Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/is_check_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";
 
 @Native("A")
 class A {}
 
 main() {
-  JS('A', '(null)'); // Class 'A' appears to be created.
-  Expect.isFalse(confuse(new Object()) is A);
+  var a = [new Object()];
+  Expect.isFalse(a[0] is A);
 }