gecko-dev/dom/workers/test/bug1132924_worker.js

onmessage = function() {
  var a = new XMLHttpRequest();
  a.open("GET", "empty.html", false);
  a.onreadystatechange = function() {
    if (a.readyState == 4) {
      postMessage(a.response);
    }
  };
  a.send(null);
};