Fix index bug in readResults

tools/gardening/lib/src/compare_failures_impl.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';
 
@@ skipping 19 matching lines @@
     }
   }
   if (buildUriList.isEmpty) {
     for (String url in args) {
       buildUriList.add(new BuildUri.fromUrl(url));
     }
   }
   Map<BuildUri, List<BuildResult>> pastResultsMap =
       <BuildUri, List<BuildResult>>{};
   List<BuildResult> buildResults = await bot.readResults(buildUriList);
-  for (int index = 0; index < buildUriList.length; index++) {
+  if (buildResults.length != buildUriList.length) {
+    print('Result mismatch: Pulled ${buildUriList.length} uris, '
+        'received ${buildResults.length} results.');
+  }
+  for (int index = 0; index < buildResults.length; index++) {
     BuildUri buildUri = buildUriList[index];
     BuildResult buildResult = buildResults[index];
     List<BuildResult> results =
         await readPastResults(bot, buildUri, buildResult, runCount);
     pastResultsMap[buildUri] = results;
   }
   return pastResultsMap;
 }
 
 /// Prints summaries for the [buildResults].
@@ skipping 156 matching lines @@
       sb.write('No errors found for ${name}');
     }
   }
 
   String get name => results.isNotEmpty
       // Use the first result as name since it most likely has an absolute build
       // number.
       ? results.first.buildUri.toString()
       : buildUri.toString();
 }