Split long-running serialization tests to avoid timeouts

tests/compiler/dart2js/serialization/model2_test.dart

Index: tests/compiler/dart2js/serialization/model2_test.dart
diff --git a/pkg/analyzer_cli/test/data/embedder_client/embedder_yaml_user.dart b/tests/compiler/dart2js/serialization/model2_test.dart
similarity index 60%
copy from pkg/analyzer_cli/test/data/embedder_client/embedder_yaml_user.dart
copy to tests/compiler/dart2js/serialization/model2_test.dart
index 2d88a2adebdfa58a10363e94f0250ef66aa8b6b6..369e4a13f1e39693e8029b9e4b6ffa74840bcbcc 100644
--- a/pkg/analyzer_cli/test/data/embedder_client/embedder_yaml_user.dart
+++ b/tests/compiler/dart2js/serialization/model2_test.dart
@@ -2,8 +2,11 @@
 // 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}']);
 }