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

tests/compiler/dart2js_native/abstract_class_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";
 
 // Native classes can have subclasses that are not declared to the program.  The
 // subclasses are indistinguishable from the base class.  This means that
 // abstract native classes can appear to have instances.
 
-abstract class A native "A" {
+@Native("A")
+abstract class A {
 }
 
-abstract class B native "B" {
+@Native("B")
+abstract class B {
   foo() native;
 }
 
 class C {}
 
 makeA() native;
 makeB() native;
 
 void setup() native """
 // This code is all inside 'setup' and so not accesible from the global scope.
@@ skipping 21 matching lines @@
   Expect.isFalse(a is B);
   Expect.isTrue(b is B);
   Expect.isFalse(c is B);
 
   Expect.isFalse(a is C);
   Expect.isFalse(b is C);
   Expect.isTrue(c is C);
 
   Expect.equals('B.foo', b.foo());
 }