Myself and a colleague just spent an embarassing amount of time tracking down a very obscure error:
'Microsoft.XLANGs.RuntimeTypes.XmlDocumentSerializationProxy' does not contain a definition for 'LoadXml'
As it turns out, this error was being thrown at compile-time for orchestration 1. The reason was that we had an unreachable send shape in orchestration 2. Please do not ask me why, I am not that smart. All I know is that we had a message construct and a send on the unreachable portion of a decision shape, whenever we moved it to the reachable side, or removed it from the decide shape entirely the error went away.
If you get this error, check your code for unreachable paths and remove message constructs or send/receive shapes from them. Again...do not ask me why I have no idea.
If you know why, please put in a comment below and enlighten the rest of us.