I wondered the same thing, it seems like 'System.Private.CoreLib.dll' is all or nothing, unless of course you write your own version such like 'Test.CoreLib' (https://github.com/dotnet/corert/tree/master/src/Test.CoreLi...)

I don't know if it's technically impossible or just hard to have 'tree-shaking' being done on it, so that you only get the functions/dependencies that are actually called?