Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/mirror_intercepted_field_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 'dart:mirrors';
-import 'native_testing.dart';
+import "dart:_js_helper";
+import "package:expect/expect.dart";
 
 @Native("B")
 class B {
   // Having this field in a native class will generate accessors with
   // the interceptor calling convention.
   var f;
 }
 
 class A {
   int f;
 }
 
 const symF = const Symbol('f');
 
 main() {
-  JS('B', '(null)'); // B appears to be created.
   var a = new A();
 
   InstanceMirror mirror = reflect(a);
   mirror.setField(symF, 42);
   Expect.equals(42, a.f);
 
   mirror = mirror.getField(symF);
   Expect.equals(42, mirror.reflectee);
 }