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

tests/compiler/dart2js_native/native_exception2_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.
 
 // Similar to native_exception_test.dart but also defines a native
 // class.  This exercises a different code path in emitter.dart.
 
 library native_exception2_test;
 
 import 'native_exception_test.dart' as other;
+import 'dart:_js_helper';
 
-class NativeClass native "NativeClass" {
+@Native("NativeClass")
+class NativeClass {
 }
 
 makeNativeClass() native;
 
 setup() native """
 function NativeClass() {}
 makeNativeClass = function() { return new NativeClass; }
 """;
 
 main() {
   setup();
   other.main();
 }