Issue
Right now if you're running postMessage({myProperty: 1}, '*'); the payload of the postMessage will actually be [object Object] (a String with this value).

What I did
I extracted the postMessage tests to another class since "Window2Test" already had enough tests and already counted up (Windows3Test also exists :))
Changing the payload in the Window.postMessage method actually did the trick.
I'm not 100% sure if Object is correct or if this needs to be specified down to something else though :-)

Potential improvements
I think we could rewrite the postMessage tests in general a bit ... didn't have time for that yet :-)