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

@abarth

Fix analyzer warnings

We had a remarkable number of analyzer failures.

I'll fix the bots to analyze across the whole project
in a follow-up patch, that should prevent this
in the future.

@abarth

The name `root` is confusing because this value isn't the root of anything.
It's just the associated `RenderObject` instance.

Add support for drag-and-drop

Widgets that want to receive drops should include a DropTarget in their build.
Currently there's no widget for initiating a drag. Components can use the
DragController directly. In the future, we'll probably want to add a Draggable
that knows how to do some of this work automatically.

Fixes #612

Trivial documentation for files in theme/.

Add a trivial README for the mojo/ directory in the SDK.

@abarth

Adds a new ActionDelay class to sprites