findExistingStream_(id) {
    const real = (obj) => shaka.test.ManifestGenerator.realObj_(obj);

    for (const maybeVariant of this.variants) {
      const variant = real(maybeVariant);
      if (variant.video && real(variant.video).id == id) {
        return variant.video;
      }
      if (variant.audio && real(variant.audio).id == id) {
        return variant.audio;
      }
    }
    for (const maybeText of this.textStreams) {
      if (real(maybeText).id == id) {
        return maybeText;
      }
    }
    return null;
  }
Gets the existing stream with the given ID.
@param {number} id
@return {?shaka.extern.Stream}
@private