Steps to reproduce:

1. Create a new project
2. Add the GTK package
3. Open Project Settings
4. Navigate to Player page
5. In the Configuration section set 

• Scripting Backend -> IL2CPP
• Editor Assemblies Compatibility Level -> .NET Standard

Expected : Unity recompiles scripts and there is no error

Actual : Unity recompiels scripts and there is the following compiler error

- /com.unity.motion/Packages/com.unity.graphtoolkit/GraphToolkitEditor/UI/ModelView/Inspector/CustomPropertyDrawerAdapter.cs(223,63): error CS1061: 'AppDomain' does not contain a definition for 'DefineDynamicAssembly' and no accessible extension method 'DefineDynamicAssembly' accepting a first argument of type 'AppDomain' could be found (are you missing a using directive or an assembly reference?)

Reported on Discussions board : [https://discussions.unity.com/t/error-cs1061-appdomain-does-not-contain-a-definition-for-definedynamicassembly/1673819]