FCS calls this (frequently). If the write time is changed, it then calls `FileStreamReadShim` where we return proper stream. 

Find all references works OK on dirty documents first time. If we change a doc further, then execute Find all references again, FCS find nothing or crashes VS. 

It seems we must change `ProjectOptions.LoadTime` if any of dirty documents is changed (I temporarily set `LoadTime = DateTime.Now` and everything works OK).

So, I'm not sure if:

* using hash code of the whole doc as last write time is a good idea
* should we calculate project's "load time" by looping over all dirty docs, summing their hash codes

Any ideas?