Can I build an “old school” PCL with DNX/DNU?

There’s been some issues with Xamarin frameworks in the past, but try doing something like this;

https://github.com/AutoMapper/AutoMapper/blob/93f276fea36dedb2efd861096f881667af880d58/src/AutoMapper/project.json

See if this works:

{
  "frameworks": {
    ".NETPortable,Version=v4.0,Profile=Profile328": {
      "frameworkAssemblies": {
        ...
      }
    }
  }
}