Step 1: Open VS 2015 RC and create a new "ASP.Net Web Application",

Step 2. Right-click the solution, add new windows "Class Library" (regular, not "Class Library (package)",

Step 3: Put the method in Class1.cs in the class library. No matter what.
Step 4: Right-click the "Links" in the web project and add a link to your class library.

Step 5: From the code file in the web project, call the method that you created in Class1.cs
So for me Class1.cs looks like this:
public class Class1
{
public void X()
{
}
}
And I added the code in the web application as follows:
var x = new ClassLibrary1.Class1();
x.X();
Step 6: try to compile, you will get this error:
CS0246 ClassLibrary1 ( using ?)
, ( ) ASP.Net 5?