Enabled displaying a scrollbar in ScrollingLists. The scrollbar is painted as an "overlay", i.e. it's painted on top of the scrolling list's visible children.

Added an abstract Painter base class that encapsulates a paint method and the renderer that it's attached to. RenderBlockViewport and HomogenousViewport now support an overlayPainter property. If specified, RenderBlockViewport attaches itself to the overlayPainter when it's attached to the rendering tree. RenderBlockViewport now calls overlayPainter.paint() after it has painted its children.

Added an abstract ScrollingListPainter class that exposes ScrollingList's state which might be needed for painting. Like its scroll direction and scrollOffset. The ScrollingListPainter is notified when a scroll starts and ends.

Defined a Material-specific ScrollingListPainter that renders a scrollbar. The scrollbar thumb is faded in/out when the scroll starts/ends.

Added onScrollStart and onScrollEnd listeners to Scrollable.

Add type annotations in many places.
Fix some identifiers to have more lint-satisfying names.
Make all operator==s consistent in style.
Reorder some functions for consistency.
Make ParentData no longer dynamic, and fix all the code around that.

Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

Previously, hitTestChildren in RenderBlockViewport just didn't support
horizontal scrolling, due to an oversight.

This reverts commit 7592213df29066cd357eaa4fffe4a19ed3bae189.

Turns out many of the functions on BoxConstraints weren't used or had callers
that could easily be updated to other functions. I've added dartdoc to all the
public functions as well as renamed some functions that had similar names but
did different things.

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

Adds a HomogeneousViewport class that works like MixedViewport but
handles only children that have all the same height.

Converts ScrollableWidgetList to use that, so that we don't waste a
frame looking at the size of the contents each time we change size.

This allows a number of seemingly pointless double-pumps in the tests
to be removed.

Other changes that were necessary to support the above:

 - RenderBlock now supports minExtent (think 'min-height' in CSS)
 - RenderBlock now supports itemExtent (forces the height of each
   child to be the same, so that the itemExtent passed to the fixed-
   height scrollables are all authoritative instead of a source of
   bugs when they don't match)
 - RenderBlockViewport now supports horizontal scrolling
 - improved the style of the isInfinite assert in box.dart
 - fixed the position of a comment in mixed_viewport.dart
 - added a test
 - made the logic for how many items to show be more precise

The core of MultiChildRenderObjectWrapper.syncRenderObject() could
apply to any subclass that uses a flat child list, so this abstracts
it out into the superclass.

(Also, instead of requiring the callbacks of RenderBlockViewport to
constrain their results, we just constrain it for them. Makes things a
bit easier for users of that class.)

- add debugDescribeSettings to a few classes that were missing it
- fix some minor bugs in RenderShrinkWrapWidth and ShrinkWrapWidth
- introduce RenderShrinkWrapHeight and ShrinkWrapHeight

Generalise RenderBlockViewport so that it can be used by a Widget that knows its children's dimensions.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.

We'll need this for horizontally scrolling lists.