Support drag-to-dismiss in persistent bottom sheets
Flinging a persistent bottom sheet downwards dismisses it, per the Material spec.
The showBottomSheet() function now returns a Future, like showModalBottomSheet() does, so that you can discover when it's been dismissed - with navigator.pop() - and with what value.
Factored the drag gesture handling code into _BottomSheetDragController
This CL was flutter/engine#1997
8a8bd0197c