Radius lerp(Radius a, Radius b, double t)

Linearly interpolate between two radii.

If either is null, this function substitutes Radius.zero instead.

Source 

static Radius lerp(Radius a, Radius b, double t) {
  if (a == null && b == null)
    return null;
  if (a == null)
    return new Radius.elliptical(b.x * t, b.y * t);
  if (b == null) {
    double k = 1.0 - t;
    return new Radius.elliptical(a.x * k, a.y * k);
  }
  return new Radius.elliptical(
    lerpDouble(a.x, b.x, t),
    lerpDouble(a.y, b.y, t)
  );
}