code: function addClass( element ) {
    const addClassNamesArray = _sliceArgs( arguments );
    if ( hasClassList ) {
      element.classList.add.apply( element.classList, addClassNamesArray );
    } else {
      var classes = element.className.split( ' ' );
      addClassNamesArray.forEach( function( name ) {
        if ( classes.indexOf( name ) === -1 ) {
          classes.push( name );
        }
      } );
      element.className = classes.join( ' ' );
    }
  
    return element;
  }
docs: Add CSS class from an element.
  
  @param {HTMLNode} element - A DOM element.
  @param {string} className - CSS selector.
  @returns {HTMLNode} element - A DOM element.