Fix index bug in readResults

tools/gardening/test/bot_test.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 'package:expect/expect.dart';
+import 'package:gardening/src/bot.dart';
+import 'package:gardening/src/buildbot_data.dart';
+import 'package:gardening/src/buildbot_structures.dart';
+import 'test_client.dart';
+
+main() async {
+  Bot bot = new Bot.internal(new DummyClient());
+  List<BuildUri> buildUriList = buildGroups
+      .firstWhere((g) => g.groupName == 'dart2js-linux')
+      .createUris(bot.mostRecentBuildNumber);
+  Expect.isTrue(buildUriList.length > Bot.maxParallel);
+  List<BuildResult> buildResults = await bot.readResults(buildUriList);
+  Expect.equals(buildUriList.length, buildResults.length);
+  bot.close();
+}