Instead of using getTotalMatrix and setMatrix, we can just use
save/restore, which is more idiomatic.
The getTotalMatrix/setMatrix pattern was introduced to improve
performance, but the original code was calling getTotalMatrix/setMatrix
at every node in the sprite tree, which is much slower than the normal
save/transform/restore pattern.
Related to #4254
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.
