Fix index bug in readResults

tools/gardening/test/test_client.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 45 matching lines @@
   }
 
   @override
   void close() {
     _client?.close();
   }
 
   @override
   int get mostRecentBuildNumber => -1;
 }
+
+class DummyClient implements BuildbotClient {
+  @override
+  Future<BuildResult> readResult(BuildUri buildUri) {
+    return new Future.value(
+        new BuildResult(buildUri, 1, const [], const [], const []));
+  }
+
+  @override
+  void close() {}
+
+  @override
+  int get mostRecentBuildNumber => -1;
+}