sendAsyncComplete_(senderId, id, error, res) {
    if (!this.player_) {
      // We've already been destroyed.
      return;
    }

    this.sendMessage_({
      'type': 'asyncComplete',
      'id': id,
      'error': error,
      'res': res,
    }, this.shakaBus_, senderId);
  }
Tell the sender that the async operation is complete.
@param {string} senderId
@param {string} id
@param {shaka.util.Error} error
@param {*} res
@private