other iframe. If the first
    // BroadcastChannel message is received by `bc1`, or if the second
    // BroadcastChannel message is received by `bc1` or `bc2` in the first
    // iframe, then the test should fail.

    window.addEventListener("message", e => {
      if(e.data == "iframe1-done") {
        let iframe2 = document.createElement("iframe");
        iframe2.src = iframe_src(channel_name, "iframe2");
        t.add_cleanup(() => { document.body.removeChild(iframe2) });
        document.body.appendC