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

tests/compiler/dart2js_native/native_constructor_name_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.
 
 // Test to see if a Dart class name is confused with dispatch tag.  Both class A
 // and class Z have a JavaScript constuctor named "A".  The dynamic native class
 // dispatch hook on Object.prototype should avoid patching the Dart class A.
 // This could be done by renaming the Dart constructor or by being able to check
 // that objects are Dart classes.
 
+import "dart:_js_helper";
 import "package:expect/expect.dart";
 
 class A {
 }
 
-class Z native "A" {
+@Native("A")
+class Z {
   foo() => 100;
 }
 
 makeZ() native;
 
 void setup() native """
 function A(){}
 makeZ = function(){return new A};
 """;
 
 main() {
   setup();
 
   var a = new A();
   var z = makeZ();
 
   Expect.equals(100, z.foo());
 
   Expect.throws(() => a.foo(),  (ex) => ex is NoSuchMethodError);
 }