code: function _fireReadFileCallbacks(filename, data, error=false) {
    const callbacks = readFileCallbacks.get(filename);
    if (callbacks) {
      if (callbacks.size) callbacks.forEach(callback=>error?callback(data, null):callback(null, data));
      callbacks.clear();
      readFileCallbacks.delete(filename);
    }
  }
docs: Fire any awaiting callbacks for given file data.
  
  @private
  @param {string} filename           The filename to fire callbacks on.
  @param {Buffer|Error} data         The received data.
  @param {boolean} [error=false]     Is this an error or file data?