Function addClass

vue.js:10846–10867

* Add class with compatibility for SVG since classList is not supported on * SVG elements in IE * *添加与SVG兼容的类，因为不支持类列表 * IE中的SVG元素 *为真实dom 元素添加class类

(el, cls)

     *为真实dom 元素添加class类
     */
    function addClass(el, cls) {
        /* istanbul ignore if */
        if (!cls || !(cls = cls.trim())) {
            return
        }

        /* istanbul ignore else */
        if (el.classList) { //如果浏览器支持classList
            if (cls.indexOf(' ') > -1) {
                cls.split(/\s+/).forEach(function (c) {
                    return el.classList.add(c);
                });
            } else {
                el.classList.add(cls);
            }
        } else { //不支持classList  直接用字符串拼接
            var cur = " " + (el.getAttribute('class') || '') + " ";
            if (cur.indexOf(' ' + cls + ' ') < 0) {
                el.setAttribute('class', (cur + cls).trim());
            }
        }
    }

    /**
     * Remove class with compatibility for SVG since classList is not supported on