Split long-running serialization tests to avoid timeouts

tests/compiler/dart2js/serialization/model_test_helper.dart

Index: tests/compiler/dart2js/serialization/model_test_helper.dart
diff --git a/tests/compiler/dart2js/serialization/model_test.dart b/tests/compiler/dart2js/serialization/model_test_helper.dart
similarity index 99%
rename from tests/compiler/dart2js/serialization/model_test.dart
rename to tests/compiler/dart2js/serialization/model_test_helper.dart
index 37b99df0e4213e3e06e39524d1fe7f3da5f2d152..2c6d86176b2d8109ef0e25b8b69bfd4fc5b20e2f 100644
--- a/tests/compiler/dart2js/serialization/model_test.dart
+++ b/tests/compiler/dart2js/serialization/model_test_helper.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.serialization_model_test;
+
 import 'dart:async';
 import 'dart:io';
 import 'package:async_helper/async_helper.dart';