Reorganize equivalence test helpers

tests/compiler/dart2js/serialization/impact_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_impact_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/filenames.dart';
 import '../memory_compiler.dart';
+import '../equivalence/check_functions.dart';
 import 'helper.dart';
-import 'test_helper.dart';
 
 main(List<String> args) {
   Arguments arguments = new Arguments.from(args);
   asyncTest(() async {
     SerializedData serializedData =
         await serializeDartCore(arguments: arguments);
     if (arguments.filename != null) {
       Uri entryPoint = Uri.base.resolve(nativeToUriPath(arguments.filename));
       await check(serializedData, entryPoint);
     } else {
@@ skipping 14 matching lines @@
 
   Compiler compilerDeserialized = compilerFor(
       memorySourceFiles: serializedData.toMemorySourceFiles(sourceFiles),
       resolutionInputs: serializedData.toUris(),
       options: [Flags.analyzeAll]);
   compilerDeserialized.resolution.retainCachesForTesting = true;
   await compilerDeserialized.run(entryPoint);
 
   checkAllImpacts(compilerNormal, compilerDeserialized, verbose: verbose);
 }