Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/inference_of_helper_methods_test.dart

 // Copyright (c) 2013, 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 "package:expect/expect.dart";
+import 'native_testing.dart';
 import 'dart:_js_helper' show intTypeCheck;
 
 bool get inCheckedMode {
   try {
     String a = 42;
   } on TypeError catch (e) {
     return true;
   }
   return false;
 }
 
 main() {
   var a = [];
   a.add(42);
   a.add('foo');
-  // By calling direclty [intTypeCheck] with an int, we're making the
+  // By calling directly [intTypeCheck] with an int, we're making the
   // type inferrer infer that the parameter type of [intTypeCheck] is
   // always an int, and therefore the method will be compiled to
   // never throw. So when the backend actually uses the helper for
   // implementing checked mode semantics (like in the check below),
   // the check won't fail at runtime.
   intTypeCheck(42);
   if (inCheckedMode) {
     int value;
     Expect.throws(() => value = a[1], (e) => e is TypeError);
   }
 }