anchor.diff:          return new Future.value(files.items.removeAt(0)); // shall only contains 1 item
         }
       })
  -    .then((GoogleDriveClient.File file) {
  -      completer.complete(file.id);
  -    })
  +    .then((GoogleDriveClient.File file) => completer.complete(file.id))
       .catchError((e, trace) {
         PinUtility.handleError(e, trace);
         completer.completeError(e, trace);
  
positive.code:     CartTag tag = tagList.addNewTag(uuid, name, timestamp: timestamp);
  
      tagList.dump()
      .then((_) {
        completer.complete(tag);
      })
      .catchError((e, trace) {
        PinUtility.handleError(e, trace);
        completer.completeError(e, trace);
  
negative.code:       postList.update(post);
        return new Future.value(true);
      })
      .then((_) => completer.complete(post)) // finish process
      .catchError((e, trace) {
        PinUtility.handleError(e, trace);
        completer.completeError(e, trace);
  