borderRadius property
final
The radii for each corner.
Each corner Radius defines the endpoints of a line segment that spans the corner. The endpoints are located in the same place as they would be for RoundedRectangleBorder, but they're connected by a straight line instead of an arc.
Negative radius values are clamped to 0.0 by getInnerPath and getOuterPath.
Implementation
final BorderRadiusGeometry borderRadius;