code: function _addReadCallbacks(filename, cache) {
    const callbacks = new Set();
    cache.set(filename, true);
    readFileCallbacks.set(filename, callbacks);
    return callbacks;
  }
docs: Add callbacks set for given file
  
  @private
  @param {string} filename     File to add for.
  @param {Map} cache           Cache to use.
  @returns {Set}               The callbacks.