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>`.
I 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.
Ah, good point - keeping it close to the JS does help make it easier to update later on. Let's keep it that way then. Thanks again!