Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_property_frog_test.dart

 // Copyright (c) 2011, 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.
 
 // Properties on hidden native classes.
 
-import "package:expect/expect.dart";
-import "dart:_js_helper";
-import 'dart:_foreign_helper' show JS;
+import 'native_testing.dart';
 
 @Native("A")
 class A {
 
   // Setters and getters should be similar to these methods:
   int getX() => JS('int', '#._x', this);
   void setX(int value) { JS('void', '#._x = #', this, value); }
 
   int get X() native;
   set X(int value) native;
 
   int get Y() native;
   set Y(int value) native;
 
   int get Z => JS('int', '#._z', this);
   set Z(int value) { JS('void', '#._z = #', this, value); }
 }
 
 A makeA() native;
 
 void setup() native """
 function A() {}
 
 Object.defineProperty(A.prototype, "X", {
   get: function () { return this._x; },
   set: function (v) { this._x = v; }
 });
 
 makeA = function(){return new A;};
+
+self.nativeConstructor(A);
 """;
 
 
 main() {
+  nativeTesting();
   setup();
 
   var a = makeA();
 
   a.setX(5);
   Expect.equals(5, a.getX());
 
   a.X = 10;
   a.Y = 20;
   a.Z = 30;
 
   Expect.equals(10, a.X);
   Expect.equals(20, a.Y);
   Expect.equals(30, a.Z);
+
+  confuse(a).setX(6);
+  Expect.equals(6, confuse(a).getX());
+
+  Expect.equals(6, confuse(a).X);
+  Expect.equals(20, confuse(a).Y);
+  Expect.equals(30, confuse(a).Z);
 }