First unit tests: adding tests for hitmap.dart

This starts to address #8. This PR includes tests for all three public methods in `hitmap.dart`. I wish I could use more features of the mock library, but the current state of isolates kind of prevents things like `.verify(happenedOnce)` etc.

The one test file has quite a few helper methods, so its a little boiler-plate heavy. Let me know what you think.