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

tests/compiler/dart2js_native/native_class_fields_2_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";
 
 // Verify that methods are not renamed to clash with native field names
 // that are known from the DOM (like x, y, z).
-class A native "A" {
+@Native("A")
+class A {
   int x;
   int y;
   int z;
   int gettersCalled;
 }
 
 
 void setup() native r"""
 function getter() {
   this.gettersCalled++;
@@ skipping 83 matching lines @@
   try { x.a21(); } catch(e) { }
   try { x.a12(); } catch(e) { }
   try { x.a23(); } catch(e) { }
   try { x.a24(); } catch(e) { }
   try { x.a25(); } catch(e) { }
   try { x.a26(); } catch(e) { }
   Expect.equals(0, x.gettersCalled);
   Expect.equals(42, x.z);
   Expect.equals(1, x.gettersCalled);
 }