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

tests/compiler/dart2js_native/native_mixin_test.dart

 // Copyright (c) 2013, 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";
 
 // Test that native classes can use ordinary Dart classes as mixins.
 
-class A native "A" {
+@Native("A")
+class A {
   foo() => "A-foo";
   baz() => "A-baz";
 }
 
-class B extends A with M native "B" {
+@Native("B")
+class B extends A with M {
   bar() => baz();
 }
 
 class M {
   foo() => "M-foo";
   bar() => "M-bar";
 }
 
 A makeA() native;
 B makeB() native;
@@ skipping 24 matching lines @@
   Expect.isTrue(b is M);
 
   M m = new M();
   Expect.equals("M-foo", m.foo());
   Expect.equals("M-bar", m.bar());
   Expect.throws(() => m.baz(), (error) => error is NoSuchMethodError);
   Expect.isFalse(m is A);
   Expect.isFalse(m is B);
   Expect.isTrue(m is M);
 }