For now I just delete MAST before building and re-import the package to do editing. Its a royal pain.
Viewing post in MAST Prevents build
I did a little digging, and I was able to get it building without having to hunt down all the #if UNITY_EDITOR placements.
I moved MAST_Prefab_Component into a new folder called Components that's along side the Scripts folder. I then added an asmdef file in both folders, I set the scripts' asmdef to be editor only, and gave it a reference to the components' asmdef.