const BorderSide({Color color: const Color(0xFF000000), double width: 1.0, BorderStyle style: BorderStyle.solid })

Creates the side of a border.

By default, the border is 1.0 logical pixels wide and solid black.

Source 

const BorderSide({
  this.color: const Color(0xFF000000),
  this.width: 1.0,
  this.style: BorderStyle.solid
});