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.

Widgets that want to receive drops should include a DropTarget in their build.
Currently there's no widget for initiating a drag. Components can use the
DragController directly. In the future, we'll probably want to add a Draggable
that knows how to do some of this work automatically.

Fixes #612