Sphere(Point3 center, num radius, [Shader shader])

Creates a new Sphere with the given center, radius and shader.

The radius of a sphere must not be zero or less.

Source 

Sphere(Point3 this.center, num this.radius, [Shader shader]) : super(shader) {
  if (radius<=0) throw new ArgumentError('Radius of sphere must not be zero or less');
}