Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/dispatch_property_initialization_test.dart

 // Copyright (c) 2016, 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.
 
 // Test for initialization of dispatchPropertyName.
 
-import "package:expect/expect.dart";
-import 'dart:_foreign_helper' show JS;
-import 'dart:_js_helper' show Native;
+import 'native_testing.dart';
 
 @Native("Foo")
 class Foo {
   String method(String x) native;
 }
 
 makeFoo() native;
 
 void setup() native r"""
 function Foo() {}
 Foo.prototype.method = function(x) { return 'Foo ' + x; }
 
 self.makeFoo = function() { return new Foo(); }
+
+self.nativeConstructor(Foo);
 """;
 
 
 main() {
+  nativeTesting();
   setup();
 
   // If the dispatchPropertyName is uninitialized, it will be `undefined` or
   // `null` instead of the secret string or Symbol. These properties on
   // `Object.prototype` will be retrieved by the lookup instead of `undefined`
   // for the dispatch record.
   JS('', r'self.Object.prototype["undefined"] = {}');
   JS('', r'self.Object.prototype["null"] = {}');
   Expect.equals('Foo A', makeFoo().method('A'));
 
   // Slightly different version that has malformed dispatch records.
   JS('', r'self.Object.prototype["undefined"] = {p: false}');
   JS('', r'self.Object.prototype["null"] = {p: false}');
   Expect.equals('Foo B', makeFoo().method('B'));
 }