code: async function _handleFileInCache(filename, cache) {
    if (cache.get(filename) !== true) return cache.get(filename);
    return _addReadCallback(filename);
  }
docs: Return cached file if already loaded or set a callback if currently loading/in-queue.
  
  
  @param {string} filename           File to get from cache.
  @param {Map} cache                 The cache to use.
  @returns {Promise.<Buffer|*>}      The file contents.