static isDummy_(stream) {
    const ContentType = shaka.util.ManifestParserUtils.ContentType;
    switch (stream.type) {
      case ContentType.TEXT:
        return !stream.language;
      case ContentType.IMAGE:
        return !stream.tilesLayout;
      default:
        return false;
    }
  }
@param {T} stream
@return {boolean}
@template T
Accepts either a StreamDB or Stream type.
@private