Reorganize equivalence test helpers

tests/compiler/dart2js/serialization/reserialization_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.reserialization_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/diagnostics/invariant.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:expect/expect.dart';
+import '../equivalence/check_functions.dart';
 import 'helper.dart';
-import 'test_helper.dart';
 import 'equivalence_test.dart';
 
 main(List<String> args) {
   // Ensure that we can print out constant expressions.
   DEBUG_MODE = true;
 
   Arguments arguments = new Arguments.from(args);
   Uri entryPoint;
   if (arguments.filename != null) {
     entryPoint = Uri.parse(arguments.filename);
@@ skipping 41 matching lines @@
         library3, "No library found for ${library1.canonicalUri}.");
     checkLibraryContent('library1', 'library3', 'library', library1, library3);
   }
 
   checkAllResolvedAsts(compiler1, compiler2);
   checkAllResolvedAsts(compiler1, compiler3);
 
   checkAllImpacts(compiler1, compiler2);
   checkAllImpacts(compiler1, compiler3);
 }