const BorderRadius.vertical({Radius top: Radius.zero, Radius bottom: Radius.zero })

Creates a vertically symmetric border radius where the top and bottom sides of the rectangle have the same radii.

Source 

const BorderRadius.vertical({
  Radius top: Radius.zero,
  Radius bottom: Radius.zero
}) : this.only(
  topLeft: top,
  topRight: top,
  bottomRight: bottom,
  bottomLeft: bottom
);