_start(props, isSpring) {
    const animation = isSpring ? new SpringAnimation(
    /** @type SpringAnimationProps */
    props) : new CSSAnimation(
    /** @type CssAnimationProps */
    props);
    this.activeAnimations.push(animation);

    animation.onFinish = () => this.stop(animation);

    return animation;
  }
@private
@param {AnimationProps} props
@param {boolean} [isSpring]
@returns {Animation}