num rotation

The rotation of this display object, in radians, from its original orientation.

// Convert from degrees to radians.
this.rotation = degrees * math.PI / 180;

// Convert from radians to degrees.
num degrees = this.rotation * 180 / math.PI;

Source 

@override
num get rotation => _rotation;
void rotation=(num value)

Source 

@override
set rotation(num value) {
  if (value is num) _rotation = value;
  _transformationMatrixRefresh = true;
}