Flutter – what is ': this.only' in dart?

Can someone explain to me the script’s (given below) constructor, what ‘: this.only’ means:

class BorderRadius extends BorderRadiusGeometry {


  const BorderRadius.all(Radius radius) : this.only(

    topLeft: radius,

    topRight: radius,

    bottomLeft: radius,

    bottomRight: radius,

  );

I am learning Dart and trying to understand the class/widgets of flutter and it’s built code.

Thank you.