This change set does exactly what it says it does (depreciates upTo:).  
However, I recommend not including it until a) we change how we respond
to depreciated and/or b) we replace a sufficient number of the upTo:
usages.  Otherwise, this will make the image very hard to use.
Tested in 3.9a. #6537. 
Right now, when we get a depreciate interrupt, clicking on proceed
appears to not allow you to ignore the error.  So IMO we can not afford
to extend theuse depreciated ... 
There are 80+ usages of upTo:, including most notablely AbstractString
and MultiByteString.  When I tried to remove the one usage in BFAV, I
found I couldnt until MBS was fixed, So BFAV became unusable.