Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_to_string_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';
 
 @Native("A")
-class A {}
+class A {
+  toString() => 'AAA';
+}
 
 makeA() native ;
 
 void setup() native """
 function A() {}
 makeA = function(){return new A;};
+self.nativeConstructor(A);
 """;
 
 main() {
+  nativeTesting();
   setup();
+
   Expect.isTrue(makeA().toString() is String);
+  Expect.equals('AAA', makeA().toString());
+
+  Expect.isTrue(confuse(makeA()).toString() is String);
+  Expect.equals('AAA', confuse(makeA()).toString());
 }