We upgraded Unity to 2018 and .NET standard 2.0.
After the .NET change, there is the following error in the class (CodeEmit).
Assets/XLua/Src/CodeEmit.cs(30,18): error CS0246: The type or namespace name `ModuleBuilder' could not be found. Are you missing an assembly reference?

How to fix this problem? Need for .NET Standard 2.0