Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/bound_closure_super_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 calling convention of property extraction closures (super edition).
 library bound_closure_super_test;
 
-import "package:expect/expect.dart";
+import 'package:expect/expect.dart';
 
 import 'bound_closure_test.dart' as bound_closure_test;
 
 import 'bound_closure_test.dart' show inscrutable, makeCC;
 
 main() {
   // Calling main from bound_closure_test.dart to set up native code.
   bound_closure_test.main();
 
   var c = inscrutable(makeCC)();
   var csfoo = inscrutable(c).superfoo;
 
   Expect.equals('BB.foo(1, B)', csfoo(1));
   Expect.equals('BB.foo(2, 3)', csfoo(2, 3));
 }