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.
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.
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.
