Refactor Dismissable to not need a SizeObserver and to generally tighten
up the code a bit.

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.

There are no clients anymore. We added it for the Drawer a while ago,
which is a use case that's now better covered by AnimatedModalBarrier,
complete with semantics.

This patch includes documentation for transitions.dart and for
scrollable.dart.

tweak from review

These should be the last references to the old "performance" name.

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.

There should be no more uses of Performance or AnimatedValue in the framework
or the examples.

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

Providing a pre-built child is more efficient because we don't need to rebuild
the child every tick of the animation.

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

Also, clean up the class hierarchy for AnimationController now that
we've renamed progress to value. That means everything in the hierarchy
now has a value, include Watchable. This patch renames Watchable to
Animated<T>, which lets us use that type almost everywhere.

I've added some ducktape to modal bottom sheets to avoid having to
refactor all of Navigator to use AnimationController. I'll remove the
ducktape in the next patch.

This patch removes state from the animation system, which was causing problems
as we were scaling the use of animated values.

Now the "tween" objects are stateless and can watch animations, which creates a
new object that holds both the tween and the animation instead of mutating the
tween every tick of the animation.

This patch ports one client as a proof-of-concept.

Fixes #215

This will allow AnimatedPositioned to reuse all the same logic.

- Add RenderFractionalTranslation, a render box that does a
  translation based on a FractionalOffset.

- Make FractionalOffset more like Offset
  - dx/dy instead of x/y
  - add /, ~/, %
  - add .zero

- Add alongOffset and alongSize to FractionalOffset so that you can
  easily apply FractionalOffset to Offsets and Sizes. (Better name
  suggestions welcome.)

- Add transformHitTests boolean to RenderTransform (also on
  RenderFractionalTranslation), and to classes based on it.

- Remove the fade from Dismissable. We can add it back using the
  builder-with-child pattern like Draggable if we need it. See #1003
  for tha feature request.

- Rename a bunch of variables in dismissable.dart.

- Change the test for dismissable to not handle leftwards dismisses
  one pixel different from rightwards dismisses, and cleaned up the
  resulting effect on the test (mostly making sure we had the right
  number of pumps, with comments explaining what each one was).

Fixes #174.

Sometimes you want to just make a transition driven straight from the
performance's progress, rather than with an explicit variable.

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.

A transition that rotates its contents.

Add a AnimatedRelativeRectValue class for animating RelativeRects.

Add a PositionedTransition class for animating Positioned using
AnimatedRelativeRectValues.

Add a test for PositionedTransition.

Fix a math bug a RelativeRect found by the test.

Fix a logic bug in the two ParentDataWidget classes found by the test.
Specifically, they were marking the child dirty, rather than the parent.
The parentData is for the parent's layout, not the child's, so they have
to mark the parent dirty. (I didn't hoist this up to the superclass
because ParentData could be used for painting, hit testing,
accessibility, or any number of other things, and I didn't want to bake
in the assumption that it needed markNeedsLayout.)

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

These are now part of material.dart.

In the vast majority of cases, folks should be interacting with the Widget
rather than its State.

Fixes #267

Fixes #1170

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

We were just missing a few type casts in tests to make the analyzer happy.

This fixes #1103.

- Add documentation for AnimationTiming.
- typo: defaules -> defaults.
- added type information to isWatching() signature.
- made Widget.toStringName() include more useful information.
- cleaned up StatefulComponent._sync(): more specific signature, change
  redundant if to else, remove redundant cast.
- change order of TransitionBase arguments for consistency.
- prevent TransitionBase from affecting the performance in its
  constructor when it didn't create it (but see #1103).
- remove TODO() from @mpcomplete... no, there is not currently a better
  way to inherit a constructor, unfortunately.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

Stop exporting framework.dart from basic.dart, since we now have widgets.dart exporting all of framework.dart.