Can't use IMarkupExtension in a .NET Standard library?!

I'm trying to migrate a PCL to .NET Standard (1.3) and classes inheriting from IMarkupExtension fail to compile with:

The type 'IServiceProvider' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.ComponentModel, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

I've got more detail on the project here:
https://stackoverflow.com/questions/45922394/iserviceprovider-not-available-in-net-standard

But reproducing this can be done with a project with one class. What's going on here? It makes no sense.