hild(iframe2);
      } else if(e.data == "iframe2-done") {
        resolve();
      } else if(e.data == "fail") {
        reject("One opaque origin received a message from the other");
      } else {
        reject("An unexpected error occurred");
      }
    });

    let iframe1 = document.createElement("iframe");
    iframe1.src = iframe_src(channel_name, "iframe1");
    t.add_cleanup(() => { document.body.removeChild(iframe1) });
    document.body.appendChild(iframe1);
    });
}, "BroadcastChannel messag