 test but with data URL dedicated workers (which
    // should have opaque origins per the HTML spec).
    const worker_name_prefix = "data-url-dedicated-worker";
    const worker_1_name = `${worker_name_prefix}-1`;
    const worker_2_name = `${worker_name_prefix}-2`;

    const handler = e => {
      if(e.data == `${worker_1_name}-done`) {
        const worker2 = new Worker(data_url_worker_src(channel_name, worker_2_name));
        t.add_cleanup(() => worker2.terminate());
        worker2.addEventListener(