Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_4_test.dart

Index: tests/compiler/dart2js_native/subclassing_4_test.dart
diff --git a/tests/compiler/dart2js_native/subclassing_4_test.dart b/tests/compiler/dart2js_native/subclassing_4_test.dart
index 3e5d4822c518804f7e1c6614338b09a659186dd3..a2841582a1046dbd3dfff481265c06386ae0ae34 100644
--- a/tests/compiler/dart2js_native/subclassing_4_test.dart
+++ b/tests/compiler/dart2js_native/subclassing_4_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 calling convention on subclasses of native classes.
@@ -39,6 +39,7 @@ getBPrototype = function(){return B.prototype;};
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));