Fixes https://github.com/flutter/flutter/issues/4910

* Change how navigator prevents redundant operations

Instead of requiring transactions, we now cancel all active pointers that are
interacting with the navigator and absorb future pointers until we get a chance
to build. This approach isn't perfect (e.g., events that trigger off the
cancelled pointers could still interact with the navigator), but it should be
better than the current transaction-based approach.

Fixes #4716

* Remove openTransaction

* test

* fixup

Turns out we weren't managing focus correct between navigator routes because we
were missing a Focus widget above the routes. However, adding this widget
caused us to explode at startup because the initial route was trying to move
focus during the build phase.

This patch teaches Focus to have an initiallyFocusedScope, which can be use to
initialize the child focus scope.

Fixes #4065

Also:
* Make PaginatedDataTable able to scroll itself horizontally.
* Make drop down buttons support having an explicit text style and icon
  size given.
* Fix a bug with drop-down buttons asserting when opened partly
  off-screen.
* Make sure to pop the drop-down button's route if the drop-down button
  is discarded while the route is up.
* Remove extraneous padding on drop-down buttons. (Couldn't figure out
  why it was there, and it breaks alignment when a drop-down is mixed
  with other text.)
* Some docs improvements.
* Add Route.isActive
* Add a setState() method to ModalRoutes.

Also, add more dartdoc to widgets.dart.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

This makes WidgetError more like RenderingError, which will aid with https://github.com/flutter/flutter/issues/2356.

Fixes https://github.com/flutter/flutter/issues/2443

Making progress documenting the widget library.

Make ancestorStateOfType() and ancestorRenderObjectOfType() support
subclassing.

Fixes https://github.com/flutter/flutter/issues/1087

Scaffold calls this to see if it should show a back arrow when there's
no drawer. With this change, everything continues to work.

Fixes styled_text and probably others.

Replace didPushNext() and didReplaceNext() with didChangeNext(), and
call it in more cases, so that a route can easily track the next route.

Use this to make TransitionRoute properly track its next route so that
you can do next-route-driven animations that work even with removes,
replaces, and other crazy manipulations of the navigator stack.

This was already supported in the underlying classes but somehow not
exposed by Navigator itself.

Instead, it should enter the world fully formed.

Also:
 - minor code reindents in places.
 - reset the widget tree between tests.
 - once you generate a route, don't let its builder change
   (previously it would keep changing as the routes table changed).
 - revert the stocks app toolbar-fading-on-forward-transition thing.

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

* NavigatorState.replaceRouteBefore()
* NavigatorState.removeRouteBefore()
* NavigatorState.popUntil()

This lets you dynamically replace one route with another.

* 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.

Now you can supply a route argument to flutter start:

  $ flutter start --route /foo

which will start the app at /foo instead of /.

Now we only use it for things related to ModalRoute and ModalBarrier.

(This is easy now that ephemeral routes are gone, so there's no other
kind of route to distinguish against.)

- Removed the concept of ephemeral routes.
- Renamed the two _MenuRoutes to _PopupMenuRoute and _DropDownRoute.
- Added type arguments in various places:
  - DropDownMenu
  - _DropDownRoute
  - _ModalBottomSheetRoute
  - PopupMenuItem
  - _PopupMenu
  - _PopupMenuRoute
- Made _ModalBottomSheetRoute, the two ex _MenuRoutes, and _DialogRoute
  all inherit from ModalRoute, via PopupRoute.
- Change "Dropdown" and "DropDown" to "DropDown" consistently.
- Made MaterialPageRoute inherit from PageRoute.
- Made ModalBarrier not create a box if it's always transparent.
- Exposed the Futures on TransitionRoutes.
- Fixed that menus were no longer dismissable by tapping the modal
  barrier.

These end up not actually being used, currently, because we don't have
generic methods, which you'd need for showDialog() and friends, and we
don't have any way to parameterise a class type at runtime, which you'd
need for MaterialApp routes, but it's a step in the right direction.

- `Scaffold.of(context).showBottomSheet(widget);`
- Returns an object with .closed Future and .close() method.
- Uses a StateRoute to handle back button.
- Take the Navigator logic out of the BottomSheet widget.
- Support showing a sheet while an old one is going away.
- Add Navigator.remove().

"showSnackBar()" is now a feature of a Scaffold. To get to a Scaffold
you either use a global key (`scaffoldKey.currentState.showSnackBar(...)`),
or you use `Scaffold.of(context)`.

Snack bars no longer have a route. They are entirely managed by the
Scaffold. Fixes #432.

Snack bars now queue up when you have several of them. Fixes #374.

Snack bars now auto-size themselves around their contents. This is step
one towards implementing multiline snack bars.

Snack bars now self-dismiss after some per-snackbar configurable period.

The self-dismissing pauses while a dialog is up above the snackbar (or
anything that uses ModalRoute). To enable this, there's now a
`ModalRoute.of(context)` API that returns the current ModalRoute, and
you will be rebuilt if you asked for this and the route's "current"
status changes. To implement this, the Navigator now rebuilds
unconditionally any time it pushes or pops a route.

Snack bars now use the curves that Android uses for snack bars.

Snack bar contents now fade in.

I'm planning on adding more .of() functions and so to avoid further
copypasta I'm providing some general utility functions here.

This makes HeroController ignore StateRoutes when deciding where to
animate heroes to and from.