static async getFirstVideoSegmentReference(manifest) {
    const variant = manifest.variants[0];
    expect(variant).not.toBe(null);
    if (!variant) {
      return null;
    }

    const video = variant.video;
    expect(video).not.toBe(null);
    if (!video) {
      return null;
    }

    await video.createSegmentIndex();
    const position = video.segmentIndex.find(0);
    goog.asserts.assert(position != null, 'Position should not be null!');

    const reference = video.segmentIndex.get(position);
    goog.asserts.assert(reference != null, 'Reference should not be null!');
    return reference;
  }
@param {shaka.extern.Manifest} manifest
@return {!Promise<shaka.media.SegmentReference>}