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

tests/compiler/dart2js_native/native_mixin_multiple_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 M1, M2 native "B" {
+@Native("B")
+class B extends A with M1, M2 {
   bar() => baz();
 }
 
 class M1 {
   foo() => "M1-foo";
   baz() => "M1-baz";
 }
 
 class M2 {
   foo() => "M2-foo";
@@ skipping 40 matching lines @@
 
   M2 m2 = new M2();
   Expect.equals("M2-foo", m2.foo());
   Expect.throws(() => m2.bar(), (error) => error is NoSuchMethodError);
   Expect.throws(() => m2.baz(), (error) => error is NoSuchMethodError);
   Expect.isFalse(m2 is A);
   Expect.isFalse(m2 is B);
   Expect.isFalse(m2 is M1);
   Expect.isTrue(m2 is M2);
 }