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: we commonly omit the type on closures like these and write it as `(e) { ... }`.

In this particualr case though, it seems that you don't use the event entirely, so you can use `(_) { ... }` instead, to indicate that the argument is not used.