const BorderRadius.only({Radius topLeft: Radius.zero, Radius topRight: Radius.zero, Radius bottomRight: Radius.zero, Radius bottomLeft: Radius.zero })

Creates a border radius with only the given non-zero values. The other corners will be right angles.

Source 

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