Split long-running serialization tests to avoid timeouts

tests/compiler/dart2js/serialization/compilation_test_helper.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_compilation_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/filenames.dart';
@@ skipping 37 matching lines @@
       entryPoint: entryPoint,
       memorySourceFiles: sourceFiles,
       resolutionInputs: resolutionInputs,
       options: [],
       outputProvider: outputCollector);
   if (verbose) {
     print(outputCollector.getOutput('', 'js'));
   }
 }