Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/native_mirror_test.dart

 // Copyright (c) 2011, 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.
 
 // Intercepted members need to be accessed in a different way than normal
 // members. In this test the native class A (thus being intercepted) has a
 // member "foo", that is final. Therefore only the getter needs to be
 // intercepted. Dart2js had a bug where it used the intercepted
 // calling-convention for parts of the compiler, and the non-intercepted
 // convention for others, making this fail.
 
-import 'native_testing.dart';
-import 'dart:_js_helper';
-import 'dart:mirrors';
+import "package:expect/expect.dart";
+import "dart:_js_helper";
+import "dart:mirrors";
 
 @Native("A")
 class A {
   final foo;
 }
 
 class B {
   String foo;
 }
 
 main() {
   var b = new B();
   reflect(b).setField(new Symbol("foo"), "bar");
   Expect.equals("bar", b.foo);
 }