Add a way of having keys based on numeric types or DateTimes by having a ValueKey<T> class.
Remove the redundant ways of declaring things, except for leaving one shorthand -- you can say `new Key(s)` instead of `new ValueKey<String>(s)`.

We're now using it at the widget layer for everything except scrolling and
flinging.

I'll write the Widget wrapper in the next CL, including adding
support for padding at the Widget layer.

@Hixie

Fixes #829

We need to implement visitChildren for the RenderObjects in the sector demo.
Also, add a test.

Fixes #790

This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.

Also, add an example for the date picker

@abarth

SnackBar's crash if you fail to provide a showing bool.

I tried to edit it in the framework, but this seemed easier for now.

The snackbar still shows behind the keyboard unfortunately.
https://github.com/domokit/sky_engine/issues/810

@collinjackson

@collinjackson

I'm not sure this is the correct approach, but it
at least mitigates https://github.com/domokit/sky_engine/issues/804
for now.

I also made it not crash if you haven't set a goalWeight.

@collinjackson

We now have Mimic tracking Mimicable through tree structure changes and while
moving around the screen.

Fixes #751 and #756

Adds itemsWrap:bool (default false) to ScrollableList and PageableList. If itemsWrap is true then scrolling past the last item wraps around to the first. Similarly, scrolling before the first item wraps around to the last.

Added abstract ExtentScrollBehavior of ScrollBehavior. Renamed fields called contentsExtents to contentExtent, containerExtents to containerExtent, contentSize to contentExtent, etc.

BoundedBehavior is now a subclass of ExtentScrollBehavior.

Added UnboundedBehavior subclass of ExtentScrollBehvaior; contentExtent and maxScrollOffset are double.INFINITY, minScrollExtent is double.NEGATIVE_INFINITY.

Mostly formating and removing arguments with default values.

These widgets were designed in CSS where you need to specify a layout model for
your children. This patch updates them to the modern style of just taking a
unique child.

Fixes #755

Block -> BlockBody
ScrollableBlock -> Block
FixedHeightScrollable -> ScrollableWidgetList
VariableHeightScrollable -> ScrollableMixedWidgetList
BlockViewport -> MixedViewport

Fixes #743