Add tests for event dispatching

[tomalec]

So far we have none, even though it's a part of public API

[tomalec]

Currently, it's hard to achieve it with <test-fixture> as it does not provide any way to hook before element is connected PolymerElements/test-fixture#51

Alternatively, we can consider refactoring the code not to use <test-fixture> but stamp fixtures manually.

[tomalec]

Or fork it and add callbacks probably somewhere in https://github.com/PolymerElements/test-fixture/blob/master/test-fixture.html#L167