static createMockStream(type, id) {
    return {
      audio: shaka.test.StreamingEngineUtil.createMockAudioStream,
      video: shaka.test.StreamingEngineUtil.createMockVideoStream,
      trickvideo: shaka.test.StreamingEngineUtil.createMockVideoStream,
      text: shaka.test.StreamingEngineUtil.createMockTextStream,
    }[type](id);
  }
Creates a mock Stream of the given type.

@param {string} type
@param {number} id
@return {shaka.extern.Stream}