code: function _flush() {
      for ( const prop in _classes ) {
        if ( _classes.hasOwnProperty( prop ) &&
             _classes[prop] !== _classes.BASE_CLASS &&
             _dom.classList.contains( _classes[prop] ) ) {
          _dom.classList.remove( _classes[prop] );
        }
      }
    }
docs: Search for and remove initial BaseTransition classes that have
  already been applied to this BaseTransition's target element.