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 run into https://code.google.com/p/dart/issues/detail?id=19938
but it's redundant now because I moved the coreMediaChangeHandler to the model class according to your suggestion.