Fixes tests.

Eventually we'll plumb positionX and positionY out so that developers can
control them.

Fixes the tests because previously we wouldn't draw any box decorations.

Add the ability to draw a foreground box decoration

- PageableList extends ScrollableList
One fixed width or height item is visible and centered at a
time. Fling and drag gestures scroll to the next/previous item.

- Scrollable.scrollTo(), Scrollable.scrollBy(), ensureWidgetIsVisible() API changed
The named animation parameter for these methods was replaced by
duration and curve. All of the methods now return a Future. The Future
completes when the scroll does.

This change eliminates the need for Scrollable to temporarily take ownership
of a ValueAnimation object (see #645).

- Using Future.then() instead of an AnimationPerformance status listener
In ensure_visible.dart _handleTap() uses ensureWidgetIsVisible() to
center the card roughly as before and then. When the implicit scroll
animation is complete, it changes the centered card's label font. The
change is made when the Future returned by ensureWidgetIsVisible()
completes.

- FixedHeightScrollable's itemHeight parameter is now itemExtent
If scrollDirection is ScrollDirection.vertical (the default) then itemExtent should
be the height of each item; otherwise it should be the width of each item.

Replaced _velocityForFlingGesture() in scrollable.dart with Scrollable._eventVelocity()
The original version clamped pixels/ms against pixels/sec constants. The new version
also deals with scrollDirection.

- Plumbed scrollDirection though FixedHeightScrollable and ScrollableList

Both classes should now support horizontal scrolling.

This feature lets you create effects similar to the "outline" property in CSS.

Pick a default theme brightness

... and reattach fitness to the build. We detached fitness from the build
because it was seeing an old version of playfair that didn't work with the most
recent version of the Sky package.

There doesn't seem to be any particular reason for us to not default the brightness to _something_.

Unhook fitness from the build to work around build break in playfair. We
updated the Sky package in an incompatible way, which means we need to roll in
a new playfair package to build fitness.

Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.

Split box.dart into many files

Sadly, box.dart has grown much longer than 1000 lines. This patch splits it up
into several files based on the class hierarchy. Fortunately, many of these
classes are loosely coupled to each other.

When given a null image, RenderImage should be as small as possible (isntead of
being NaNxNaN).

Also, plumb ImageFit and ImageRepeat through the various image widgets.

This CL inflates the padding of Container to account for the borders so that
the borders are allocated space in the layout and don't draw behind the
Container's child.

Request gridlines for fitness chart

We now fold the width and height properties into the constraints and try to
find a size for the image that perserves the image's intrinsict aspect ratio
while fitting within the given constraints.

Apply ImageFit and ImageRepeat to RenderImage

These properties should apply to foreground images as well as background
images. Also, rename these types from BackgroundFit and BackgroundRepeat
because they apply to things other than backgrounds.

Container should expand in dimensions without constraints

... as long as it doesn't have a child. If it has a child, it should size
according to that child. Essentially, if Container doesn't have a child, it
pretends like it has a child that expands to fill its constraints.

Adds bounce and elastic easing to animation curves

- Add Canvas.getSaveCount()
- Make RenderClipRect call context.paintChildWithClip instead of doing the clipping itself
- Make ClipLayer take a Rect instead of a Size
- Make PaintingContext.canvas read-only
- Add PaintingContext.paintChildWithClip()
- Minor rearrangings of code and style tweaks

@collinjackson

This was the last instance of loading assets off of Github.

@abarth

Teach Asteroids Sounds how to load from AssetBundle

Trivial code changes for style and readability in animation/ directory.

Update READMEs for painting/ and animation/ to be up to date with current dependencies.

Improve style in lerp.dart and box_painter.dart