* Split didPush() into didPush() and install(), so that we can install
  the overlays without triggering the push logic. This will be used in a
  subsequent patch to implement route replacement.

* Split didPop() into didPop() and dispose(), so that we can remove
  overlays without triggering the pop logic. Also for a subsequent patch
  that implements replacement.

* Clean up _navigator on the routes when the Navigator itself is
  disposed.

* Drop the forwarding logic on willPushNext() -- now didPushNext() --
  and didPopNext(), since we no longer have StateRoutes to get in the
  way.

* Implement isCurrent more broadly and without having to keep track of
  state.

* Provide some toString()s on NamedRouteSettings and ModalRoutes.

* Make OverlayState.initState() use the insertAl functionality.

* Make OverlayRoute.builders abstract since that way you'll catch when
  you forget to do it. If you don't want overlays, don't inherit from
  this class.

* Made handleStatusChanged() on TransitionRoute public so that it can be
  overridden by subclasses.