src/fs.js
function _addReadCallbacks(filename, cache) {
  const callbacks = new Set();
  cache.set(filename, true);
  readFileCallbacks.set(filename, callbacks);
  return callbacks;
}
function _addReadCallbacks(filename, cache) {
  const callbacks = new Set();
  cache.set(filename, true);
  readFileCallbacks.set(filename, callbacks);
  return callbacks;
}
function _addReadCallbacks ( filename , cache ) { const callbacks = new Set ( ) ; cache . set ( filename , true ) ; readFileCallbacks . set ( filename , callbacks ) ; return callbacks ; }
Add callbacks set for given file

@private
@param {string} filename     File to add for.
@param {Map} cache           Cache to use.
@returns {Set}               The callbacks.
Add callbacks set for given file