... 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.

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.

Add package:sky/rendering.dart

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.

RenderImage should handle the case of a null image

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.

Avoid warnings on iOS due to surface resize events not being handled

Borders on Containers shouldn't overlap the content

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

Improve RenderImage constraint solving

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.

Set default active arch on iOS to armv7

Fix crashes on iOS due to EXC_ARM_DA_ALIGN

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.

Fixes loading of assets in demo game

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

Port clipping to the new layer world.

- 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

Make Asteroids.apk use background music from its bundle

Fix analyzer to not change the working directory.