HTML: assert ErrorEventInit defaults

This accounts for the change in whatwg/html#7983 as well.

html/webappapis/scripting/events/event-handler-processing-algorithm-error/document-synthetic-errorevent.html

@@ -21,10 +21,24 @@
   const eventWatcher = new EventWatcher(t, document, "error");
   const promise = eventWatcher.wait_for("error").then(e => {
     assert_equals(e.defaultPrevented, false);
+    assert_equals(e.message, "");
+    assert_equals(e.filename, "");
+    assert_equals(e.lineno, 0);
+    assert_equals(e.colno, 0);
+    assert_equals(e.error, undefined);
   });
 
   document.dispatchEvent(new ErrorEvent("error", { cancelable: true }));
 
   return promise;
 }, "error event is normal (return true does not cancel; one arg) on Document, with a synthetic ErrorEvent");
+
+test(() => {
+  const e = new ErrorEvent("error");
+  assert_equals(e.message, "");
+  assert_equals(e.filename, "");
+  assert_equals(e.lineno, 0);
+  assert_equals(e.colno, 0);
+  assert_equals(e.error, undefined);
+}, "Initial values of ErrorEvent members")
 </script>