Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_window1_frog_test.dart

Index: tests/compiler/dart2js_native/native_window1_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_window1_frog_test.dart b/tests/compiler/dart2js_native/native_window1_frog_test.dart
index 80c56cb4ad82e1a43710dbd71fc01adf8465649b..fd186416badc9de3d58aa7be83cfa92d102fb7ad 100644
--- a/tests/compiler/dart2js_native/native_window1_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_window1_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 typing this variable to 'Window', Frog will optimize calls on
   // it.
   Window win = new Win();