code: function _sortDirectoryContents(contents, dirsFirst) {
          if (dirsFirst) {
              return contents.keySeq().sort(_buildDirsFirstComparator(contents));
          } else {
              return contents.keySeq().sort(FileUtils.compareFilenames);
          }
      }
docs: @private
  
  Sort a directory either alphabetically or with subdirectories listed first.
  
  @param {Immutable.Map} contents the directory's contents
  @param {boolean} dirsFirst true to sort subdirectories first
  @return {Immutable.Map} sorted mapping