Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_1_test.dart

Index: tests/compiler/dart2js_native/subclassing_1_test.dart
diff --git a/tests/compiler/dart2js_native/subclassing_1_test.dart b/tests/compiler/dart2js_native/subclassing_1_test.dart
index 77ca191f3b5393aed08ee2b38b71c3162976c0e3..69a2aa9dcbea8fe1fd27d6cbda886a55cef4252e 100644
--- a/tests/compiler/dart2js_native/subclassing_1_test.dart
+++ b/tests/compiler/dart2js_native/subclassing_1_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 findInterceptorForType;
 
 // Test that subclasses of native classes can be defined by setting the dispatch
@@ -43,6 +43,7 @@ getCPrototype = function(){return C.prototype;};
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));