drupal/core/misc/dialog/dialog.jquery-ui.js

/**
 * @file
 * Adds default classes to buttons for styling purposes.
 */
(function ($) {

  "use strict";

  $.widget('ui.dialog', $.ui.dialog, {
    options: {
      buttonClass: 'button',
      buttonPrimaryClass: 'button--primary'
    },
    _createButtons: function () {
      var opts = this.options;
      var primaryIndex;
      var $buttons;
      var index, il;
      for (index = 0, il = opts.buttons.length; index < il; index += 1) {
        if (opts.buttons[index].primary && opts.buttons[index].primary === true) {
          primaryIndex = index;
          delete opts.buttons[index].primary;
          break;
        }
      }
      this._super();
      $buttons = this.uiButtonSet.children().addClass(opts.buttonClass);
      if (typeof primaryIndex !== 'undefined') {
        $buttons.eq(index).addClass(opts.buttonPrimaryClass);
      }
    }
  });

})(jQuery);