anchor.diff:      tagList.remove(uuid, postList);
   
       tagList.dump()
  -    .then((_) {
  -      completer.complete(true);
  -    })
  +    .then((_) => completer.complete(true))
       .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);
  