UnityEditor.AI.NavMeshBuilder source available?

I'm trying to figure out why some objects are not included when
NavMeshBuilder.CollectSources() is called, for example from NavMeshSurface (from the GitHub NavMeshComponents).

Is the UnityEditor.AI.NavMeshBuilder source closed?

Edit: Think I've found it:
https://github.com/Unity-Technologi...Modules/AI/Builder/NavMeshBuilder.bindings.cs

 

https://github.com/Unity-Technologies/NavMeshComponents