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.

Fixes #173.

- 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.

These docs cover some undocumented enums and some of the basic layout widgets.

- drag and drop was using the wrong draggables (long press vs short
  press).

- navigation.dart wasn't using material correctly and so was ending up
  with some red text.

- piano wasn't followig our style guide even a little.

- date picker was not scrollable.

Fixes #1807

We still use special-purpose typedefs in the gesture code for symmetry with
other gesture callbacks.

Fixes #1827

The style we use for callbacks in widgets is "onFoo". These classes were using
an order naming convention and just called their callbacks "callback".

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

This patch simplifies AnimationTiming and all the AnimatedValue base classes.
Also, make PopupMenu a stateless component because it has no state.

Fixes #1168

Fixes #1170

We already forced hitting end on 1.0.

Fixes #1358

This fixes #1103.

This fixes an issue in the stocks app in horizontal mode where you could both
scroll and drag the drawer at the same time.

Move _activeCardDragEndPoint near build() so it's more obvious that it
is part of the build state.

Make a couple of functions use setState() since they modify variables
that are used by build().

Add a more detailed comment to the empty setState() call, since those
are dubious in general and need explaining when they occur, lest people
start using them as magic incantations to Make Things Work.

Dismissable now only depends on GestureDetector.

Added a unit test that verifies that issue #1068 has been fixed. It's commented out for now.

Cleaned up VelocityTracker.cc et al a little.

That way the fling engages in the same direction as the scroll. For example, if
you have a horizontal scroll nested inside a vertical scroll, the fling will
take place in the same direction as the scroll.

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.

```
enum DismissDirection {
  vertical,
  horizontal,
  left,
  right,
  up,
  down
}
```

To only enable dismissing to the right create the `Dismissable` with `direction: DismissDirection.right`. By default direction is `DismissDirection.horizontal` (left or right).

Updated the card_collection "Swipe Away" demo with a drawer that can be used to select one of the three X axis dismiss directions. Currently the MixedViewport class doesn't support horizontal scrolling, so the demo doesn't support the X axis dismiss directions.

Alternating scroll gestures would sometimes be ignored because _ScrollGestureRecognizer didn't always reset its _state when the pointer[s] went up.

A Dismissable dismiss triggered by a drag and then a fling could cause the next attempt to drag-dimiss to fail.

Fixed the definition of lerpColor().

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.

Convert Dismissable to use the ScrollStart, ScrollUpdate, and ScrollEnd gestures. Support for fling gestures is TBD.

Included a basic unit test that checks that one item can be dismissed with a press-drag-release gesture.

Fixed the scroll gesture recognizer: if the last pointer goes up and candidate recognizers still exist, then reject the gesture.

Just infer it from the sign of the velocity. That's what odeon does.

It was confusing to have both widget.dart and widgets.dart

This introduces the notion of event disposition and allows event
targets (widgets and render objects) to consume events that should not
be processed further. This is needed by the Switch component in the
Drawer in the stocks example. The Switch is embedded in a DrawerItem.
The Switch handles the gesture tap event to toggle its state and should
handle pointer events to allow swiping and draw its own radial
reaction. The DrawerItem also handles gesture taps to allow toggling
the switch value when tapping anywhere on the drawer and to draw its
own ink splash. When tapping on the switch, both the switch's render
object and the DrawerItem's listener are in the event dispatch path.
The Switch needs to signal in some fashion that it consumed the event
so the DrawerItem does not also try to toggle the switch's state.