play (startTime) {

    if (this.invert_ === null)
      throw new Error('invert() must be called before play()');

    if (typeof this.play_ === 'undefined')
      throw new Error('No player specified.');

    this.play_(startTime);
  }
Plays the animation.

@param {Number} [startTime] - The time the animation should start (using
 <code>window.performance.now</code> as the source of truth).