Navigator.of(context).push(new StateRoute())
...is now:
ModalRoute.of(context).addLocalHistoryEntry(new LocalHistoryEntry())
StateRoute and LocalHistoryEntry are similar, except onPop is now onRemove and LocalHistoryEntry has a remove() method that causes the local history entry to be removed.

Route.didPop() and NavigatorState.pop() now return a boolean saying whether or not the route or navigator (respectively) handled the pop itself.

Other changes:

• Heroes now trigger on PageRoutes, not ModalRoutes.
• hasPreviousRoute is gone.
• NavigatorState.remove() is gone.

A subsequent patch will simplify the isCurrent stuff which is now overengineered.

Fixes #553.