Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_constructor_2_test.dart

Index: tests/compiler/dart2js_native/subclassing_constructor_2_test.dart
diff --git a/tests/compiler/dart2js_native/subclassing_constructor_2_test.dart b/tests/compiler/dart2js_native/subclassing_constructor_2_test.dart
index 2b9f6c527d1cc6b9f58458ee0bbbbc5e17341b58..f1c8dd1fce31b6b87ad9742a2eee05a315ac0b8b 100644
--- a/tests/compiler/dart2js_native/subclassing_constructor_2_test.dart
+++ b/tests/compiler/dart2js_native/subclassing_constructor_2_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";
 
 // Test that native classes and subclasses of native classes cannot be directly
 // created via a generative constructor.