public void Rotate( 
   SelectionSet features,
   MapPoint origin,
   double angle
)

var rotateFeatures = new EditOperation() { Name = "Rotate Features" };

//Rotate works on a selected set of features
//Get all features that intersect a polygon

//at 2.x - var rotateSelection = MapView.Active.GetFeatures(polygon).Select(
//    k => new KeyValuePair<MapMember, List<long>>(k.Key as MapMember, k.Value));
//rotateFeatures.Rotate(rotateSelection, origin, Math.PI / 2);

//Rotate selected features 90 deg about "origin"
rotateFeatures.Rotate(MapView.Active.GetFeatures(polygon), origin, Math.PI / 2);

//Execute to execute the operation
//Must be called within QueuedTask.Run
if (!rotateFeatures.IsEmpty)
{
  var result = rotateFeatures.Execute(); //Execute and ExecuteAsync will return true if the operation was successful and false if not
}

//or use async flavor
//await rotateFeatures.ExecuteAsync();