Sends data on the BC.
<div id=sendMsg>
  <button>click to send <span id=msgNumSpan><span>
  </button>
</div>
<script>
  let msgNum = 1;
  let bc = new BroadcastChannel("bfcacheTest");
  function update() {
    msgNumSpan.textContent = msgNum;
  }
  console.log("here");
  update();
  sendMsg.onclick = () => {
    console.log("sending", msgNum);
    bc.postMessage(msgNum++);
    update();
  }
</script>