Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_exception_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.
 
 // Test that dart2js can handle unexpected exception types.
 
 library native_exception_test;
 
-import 'dart:_foreign_helper' show JS;
-import 'dart:_js_helper';
-import 'package:expect/expect.dart';
+import 'native_testing.dart';
 
 main() {
   var previous;
   check(e) {
     print('$e');
     Expect.equals(e, e);
     Expect.notEquals(e, new Object());
     Expect.notEquals(e, previous);
     previous = e;
     return '$e' != '[object Object]';
@@ skipping 14 matching lines @@
   Expect.throws(() {
     JS('void', 'throw void 0');
   }, check);
   Expect.throws(() {
     JS('void', 'throw "a string"');
   }, check);
   Expect.throws(() {
     JS('void', 'throw null');
   }, check);
 }