We use the term "modal" to mean two related but not identical things:

• Routes that go into the navigator's "modal" list (as opposed to the ephemeral list); this is exposed in particular on the NavigatorObserver "didPushModal" method. These could be ModalRoute subclasses but need not be (in particular, StateRoute isn't).
• Routes that subclass ModalRoutes (which could be modal or ephemeral).

We should clean that up.