rce.canConstructInDedicatedWorker,
      'MediaSource.canConstructInDedicatedWorker');

  const video = document.createElement('video');
  document.body.appendChild(video);
  video.onerror = t.unreached_func('video element error');
  video.onended = t.step_func_done();

  let worker = new Worker('mediasource-worker-play.js');
  worker.onerror = t.unreached_func('worker error');
  worker.onmessage = t.step_func(e => {
    let subject = e.data.subject;
    assert_true(subject != undefined, 'message must have 