Split long-running serialization tests to avoid timeouts

tests/compiler/dart2js/serialization/model2_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.
 
-import 'dart:bear'; // Defined in package_with_embedder_yaml/lib/_embedder.yaml
+library dart2js.serialization.model2_test;
+
+import 'model_test_helper.dart' as test;
+import 'test_data.dart';
 
 main() {
-  print(new Bear()); // Defined in 'dart:bear'
+  test.main(['${TESTS.length ~/ 2}']);
 }