wrapBackdrop abstract method
Wraps child (the backdrop content) with the visual transform for
progress t (0 = closed, 1 = fully open).
isRoot is true for the bottom-most layer in a stack of sheets (the one
that wraps the actual app content); root layers may additionally clip
their corners as they scale in.
Implementation
Widget wrapBackdrop(BuildContext context, Widget child, double t,
{bool isRoot = true});