Visual Studio 2017 won't launch custom tool in .NET Standard Library

I am migrating all my development to Visual Studio 2017. I used Portable Libraries extensively in 2015, but it seems .NET Standard is the way to deal with cross-environment issues in the future. I have one file code generation extension which I have updated. In Visual Studio 2017, it works fine with traditional .NET libraries and portable libraries, but it doesn't work with the new .NET Standard library I'm trying to create. I'm glad I did something wrong, but the GITHUB example doesn't work either:

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

Does anyone know why Visual Studio extensions (at least code extensions) don't work on this type of project?