Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_type_test.dart

Index: tests/compiler/dart2js_native/subclassing_type_test.dart
diff --git a/tests/compiler/dart2js_native/subclassing_type_test.dart b/tests/compiler/dart2js_native/subclassing_type_test.dart
index a4e14e8937b8632a62e00a6b01d5de6374b6d493..3ab4af9e3664a5583120ba754e3fc9d555650bb0 100644
--- a/tests/compiler/dart2js_native/subclassing_type_test.dart
+++ b/tests/compiler/dart2js_native/subclassing_type_test.dart
@@ -2,8 +2,8 @@
 // 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 "package:expect/expect.dart";
-import 'dart:_js_helper' show Native, Creates, setNativeSubclassDispatchRecord;
+import 'native_testing.dart';
+import 'dart:_js_helper' show setNativeSubclassDispatchRecord;
 import 'dart:_interceptors' show Interceptor, findInterceptorForType;
 
 // Test that type checks and casts work for subclasses of native classes and