Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_window2_frog_test.dart

Index: tests/compiler/dart2js_native/native_window2_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_window2_frog_test.dart b/tests/compiler/dart2js_native/native_window2_frog_test.dart
index d52366775b03b75520faae307591c787d1e01541..77846c3470e63f0c35b3cbbb23cc36c182c1c004 100644
--- a/tests/compiler/dart2js_native/native_window2_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_window2_frog_test.dart
@@ -2,8 +2,7 @@
 // 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;
@@ -19,6 +18,7 @@ class _DOMWindowJs implements Window {
 class Win implements Window {}
 
 main() {
+  nativeTesting();
   // By not typing the variable, Frog does not try to optimize calls
   // on it.
   var win = new Win();