg1) => arg1, 43));
   Expect.equals(0, aa.foo1((arg1) => arg1));
 
   Expect.equals(44, aa.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
   Expect.equals(22, aa.foo2((arg1, arg2) => arg1 + arg2, 21));
 }