close(cb) {
    if (!this._stream) {
      return;
    }

    this._stream.end(() => {
      if (cb) {
        cb(); // eslint-disable-line callback-return
      }
      this.emit('flush');
      this.emit('closed');
    });
  }
Closes the stream associated with this instance.
@param {function} cb - TODO: add param description.
@returns {undefined}