My DLL is giving an error in Unity5 no matter what I have in it, even though in Unity4 things worked fine

I've made a .dll for use in my project, and it worked fine in Unity 5. However in Unity5 the dll (or any other dll I build), always gives the following error when I have it somewhere in the assets folder:

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 
 namespace TestClassLibrary1
 {
     public class Class1
     {
 
     }
 }
 

What's up with this? Why doesn't Unity5 understand what Unity4 did understand? How do I fix this?