PageRoute is now MaterialPageRoute.
This also changes the following:
- Now the HeroController is a Navigator observer, rather than a feature
of HeroPageRoutes, which are gone. This means heroes can work between
any kind of ModalRoute now.
- ModalPageRoute is moved from modal_barrier.dart to routes.dart.
- It allows routes to opt-out of their modal barrier being a shortcut to
popping the route.
- Features of PageRoute that aren't Material-specific get promoted to
ModalRoute features: storage, the subtree key, offstageness...
The AnimatedModalBarrier is still a ModalRoute feature.
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