BorderSide copyWith({Color color, double width, BorderStyle style })

Creates a copy of this border but with the given fields replaced with the new values.

Source 

BorderSide copyWith({
  Color color,
  double width,
  BorderStyle style
}) {
  return new BorderSide(
    color: color ?? this.color,
    width: width ?? this.width,
    style: style ?? this.style
  );
}