Add some stricter analysis (#64)

- Enable errors that would fail internally
- Add `comment_references` lint - this makes DartDocs output more usable
  and there isn't any reason to have unclickable references.
- Add `prefer_typing_uninitialized_variables` lint - this prevents some
  unintentionally dynamic behavior.
- Disable implicit casts.
- Use function type syntax for the ManuallyClosedWatcher factory.
- Remove some unused utilities.