Sure, use a box decoration with one rounded corner, with the radius of the rounding being the same as the size of the container.

Container(
  width: 100.0,
  height: 100.0,
  decoration: BoxDecoration(
    border: Border.all(width: 4.0, color: Colors.black),
    borderRadius: BorderRadius.only(topLeft: Radius.circular(100.0)),
  )
)