LayoutTests/wasm/resources/frame-parent.html

<script>
function listener(event) {
  var mod = event.data;
  try {
    var i = new WebAssembly.Instance(mod);
    var ans = i.exports.calc(5);
    window.parent.postMessage(ans, event.origin);
  } catch (e) {
    window.parent.postMessage(e, event.origin);
  }
}

addEventListener("message", listener, false);

window.parent.postMessage("ready", "*");
</script>