Reorganize equivalence test helpers

tests/compiler/dart2js/serialization/native_data_test.dart

 // Copyright (c) 2016, 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.
 
 library dart2js.serialization.native_data_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/common/names.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/js_backend/native_data.dart';
 import 'package:compiler/src/filenames.dart';
 import 'package:compiler/src/serialization/equivalence.dart';
 import 'package:compiler/src/world.dart';
+import '../equivalence/check_helpers.dart';
 import '../memory_compiler.dart';
 import 'helper.dart';
-import 'test_helper.dart';
 
 main(List<String> args) {
   asyncTest(() async {
     Arguments arguments = new Arguments.from(args);
     Uri uri = Uris.dart_html;
     if (arguments.filename != null) {
       uri = Uri.base.resolve(nativeToUriPath(arguments.filename));
     }
     await checkNativeData(uri, verbose: arguments.verbose);
   });
@@ skipping 70 matching lines @@
       verbose: verbose);
 
   checkMaps(
       nativeData1.nativeFieldStoreBehavior,
       nativeData2.nativeFieldStoreBehavior,
       "NativeData.nativeFieldStoreBehavior",
       areElementsEquivalent,
       testNativeBehavior,
       verbose: verbose);
 }