fix final

@kevmoo

Hey Kevin, thanks for the pull request! I know you're super committed to `const` and `final` :)

Here's a tweak to your PR, which addresses the concerns that @ussuri raised. I don't believe you were assigning the ctor param values into the instance fields.

And, the tests do run. To run them locally, run `./grind mode-test`. The next time you run the app, you'll see a 'Run Tests...' menu off the overflow menu.

You can also run them from the command line. `dart tools/test_tunner.dart --dartium`.