If you see an error like this: when running AppVeyor it is because the version of fsharp.compiler.dll is loaded from the GAC rather than the LKG directory.
C:\Microsoft>"C:\Microsoft\visualfsharp\tests\..\lkg\FSharp-14.0.23413.0\bin\Fsi.exe" --exec --nologo "C:\Microsoft\visualfsh
arp\tests\\Convert-NUnit2Args-to-NUnit3Where.fsx" -- "Smoke" "ReqPP,NOOPEN,NO_CI,Expensive"
error FS0193: internal error: Method not found: 'FileSystem Shim.get_FileSystem()'.
When we created the LKG we used a preview of the compiler that had some of Don's recent reformatting PR's in it, rather than the DEV15 RTM version, it had a bug fix we wanted to use.
I think the solution is to for us to downgrade the LKG to the RTM version of the compiler:
Kevin