Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_4_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 class M {
   miz() => 'M';
 }
 
 @Native("N")
 class N {}
@@ skipping 15 matching lines @@
 @Creates('=Object')
 getBPrototype() native ;
 
 void setup() native r"""
 function B() {}
 makeB = function(){return new B;};
 getBPrototype = function(){return B.prototype;};
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));
 
   B b = makeB();
   Expect.equals('M', b.callMiz());
 }