onStatusChange_() {
    const currentStatus = this.xhrReader_.getStatus();
    const EventType = NodeReadableStream.EventType;

    switch (currentStatus) {
      case XhrStreamReaderStatus.ACTIVE:
        this.doStatus_(EventType.READABLE);
        break;

      case XhrStreamReaderStatus.BAD_DATA:
      case XhrStreamReaderStatus.HANDLER_EXCEPTION:
      case XhrStreamReaderStatus.NO_DATA:
      case XhrStreamReaderStatus.TIMEOUT:
      case XhrStreamReaderStatus.XHR_ERROR:
        this.doStatus_(EventType.ERROR);
        break;

      case XhrStreamReaderStatus.CANCELLED:
        this.doStatus_(EventType.CLOSE);
        break;

      case XhrStreamReaderStatus.SUCCESS:
        this.doStatus_(EventType.END);
        break;
    }
  }
Handles any state changes from XHR.

@private