function helperCallback(err) {
                if (--counter === 0) {
                    callback(null);
                }
            }
Private helper function for FileSystemEntry.visit that requires sanitized options.

@private
@param {FileSystemStats} stats - the stats for this entry
@param {{string: boolean}} visitedPaths - the set of fullPaths that have already been visited
@param {function(FileSystemEntry): boolean} visitor - A visitor function, which is
     applied to descendent FileSystemEntry objects. If the function returns false for
     a particular Directory entry, that directory's descendents will not be visited.
@param {{maxDepth: number, maxEntriesCounter: {value: number}, sortList: boolean}} options
@param {function(?string)=} callback Callback with single FileSystemError string parameter.