Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_closure_identity_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 MyFunctionType();
 
 @Native("A")
 class A {
   setClosure(MyFunctionType f) native ;
   check(MyFunctionType f) native ;
   invoke() native ;
 }
 
 makeA() native ;
 
 void setup() native """
 function A() {}
 A.prototype.setClosure = function(f) { this.f = f; };
 A.prototype.check = function(f) { return this.f === f; };
 A.prototype.invoke = function() { return this.f(); };
 makeA = function(){return new A;};
+
+self.nativeConstructor(A);
 """;
 
 var staticClosure;
 staticMethod() => 42;
 
 class B {
   var instanceClosure;
   instanceMethod() => 43;
 }
 
 checkUntyped(a, closure) {
   a.setClosure(closure);
   Expect.isTrue(a.check(closure));
   Expect.equals(closure(), a.invoke());
 }
 
 checkTyped(A a, MyFunctionType closure) {
   a.setClosure(closure);
   Expect.isTrue(a.check(closure));
   Expect.equals(closure(), a.invoke());
 }
 
 main() {
+  nativeTesting();
   setup();
 
   staticClosure = () => 44;
   B b = new B();
   b.instanceClosure = () => 45;
 
   closureStatement() => 46;
   var closureExpression = () => 47;
 
   checkUntyped(makeA(), staticClosure);
   checkTyped(makeA(), staticClosure);
 
   checkUntyped(makeA(), staticMethod);
   checkTyped(makeA(), staticMethod);
 
   checkUntyped(makeA(), b.instanceClosure);
   checkTyped(makeA(), b.instanceClosure);
 
   checkUntyped(makeA(), b.instanceMethod);
   checkTyped(makeA(), b.instanceMethod);
 
   checkUntyped(makeA(), closureStatement);
   checkTyped(makeA(), closureStatement);
 
   checkUntyped(makeA(), closureExpression);
   checkTyped(makeA(), closureExpression);
 }