Split serialization tests further

tests/compiler/dart2js/serialization/compilation2_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.compilation2_test;
 
 import 'compilation_test_helper.dart' as test;
-import 'test_data.dart';
+import 'test_helper.dart';
 
 main() {
-  test.main(['${TESTS.length ~/ 4}', '${2 * TESTS.length ~/ 4}']);
+  test.main(testSegment(2, test.SPLIT_COUNT, test.SKIP_COUNT));
 }