Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_method_with_keyword_name_test.dart

 // Copyright (c) 2012, 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 "dart:_js_helper";
-import "package:expect/expect.dart";
+import 'native_testing.dart';
 
 // Make sure we can have a native with a name that is a JavaScript keyword.
 
 @Native("A")
 class A {
   int delete() native ;
 }
 
 A makeA() native ;
 
 void setup() native """
 function A() {}
 A.prototype.delete = function() { return 87; };
 
 makeA = function(){return new A;};
+self.nativeConstructor(A);
 """;
 
 main() {
+  nativeTesting();
   setup();
 
-  var a = makeA();
-  Expect.equals(87, a.delete());
-  A aa = a;
-  Expect.equals(87, aa.delete());
+  var a1 = confuse(makeA());
+  Expect.equals(87, a1.delete());
+  A a2 = makeA();
+  Expect.equals(87, a2.delete());
 }