Split long-running serialization tests to avoid timeouts

tests/compiler/dart2js/serialization/analysis_test_helper.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_analysis_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 45 matching lines @@
         test.expectedWarningCount,
         diagnosticCollector.warnings.length,
         "Unexpected warning count.");
     Expect.equals(test.expectedHintCount, diagnosticCollector.hints.length,
         "Unexpected hint count.");
     Expect.equals(test.expectedInfoCount, diagnosticCollector.infos.length,
         "Unexpected info count.");
   }
 }