Function addClass

static/vuejs/vue.js:6138–6157

* Add class with compatibility for SVG since classList is not supported on * SVG elements in IE

(el, cls)

 * SVG elements in IE
 */
function addClass (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.add(c); });
    } else {
      el.classList.add(cls);
    }
  } else {
    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