add unittest for core_collapse

related to #41

I read this issue and thought I could take a stab (we had already several tests ported for polymer-elements).

First I tried the test for `<core-ajax-dart>` but I wasn't able to get anything but `<debug target crashed>`.
This test is checked in this test to my cloned repo.
 
The test for `<core-collapse>` works fine so far.

If this is what you want, I can add/port the other tests too.
for these async tests, the unittest framework now supports that if you return a 'future' it will wait until the future completes. That means that you can flatten the code a bit and delete `done` below. For example, it could look something like this:

```dart
...
test('core collapse', () {
  ...
  return Polymer.onReady
      .then(_nextMicrotask)
      .then((_) => expect(c.opened, isTrue))
      .then(_delay)
      .then((_) {
          var h = ....;
          expect(h, ...);
          c.opened = false;
      })
      .then(_delay)
      .then((_) => expect ...)
      .then(_delay)
      .then((_) => expect ...);
  });
});

...

Future _nextMicrotask(_) => new Future(() {});
Future _delay(_) => new Future.delayed(200, () {});
```