Visual Studio 2017 - Could not load file or assembly 'System.IO.FileSystem

In my particular case, I did a Clean Solution, Tools/options/NuGet Package Manager/Clear All NuGet Cache(s), then a reboot. Loading/building the project then worked.

Not sure how repeatable this is. I'm quite sure my project worked the first time I loaded it, then failed, then worked again after these steps. It has worked on 2 computers.