The needsLayout getter is prone to misuse. See discussion on #3083.

1: If a route is already dismissed when it's popped, there's no point
trying to animate heroes, because it's going to be gone before the
heroes code can look at it.

2: If a hero animation finishes just as a new one is starting, we
previously blew away the state for the starting one. Now we correctly
segregate the "starting up quest" variables from the "actively ongoing
quest" variables.

Doesn't do any of the fancy effects. Just lets the user control the
back transition by sliding from the left, like a drawer. Hero
transitions are disabled during the gesture.

BUG=https://github.com/flutter/flutter/issues/4817

* Removed unnecessary Shrine hero logic

After this patch, they ignore pointers.

This patch improves the Post and Shrine transitions by making the AppBar
into a Hero and changing the default MaterialPageTransition. Now the
AppBar transitions smoothly between screens and the
MaterialPageTransition doesn't involve a fade effect.

Also, rejigger the bounds of the image header in Pesto to avoid the
"pop" at the end of the animation by laying out the image header at its
final visual size instead of relying on occlusion to size the image
header.

Fixes #5202
Fixes #5204

Fixes https://github.com/flutter/flutter/issues/5178

This patch includes a number of improvements:

 * Material page routes now put a repaint boundary inside their transition so
   they don't repaint during the transition.
 * Heroes that are on a quest now get a repaint boundary so we repaint them
   individually.
 * I've hoisted the transparent material for the product items up in the widget
   tree, which doesn't affect performance but makes the ink splashes reach the
   edge of the product cards.
 * I've changed the repaint rainbow visualization to make it easier to see
   what's going on.

* 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

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

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

This concept is now private to AnimationController. All the clients actually
want the AnimationStatus.

Somehow this actually made heroes way simpler.

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.

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.

- Remove unused FocusChanged typedef.
- Remove unused centerOfAttentionHeroTag.
- Modernize static functions for interacting with Scrollable by moving them
  into the Scrollable class.