static getAnimationEvent(optEl) {
    if (animationEvent && !optEl) {
      return animationEvent;
    }

    animationEvent = 'animationend';

    const el = optEl || document.createElement('ws');
    const animations = {
      'animation': 'animationend',
      'OAnimation': 'oAnimationEnd',
      'MozAnimation': 'animationend',
      'WebkitAnimation': 'webkitAnimationEnd'
    };
    const animationNames = Object.keys(animations);

    for (let i = 0, length = animationNames.length;
          i < length; i++) {
      const animationName = animationNames[i];

      if (typeof el.style[animationName] !== 'undefined') {
        animationEvent = animations[animationName];
        break;
      }
    }

    return animationEvent;
  }
Gets the prefixed animation end event.
@param {?Element} optEl Element to check
@return {string}