Revert "Don't assume that there is a Source value"
Revert "Don't assume that there is a Source value"

This reverts commit e000770f80355b0c22969cd6f68b4e74f61f44c8.

migueldeicaza/MonoTouch.Dialog,hisystems/MonoTouch.Dialog,benoitjadinon/MonoTouch.Dialog,danmiser/MonoTouch.Dialog,newky2k/MonoTouch.Dialog,Milan1992/MonoTouch.Dialog,ClusterReplyBUS/MonoTouch.Dialog,jstedfast/MonoTouch.Dialog