Split serialization tests further

tests/compiler/dart2js/serialization/compilation_test_helper.dart

Index: tests/compiler/dart2js/serialization/compilation_test_helper.dart
diff --git a/tests/compiler/dart2js/serialization/compilation_test_helper.dart b/tests/compiler/dart2js/serialization/compilation_test_helper.dart
index 4445f9dcfedbb0b3c33a4ad1bca2b27a02bae788..5eda517245de8ecefa6574274e6febecdb82cc43 100644
--- a/tests/compiler/dart2js/serialization/compilation_test_helper.dart
+++ b/tests/compiler/dart2js/serialization/compilation_test_helper.dart
@@ -13,6 +13,12 @@ import 'helper.dart';
 import 'test_data.dart';
 import '../output_collector.dart';
 
+/// Number of tests that are not part of the automatic test grouping.
+int SKIP_COUNT = 2;
+
+/// Number of groups that the [TESTS] are split into.
+int SPLIT_COUNT = 4;
+
 main(List<String> args) {
   asyncTest(() async {
     Arguments arguments = new Arguments.from(args);