We're now using it at the widget layer for everything except scrolling and flinging.
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.
Block -> BlockBody ScrollableBlock -> Block FixedHeightScrollable -> ScrollableWidgetList VariableHeightScrollable -> ScrollableMixedWidgetList BlockViewport -> MixedViewport
