Finished polyfill.js

if (document.body.animate && typeof Animation === 'undefined') {
  window.Animation = document.body.animate({}).constructor;
}

if (typeof Animation !== 'undefined' && Animation.prototype.finished === undefined) {
  Object.defineProperty(Animation.prototype, 'finished', {get: function () {
    if (this.__finishPromise) return this.__finishPromise;
    if (this.playState === 'finished') {
      this.__finishPromise = Promise.resolve();
    } else {
      this.__finishPromise = new Promise(function (resolve) {
        this.addEventListener('finish', resolve, {once: true});
      }.bind(this));
    }
    return this.__finishPromise;
  }});
}

Test: http://jsbin.com/qiciyuboqo/edit?html,js,output

Thanks Šime Vidas for the Chrome fix :)