Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/subclassing_constructor_2_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 "native_testing.dart";
+import "dart:_js_helper";
+import "package:expect/expect.dart";
 
 // Test that native classes and subclasses of native classes cannot be directly
 // created via a generative constructor.
 
 @Native("A")
 class A {
   A.one();
 }
 
 class B extends A {
   B.one() : super.one();
 }
 
 main() {
   Expect.throws(() => new A.one());
   Expect.throws(() => new B.one());
 }