Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/jsobject_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
         JSObject, // The interface, which may be re-exported by a
         // js-interop library.
         JavaScriptObject, //   The interceptor abstract class.
         PlainJavaScriptObject, //     The interceptor concrete class.
         UnknownJavaScriptObject, //     The interceptor concrete class.
         Interceptor;
 
 // Test for JavaScript objects from outside the Dart program.  Although we only
 // export the interface [JSObject] to user level code, this test makes sure we
 // can distinguish plain JavaScript objects from ones with a complex prototype.
 
 @Native('QQ')
 class Q {}
 
 makeA() native ;
 makeB() native ;
 makeQ() native ;
 
 void setup() native r"""
 makeA = function(){return {hello: 123};};
 
 function BB(){}
 makeB = function(){return new BB();};
 
 function QQ(){}
 makeQ = function(){return new QQ();};
+
+self.nativeConstructor(QQ);
 """;
 
 class Is<T> {
   bool check(x) => x is T;
 }
 
 static_test() {
   var x = makeA();
   Expect.isTrue(x is JSObject);
   Expect.isTrue(x is JavaScriptObject);
@@ skipping 41 matching lines @@
   x = makeQ();
   Expect.isFalse(isJSObject(x));
   Expect.isFalse(isJavaScriptObject(x));
   Expect.isFalse(isPlainJavaScriptObject(x));
   Expect.isFalse(isUnknownJavaScriptObject(x));
   Expect.isTrue(isQ(x));
   Expect.isFalse(x.runtimeType == JSObject);
 }
 
 main() {
+  nativeTesting();
   setup();
 
   dynamic_test();
   static_test();
 }