add test for core-media-query

I tried to do the test `core-media-query` within the main browser window but I learned that it is not supported to resize the current window (except once in onLoad) and I wasn't able to find another way to make `core-media-query` fire a change event, events so I went with the dialog window.
The test runs fine so far, didn't even need a `new Future.delayed` for synchronization.
minor nit from our styleguide: we add a space after the if (`if (e.data...)`)

(here and the following ifs below)