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.
Thinking more about this - we could move a lot of these definitions directly to the MyModel class, since it's only used for this test. For example

```dart
class MyModel extends Object with Observable {
  @observable
  String phoneQuery = '(min-width: 600px)';

  @observable
  String tabletQuery = '(max-width: 599px)';

  @observable
  bool matchesPhone;

  @observable
  bool matchesTablet;

  int _counter = 0;
  coreMediaChangeHandler() {
     window.postMessage(...);
     counter++;
  }
```