- 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.

This introduces the concept of an Anchor, which you can use to link
transitions together. I've used this in the Fitness and Stocks apps to
link the FAB and SnackBar to animate together by sharing the
SlideTransition.

I also fixed the Scaffold hit testing code to apply sub-widget
transforms, so it works with Transformed nodes.

Also add intervals and curves to AnimationPerformance, which affect all
variables used by the performance.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

* Intentions => Behaviors
* SlideIn => SlideTransition
* FadeIn => FadeTransition
* removed unused AnimatedContainer intentions

Simplifies the code a bit.

Use those in SnackBar, Drawer, navigator instead of AnimatedContainer's
intentions.