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

Don't grey out Sky code in Atom

For some reason having |packages| in this .gitignore file is causing Atom to
grey out all the files in the Sky package. Moving the entry up to the root of
the git repo fixes the issue.

Add iOS sky_app target for the fitness app example

Reset the Dimissable fade animation on slow flings

Refactor RenderParagraph to do more work at the painting layer