The source code

    /*!
     * Ext JS Library 3.4.0
     * Copyright(c) 2006-2011 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    /**
     * @class Ext.menu.Separator
     * @extends Ext.menu.BaseItem
     * Adds a separator bar to a menu, used to divide logical groups of menu items. Generally you will
     * add one of these by using "-" in you call to add() or in your items config rather than creating one directly.
     * @constructor
     * @param {Object} config Configuration options
     * @xtype menuseparator
     */
    Ext.menu.Separator = Ext.extend(Ext.menu.BaseItem, {
        /**
         * @cfg {String} itemCls The default CSS class to use for separators (defaults to "x-menu-sep")
         */
        itemCls : "x-menu-sep",
        /**
         * @cfg {Boolean} hideOnClick True to hide the containing menu after this item is clicked (defaults to false)
         */
        hideOnClick : false,

        /**
         * @cfg {String} activeClass
         * @hide
         */
        activeClass: '',

        // private
        onRender : function(li){
            var s = document.createElement("span");
            s.className = this.itemCls;
            s.innerHTML = " ";
            this.el = s;
            li.addClass("x-menu-sep-li");
            Ext.menu.Separator.superclass.onRender.apply(this, arguments);
        }
    });
    Ext.reg('menuseparator', Ext.menu.Separator);