Use Bot in compare_failures

tools/gardening/lib/src/bot.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.
 
 import 'dart:async';
 
 import 'buildbot_data.dart';
 import 'buildbot_structures.dart';
 import 'client.dart';
 import 'util.dart';
 
 class Bot {
-  final bool usesLogdog;
   final BuildbotClient _client;
 
   /// Instantiates a Bot.
   ///
   /// Bots must be [close]d when they aren't needed anymore.
   Bot({bool logdog = false})
-      : usesLogdog = logdog,
-        _client =
-            logdog ? new LogdogBuildbotClient() : new HttpBuildbotClient();
+      : this.internal(
+            logdog ? new LogdogBuildbotClient() : new HttpBuildbotClient());
+
+  Bot.internal(this._client);
 
   int get mostRecentBuildNumber => _client.mostRecentBuildNumber;
 
   /// Reads the build result of [buildUri] and the [previousCount] earlier
   /// builds.
   Future<List<BuildResult>> readHistoricResults(BuildUri buildUri,
       {int previousCount = 0}) {
     log("Fetching $buildUri and $previousCount previous builds in parallel");
     var uris = [buildUri];
     for (int i = 0; i < previousCount; i++) {
@@ skipping 37 matching lines @@
     List<BuildUri> result = [];
     for (BuildGroup group in buildGroups) {
       result.addAll(group.createUris(mostRecentBuildNumber));
     }
     return result;
   }
 
   /// Closes the bot.
   void close() => _client.close();
 }