src/fs.js
function _addReadCallback(filename) {
  return new Promise((resolve, reject)=> {
    readFileCallbacks.get(filename).add((err, data)=> {
      if (err) return reject(err);
      return resolve(data);
    });
  });
}
function _addReadCallback(filename) {
  return new Promise((resolve, reject)=> {
    readFileCallbacks.get(filename).add((err, data)=> {
      if (err) return reject(err);
      return resolve(data);
    });
  });
}
function _addReadCallback ( filename ) { return new Promise ( ( resolve , reject ) => { readFileCallbacks . get ( filename ) . add ( ( err , data ) => { if ( err ) return reject ( err ) ; return resolve ( data ) ; } ) ; } ) ; }
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.
Add a callback for reading of given file .