If you look inside UIDrawCall.cs line 105, you will notice that sortingOrder is wrapped in
#if !UNITY_3_5 && !UNITY_4_0 && !UNITY_4_1 && !UNITY_4_2
So for some reason one of the defines here is valid for you -- possibly due to generation of the solution in an older version of Unity, or due to a bug in Unity. Either way, figure out which define it is (such as UNITY_4_2) and get rid of it from your solution's list of defines.