TableBorder.symmetric({BorderSide inside: BorderSide.none, BorderSide outside: BorderSide.none })

Creates a border for a table where all the interior sides use the same styling and all the exterior sides use the same styling.

Source 

factory TableBorder.symmetric({
  BorderSide inside: BorderSide.none,
  BorderSide outside: BorderSide.none
}) {
  return new TableBorder(
    top: outside,
    right: outside,
    bottom: outside,
    left: outside,
    horizontalInside: inside,
    verticalInside: inside
  );
}