[MLton] Problems with Primitive.MLton visibility

[Brent A. Fulgham]

I'm trying to deal with path issues in basis-library/system/path.sml.
To do so, I was trying to access the Primitive.MLton.Platform.OS value,
but for some reason, MLton fails during the compile complaining that
MLton is an unknown value.

val isWin32 = let open Primitive.MLton.Platform.OS in
                        case host of
                            Mingwin => true
                        |   _ => false

I see primitive is accessed from real/pack.real (and various mlton/* 
entries) this way.  Do I have to do something fancy to get the compiler 
to incorporate the MLton structure in its environment?

-Brent