void close([DSError err = null ])

close the request from responder side and also notify the requester

Source 

void close([DSError err = null]) {
  if (err != null) {
    _err = err;
  }
  if (!pendingData.isEmpty) {
    pendingData.last.status = StreamStatus.closed;
  } else {
    pendingData.add(
      new _InvokeResponseUpdate(StreamStatus.closed, null, null, null)
    );
    prepareSending();
  }
}