To make it easier to avoid pushing twice in one frame, provide a
transaction mechanism for the navigator.

This package contains WidgetTester, which is very useful when writing tests for
widgets.

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.

- Change RouteArguments to pass the route's BuildContext rather than
  the Navigator. This caused the bulk of the examples/ and .../test/
  changes (those are mostly mechanical changes). It also meant I could
  simplify Navigator.of().

- Make initState() actually get called when the State's Element is in
  the tree, so you can use Foo.of() functions there. Added a test for
  this also.

- Provide a RouteWidget so that routes have a position in the Widget
  tree. The bulk of the route logic is still in a longer-lived Route
  object for now.

- Make Route.setState() only rebuild the actual route, not the whole
  navigator.

- Provided a Route.of().

- Provided a Route.writeState / Route.readState API that tries to
  identify the clients by their runtimeType, their key, and their
  ancestors keys, up to the nearest ancestor with a GlobalKey.

- Made scrollables hook into this API to track state. Added a test to
  make sure this works.

- Fix the debug output of GestureDetector and the hashCode of
  MixedViewport.

- Fixed ScrollableWidgetListState<T> to handle infinite lists.