textStream(uri) {
    goog.asserts.assert(this.manifest_,
        'A top-level generated Manifest is required to use this method!');

    const duration = this.manifest_.presentationTimeline.getDuration();
    this.segmentIndex =
        this.manifest_.shaka_.media.SegmentIndex.forSingleSegment(
            /* startTime= */ 0, duration, [uri]);
  }
Sets the current stream to use the given text stream.  It will serve a
single media segment at the given URI for the entire presentation.

@param {string} uri