Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_3_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 of methods introduced on subclasses of native
 // class of mixin.
 
 doFoo(r, x) => '$x,${r.oof()},${r.miz()}';
 
 class M {
   miz() => 'M';
 }
@@ skipping 18 matching lines @@
 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('1,B,M', b.foo(1));
 }