Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/subclassing_type_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' show setNativeSubclassDispatchRecord;
+import "package:expect/expect.dart";
+import 'dart:_js_helper' show Native, Creates, setNativeSubclassDispatchRecord;
 import 'dart:_interceptors' show Interceptor, findInterceptorForType;
 
 // Test that type checks and casts work for subclasses of native classes and
 // mixins on native classes.
 
 class M {}
 
 @Native("N")
 class N {}
 
@@ skipping 127 matching lines @@
     checkId(setB)(null);
     checkId(setC)(null);
     checkId(setM)(null);
 
     Expect.isNull(gA);
     Expect.isNull(gB);
     Expect.isNull(gC);
     Expect.isNull(gM);
   }
 }