The test has:

  try {
    postMessage(1, null);
  } catch(e) {
    postMessage(''+e);
  }

and asserts that 1 is posted.

But the spec for postMessage is:

  void postMessage(any message, optional sequence<Transferable> transfer);

and passing null to the second argument of that should throw, per webidl.

I recommend we do three things:

1)  Add a copy of this test that replaces the first postMessage line with:

     postMessage(1);

2)  Add a copy of this test that replaces the first postMessage line with:

     postMessage(1, undefined);

3)  Modify this test to check that postMessage(1, null) in fact throws.

#1 and #2 would test correct handling of the optional argument when it's not actually passed and #3 would test correct handling of null here.