Use package:testing for tools/gardening

tools/gardening/bin/compare_failures.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 /// Compares the test log of a build step with previous builds.
 ///
 /// Use this to detect flakiness of failures, especially timeouts.
 
+import 'dart:async';
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:gardening/src/bot.dart';
 import 'package:gardening/src/compare_failures_impl.dart';
 import 'package:gardening/src/util.dart';
 
 void help(ArgParser argParser) {
   print('Given a <log-uri> finds all failing tests in that stdout. Then ');
   print('fetches earlier runs of the same bot and compares the results.');
   print('This tool is particularly useful to detect flakes and their ');
   print('frequency.');
   print('Usage: compare_failures [options] ');
   print('  (<log-uri> [<log-uri> ...] | <build-group> [<build-group> ...])');
   print('where <log-uri> is the uri the stdio output of a failing test step ');
   print('and <build-group> is the name of a buildbot group, for instance ');
   print('`vm-kernel`, and options are:');
   print(argParser.usage);
 }
 
-main(List<String> args) async {
+Future main(List<String> args) async {
   ArgParser argParser = createArgParser();
   argParser.addOption("run-count",
       defaultsTo: "10", help: "How many previous runs should be fetched");
   ArgResults argResults = argParser.parse(args);
   processArgResults(argResults);
 
   var runCount = int.parse(argResults['run-count'], onError: (_) => null);
 
   if (argResults.rest.length < 1 || argResults['help'] || runCount == null) {
     help(argParser);
     if (argResults['help']) return;
     exit(1);
   }
 
   Bot bot = new Bot(logdog: argResults['logdog']);
   await mainInternal(bot, argResults.rest, runCount: runCount);
   bot.close();
 }