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

tests/compiler/dart2js_native/native_mixin_with_plain_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 and ordinary Dart classes can both use the same
 // ordinary Dart classes as a mixin.
 
-class A native "A" {
+@Native("A")
+class A {
   final String aa;
   foo() => "A-foo $aa";
   baz() => "A-baz $aa";
 }
 
-class B extends A with M native "B" {
+@Native("B")
+class B extends A with M {
   bar() => 'B-bar -> ${baz()}';
   get mm => 'B.mm($aa)';
 }
 
 class M {
   foo() => "M-foo ${this.mm}";
   bar() => "M-bar ${this.mm}";
   get mm => 'M.mm';
 }
 
@@ skipping 69 matching lines @@
   Expect.equals("M-foo D.mm(cc)", d.foo());
   Expect.equals("D-bar -> C-baz cc", d.bar());
   Expect.equals("C-baz cc", d.baz());
   Expect.isFalse(d is A);
   Expect.isFalse(d is B);
   Expect.isTrue(d is C);
   Expect.isTrue(d is D);
   Expect.isTrue(d is M);
 
 }