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.
I tried to be consistent with double quotes but
I myself prefer single quotes - this is the reason I fail often with double quotes - see above.

From a comment on another PR I understand that you prefer single quotes too.

Should I change " to ' or did I misunderstand the comment?