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

tests/compiler/dart2js_native/native_field_invocation6_test.dart

 // Copyright (c) 2014, 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";
 
 makeA() native;
 nativeFirst(x, y) native;
 
 void setup() native """
 nativeFirst = function(x, y) { return x; }
 
 function A() {}
 makeA = function() { return new A; }
 """;
 
 
-class A native "A" {
+@Native("A")
+class A {
   var _foo;
 
   get foo => _foo;
 
   init() {
     _foo = () => 42;
   }
 }
 
 class B {
@@ skipping 15 matching lines @@
   var b = new B();
   var c = new C();
   var d = new D();
   a = nativeFirst(a, b);
   a = nativeFirst(a, c);
   a = nativeFirst(a, d);
   // The variable a is still an instance of class A, but dart2js can't infer
   // that anymore.
   Expect.equals(42, a.foo());
 }