<!DOCTYPE html>
<script>
  var x = 0;
</script>
<!-- This script's URI is:
  var xhr = new XMLHttpRequest();
  xhr.open('GET', 'data:text/plain,aaa', false);
  xhr.send();
  x=1;
  -->
<script defer src="data:application/javascript,var%20x%20=%200;%20var%20xhr%20=%20new%20XMLHttpRequest();%20xhr.open('GET',%20'data:text/plain,aaa',%20false);%20xhr.send();%20x=1"></script>

<!-- This script's URI is:
  parent.postMessage(x, '*');
-->
<script defer src="data:application/javascript,parent.postMessage(x, '*');"></script>