visitor = function (child) {
                    if (watchedRoot.filter(child.name, child.parentPath)) {
                        if (child.isDirectory || child === watchedRoot.entry) {
                            entriesToWatch.push(child);
                        }
                        return true;
                    }
                    return false;
                }
Helper function to watch or unwatch a filesystem entry beneath a given
watchedRoot.

@private
@param {FileSystemEntry} entry - The FileSystemEntry to watch. Must be a
     non-strict descendent of watchedRoot.entry.
@param {WatchedRoot} watchedRoot - See FileSystem._watchedRoots.
@param {function(?string)} callback - A function that is called once the
     watch is complete, possibly with a FileSystemError string.
@param {boolean} shouldWatch - Whether the entry should be watched (true)
     or unwatched (false).