Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_window2_frog_test.dart

 // Copyright (c) 2012, 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";
+import "native_testing.dart";
 
 abstract class Window {
   final int document;
 }
 
 // Defining this global object makes Frog eager on optimizing
 // call sites where the receiver is typed 'Window'.
 @Native("@*DOMWindow")
 class _DOMWindowJs implements Window {
   final int document;
 }
 
 class Win implements Window {}
 
 main() {
+  nativeTesting();
   // By not typing the variable, Frog does not try to optimize calls
   // on it.
   var win = new Win();
   Expect.throws(() => win.document, (e) => e is NoSuchMethodError);
 }