Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_wrapping_function_frog_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";
 
 typedef void Callback0();
 typedef void Callback1(arg1);
 typedef void Callback2(arg1, arg2);
 
 @Native("A")
 class A {
   foo0(Callback0 closure) native ;
   foo1(Callback1 closure, arg1) native ;
   foo2(Callback2 closure, arg1, arg2) native ;
 }
 
 makeA() native ;
 
 void setup() native """
 function A() {}
 A.prototype.foo0 = function(closure) { return closure(); };
 A.prototype.foo1 = function(closure, arg1) { return closure(arg1); };
 A.prototype.foo2 = function(closure, arg1, arg2) {
   return closure(arg1, arg2);
 };
 makeA = function(){return new A;};
+self.nativeConstructor(A);
 """;
 
 main() {
+  nativeTesting();
   setup();
   var a = makeA();
   Expect.equals(42, a.foo0(() => 42));
   Expect.equals(43, a.foo1((arg1) => arg1, 43));
   Expect.equals(44, a.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
 
   A aa = a;
   Expect.equals(42, aa.foo0(() => 42));
   Expect.equals(43, aa.foo1((arg1) => arg1, 43));
   Expect.equals(44, aa.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
 }