code: function _addReadCallback(filename) {
    return new Promise((resolve, reject)=> {
      readFileCallbacks.get(filename).add((err, data)=> {
        if (err) return reject(err);
        return resolve(data);
      });
    });
  }
docs: Add a callback for reading of given file.
  
  @private
  @param {string} filename           File to set callback for.
  @returns {Promise.<Buffer|*>}      The file contents, once loaded.