* add import guidance to library-level docs
* add import docs to other packages
* fix review comment
* clarify which libraries aren't meant to be directly imported
To be more consistent with other parts of the platform:
* put the binding in a binding.dart file.
* rearrange some members of the Scheduler class to be more close to
execution order.
* factor out Priority class into its own file.
* add more dart docs.
Each layer is supposed to reexport the parts of the previous layer
that are part of its API.
- In painting.dart, export from dart:ui all the Canvas-related APIs
that make sense to be used at higher levels, e.g. PaintingStyle.
- Delete painting/shadows.dart. It was dead code.
- In rendering/object.dart, export all of painting.dart.
- In widgets/basic.dart, export all of painting.dart and
animation.dart. Some classes in animation/ are renamed to make this
less disruptive and confusing to the namespace.
- Split out Stocks back into an import model rather than a part model,
so that it's easier to manage its dependencies on a per-file basis.
- Move Ticker to scheduler library.
- Remove as many redundant imports as possible now.
- Some minor nit picking cleanup in various files.
