I recommend to “Dump Tree” (File menu) to a test file, then search for the referenced assembly (“[System]”) or namespace. It will show you the class and probably the method referencing the assembly.
As of 4.3, you don’t have to care about
“[System.Core]System.Runtime.CompilerServices.ExtensionAttribute”
which is caused by using extension methods. This is not a “real” reference, and you can use your build without System.Core.dll.