Function removeClass

vue.js:10877–10908

* Remove class with compatibility for SVG since classList is not supported on * SVG elements in IE *删除与SVG兼容的类，因为不支持类列表 * IE中的SVG元素 删除真实dom的css类名

(el, cls)

     删除真实dom的css类名
     */
    function removeClass(el, cls) {
        /* istanbul ignore if */
        if (!cls || !(cls = cls.trim())) {
            return
        }

        /* istanbul ignore else */
        if (el.classList) {
            if (cls.indexOf(' ') > -1) {
                cls.split(/\s+/).forEach(function (c) {
                    return el.classList.remove(c);
                });
            } else {
                el.classList.remove(cls);
            }
            if (!el.classList.length) {
                el.removeAttribute('class');
            }
        } else {
            var cur = " " + (el.getAttribute('class') || '') + " ";
            var tar = ' ' + cls + ' ';
            while (cur.indexOf(tar) >= 0) {
                cur = cur.replace(tar, ' ');
            }
            cur = cur.trim();
            if (cur) {
                el.setAttribute('class', cur);
            } else {
                el.removeAttribute('class');
            }
        }
    }

    /*
    *