Add support for merging results from plugins

pkg/analysis_server/test/src/plugin/test_all.dart

Index: pkg/analysis_server/test/src/plugin/test_all.dart
diff --git a/pkg/analysis_server/test/src/plugin/test_all.dart b/pkg/analysis_server/test/src/plugin/test_all.dart
index c179a26af80e759ba91a786dcb88bb9b303d1bac..c942661d9f5da698fe7c8b35c71a9a586512843b 100644
--- a/pkg/analysis_server/test/src/plugin/test_all.dart
+++ b/pkg/analysis_server/test/src/plugin/test_all.dart
@@ -5,9 +5,11 @@
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'result_collector_test.dart' as result_collector_test;
+import 'result_merger_test.dart' as result_merger_test;
 
 main() {
   defineReflectiveSuite(() {
     result_collector_test.main();
+    result_merger_test.main();
   });
 }