Add summary command to bot.

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';
@@ skipping 39 matching lines @@
   }
   Uri uri = Uri.parse(url);
   BuildUri buildUri = new BuildUri(uri);
   List<BuildResult> results =
       await readBuildResults(client, buildUri, runCount);
   print(generateBuildResultsSummary(buildUri, results));
   client.close();
 }
 
 /// Creates a [BuildResult] for [buildUri] and, if it contains failures, the
-/// [BuildResult]s for the previous 5 builds.
+/// [BuildResult]s for the previous [runCount] builds.
 Future<List<BuildResult>> readBuildResults(
     BuildbotClient client, BuildUri buildUri, int runCount) async {
   List<BuildResult> summaries = <BuildResult>[];
   BuildResult summary = await client.readResult(buildUri);
   summaries.add(summary);
   if (summary.hasFailures) {
     for (int i = 0; i < runCount; i++) {
       buildUri = summary.buildUri.prev();
       summary = await client.readResult(buildUri);
       summaries.add(summary);
@@ skipping 78 matching lines @@
           sb.write(' / ');
           sb.write(padRight('-- OK --', 10));
         }
         sb.write('\n');
       }
       sb.write('\n');
     });
   }
   return sb.toString();
 }