Reorganize equivalence test helpers

tests/compiler/dart2js/serialization/equivalence_test.dart

 // Copyright (c) 2015, 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_test;
 
 import 'dart:io';
 import '../memory_compiler.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/common/resolution.dart';
 import 'package:compiler/src/constants/constructors.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/diagnostics/invariant.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/resolution_types.dart';
 import 'package:compiler/src/elements/visitor.dart';
 import 'package:compiler/src/filenames.dart';
 import 'package:compiler/src/library_loader.dart';
 import 'package:compiler/src/ordered_typeset.dart';
 import 'package:compiler/src/serialization/element_serialization.dart';
 import 'package:compiler/src/serialization/equivalence.dart';
 import 'package:compiler/src/serialization/json_serializer.dart';
 import 'package:compiler/src/serialization/serialization.dart';
 import 'package:expect/expect.dart';
-import 'test_helper.dart';
+import '../equivalence/check_helpers.dart';
 
 const TEST_SOURCES = const <String, String>{
   'main.dart': '''
 import 'library.dart';
 import 'deferred_library.dart' deferred as prefix;
 
 asyncMethod() async {}
 asyncStarMethod() async* {}
 syncStarMethod() sync* {}
 get asyncGetter async {}
@@ skipping 798 matching lines @@
     });
   }
 
   @override
   void visitErroneousElement(
       ErroneousElement element1, ErroneousElement element2) {
     check(element1, element2, 'messageKind', element1.messageKind,
         element2.messageKind);
   }
 }