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

tests/compiler/dart2js_native/native_field_rename_2_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.
 
 // A native method prevents other members from having that name, including
 // fields.  However, native fields keep their name.  The implication: a getter
 // for the field must be based on the field's name, not the field's jsname.
 
 import "package:expect/expect.dart";
-import 'dart:_js_helper' show JSName;
+import 'dart:_js_helper' show Native, JSName;
 
 abstract class I {
   int key;
 }
 
-class A implements I native "A" {
+@Native("A")
+class A implements I {
   int key;                    //  jsname is 'key'
   int getKey() => key;
 }
 
 class B implements I {
   int key;                    //  jsname is not 'key'
   B([this.key = 222]);
   int getKey() => key;
 }
 
-class X native "X" {
+@Native("X")
+class X {
   @JSName('key')
   int native_key_method() native;
   // This should cause B.key to be renamed, but not A.key.
   @JSName('key')
   int key() native;
 }
 
 A makeA() native;
 X makeX() native;
 
@@ skipping 51 matching lines @@
   Expect.equals(222, b.getKey());
 }
 
 main() {
   setup();
 
   testTyped();
   testPartial();
   testDynamic();
 }