code: function setupBackdrop() {
          if (!opts.backdrop) return angular.noop;
  
          opts.backdrop.on('click', onBackdropClick);
  
          return function() {
            opts.backdrop.off('click', onBackdropClick);
          };
        }
docs: Sets up the backdrop and listens for click elements.
  Once the backdrop will be clicked, the menu will automatically close.
  @returns {!Function} Function to remove the backdrop.