add a postMessage test

html/dom/dynamic-markup-insertion/opening-the-input-stream/tasks.window.js

@@ -9,4 +9,22 @@ async_test(t => {
   }, 200);
   // Ensure the load event fires and testharness doesn't timeout
   frame.contentDocument.close();
-}, "document.open() and tasks");
+}, "document.open() and tasks (timeout)");
+
+async_test(t => {
+  const frame = document.body.appendChild(document.createElement("iframe"));
+  let counter = 0;
+  frame.contentWindow.onmessage = t.step_func(e => {
+    assert_equals(e.data, undefined);
+    counter++;
+    alert(counter);
+    if (counter == 2) {
+      t.done();
+    }
+  });
+  frame.contentWindow.postMessage(undefined, "*");
+  frame.contentDocument.open();
+  frame.contentWindow.postMessage(undefined, "*");
+  // Ensure the load event fires and testharness doesn't timeout
+  frame.contentDocument.close();
+}, "document.open() and tasks (message)");