mdat_(streamInfo) {
    const Mp4Generator = shaka.util.Mp4Generator;
    const samples = streamInfo.data ? streamInfo.data.samples : [];
    const allData = samples.map((sample) => sample.data);
    const bytes = shaka.util.Uint8ArrayUtils.concat(...allData);
    return Mp4Generator.box('mdat', bytes);
  }
Generate a MDAT box

@param {shaka.util.Mp4Generator.StreamInfo} streamInfo
@return {!Uint8Array}
@private