Visual Studio 2017 Won't run Custom Tool in .N

Question

I'm porting all my development to Visual Studio 2017. I made heavy use of Portable Libraries in 2015, but it appears that .NET Standard is the way to handle cross-environment issues going forward. I've got a single file code generation extension that I've upgraded. In Visual Studio 2017, it works great against the traditional .NET libraries and portable libraries, but it doesn't work with the new .NET Standard library I'm trying to build. I'm happy to consider that I've done something wrong, but the GITHUB example doesn't work either:

https://github.com/Microsoft/VSSDK-Extensibility-Samples/tree/master/Single_File_Generator

Anyone know why Visual Studio extensions (at least code generation extensions) don't work on this project type?