code: function addClasses(el, classes) {
    var newClasses = convertToArray(classes);
    var classList;
  
    if (el.className instanceof SVGAnimatedString) {
      classList = convertToArray(el.className.baseVal);
    } else {
      classList = convertToArray(el.className);
    }
  
    newClasses.forEach(function (newClass) {
      if (classList.indexOf(newClass) === -1) {
        classList.push(newClass);
      }
    });
  
    if (el instanceof SVGElement) {
      el.setAttribute('class', classList.join(' '));
    } else {
      el.className = classList.join(' ');
    }
  }
docs: Add classes to an element.
  This method checks to ensure that the classes don't already exist before adding them.
  It uses el.className rather than classList in order to be IE friendly.
  @param {object} el - The element to add the classes to.
  @param {classes} string - List of space separated classes to be added to the element.