Use package:testing for tools/gardening

tools/gardening/bin/summary.dart

Index: tools/gardening/bin/summary.dart
diff --git a/tools/gardening/bin/summary.dart b/tools/gardening/bin/summary.dart
index 15a77e76c60d4c424ac114eb553c111576074d42..2fa018927b188766567b07d37cf72514871f24f7 100644
--- a/tools/gardening/bin/summary.dart
+++ b/tools/gardening/bin/summary.dart
@@ -2,6 +2,7 @@
 // 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:async';
 import 'package:args/args.dart';
 import 'package:gardening/src/bot.dart';
 import 'package:gardening/src/buildbot_data.dart';
@@ -16,7 +17,7 @@ void help(ArgParser argParser) {
   print(argParser.usage);
 }
 
-main(List<String> args) async {
+Future main(List<String> args) async {
   ArgParser argParser = createArgParser();
   ArgResults argResults = argParser.parse(args);
   processArgResults(argResults);