<script>
function xhr_send(method, data) {
  return new Promise(function(resolve, reject) {
      var xhr = new XMLHttpRequest();
      xhr.onload = function() {
        resolve();
      };
      xhr.onerror = function() {
        reject('XHR must succeed.');
      };
      xhr.responseType = 'text';
      xhr.open(method, './dummy?test', true);
      xhr.send(data);
    });
}

window.addEventListener('message', function(evt) {
    var port = evt.ports[0];
    xhr_send('POST', 'test string')
      .then(function() { port.postMessage({results: 'finish'}); })
      .catch(function(e) { port.postMessage({results: 'failure:' + e}); });
  });
</script>