The type or namespace name 'ILGenerator' could not be found

we use the XLUA plugin in our project, which compile error when upgrading from .Net3.5 to .Net4.x in Unity2018.3.0.f2.

 using System.Collections.Generic;
 using System.Reflection.Emit;
 using System.Reflection;
 using System;
 using System.Linq;
 
 // ...
 
 private void emitPush(ILGenerator il, Type type, short dataPos, bool isParam, LocalBuilder L, LocalBuilder translator, bool isArg)
 {
    // ...
 }