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.

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

When introducing Focus widgets for the Drawer (and ModalRoutes), we weren't
actually giving them the focus. Now we move the focus scope when pushing modal
routes.

Fixes #184

Some of the Animation classes that we converted from performances use
the term "master" to refer to the animation upon which they're based.
This patch changes them to use the term "parent", which is consistent
with the rest of the animation classes.

This patch removes Performance and AnimationValue now that we've ported the
framework over to AnimationController and Tween. This patch also cleans up the
names of the AnimationController classes now that they don't have to avoid
conflicts with the old animation API. Specifically, I've made the following
renames:

 * Animated -> Animation
 * Evaluatable -> Animatable
 * PerformanceStatus -> AnimationStatus

This patch is just renames and moving code around. There aren't any changes in
behavior.

I've left transitions and enter_exit_transition out of this patch, but I've
converted the rest.

This patch moves Navigator and related code over to using
AnimationController.

Make ancestorStateOfType() and ancestorRenderObjectOfType() support
subclassing.

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

 - Adds dartdoc for all the `of` functions.
 - Renames Image to RawImage. This widget is rarely used and shouldn't take up
   such a nice global name.

Fixes #361

Now we just use the state from the Route.

Fixes #744

Now we ignore the pointer after we start popping the route.

Fixes #686

Now use use the route's getPosition function to position the drop-down menu.
Also, fix a number of other related bugs that blocked the dropdown button from
working correctly. The dropdown menu still has the following issues:

1) In the exit animation, the background of the menu disappears too quickly
   because of incorrect paint bounds computations in the layer tree.
2) The drop down menu isn't positioned correctly after the device rotates.
   We'll need to address this issue in a separate patch.

Fixes #630

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.

Also, replace places that use the term "create performance" with
"create performance controller".

Instead of separate functions for the 0->1 and 1->2 transitions, just
have one function.

So you don't fade out when going to a popup menu, for example.

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.

This patch restructures how we handle drawer. The drawer is now a child of the
Scaffold, which wraps the Drawer in a DrawerController. The DrawerController
manages the interaction with the navigator as well as the edge swiping. The
DrawerController's state machine is driven almost entirely off its Performance,
which it now owns completely.

Fixes #90
Fixes #187
Fixes #192
Fixes #194
Fixes #604

For those times when you want to do something as you move away from a
route into the next one, as well as when you move into it from the
previous one.

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

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.

"Fixes" #389.

- 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.

Rather than have delayed calls to super.didPop(), which raises my
eyebrow every time I see it, this provides a separate finished()
function to call, and uses the convention that if you want to call it
yourself, you just don't call super.didPop().

Previously, we were putting a ForcedLayer just below the overlay, but that
causes trouble for routes like the popup menu that want to position themselves
inside the overlay. Instead, I've moved the ForcedLayer down into ModalRoute.

Also, rename ForcedLayer to RepaintBoundary, which is more descriptive of what
this widget does.

Fixes #485

"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.

Now we use a ForcedLayer in the navigator overlay to cache the recording for
each entry in the overlay. This mechanism just caches the display list, not the
underlying pixels.

Also, remove the "dispose" notification in the Layer tree because it was
disposing layer too eagerly. We don't actually need this notification for
anything other than eagerly freeing some C++ memory.