Thank you folks for your answers. But, I finally found a solution :D The problem was in how Maps work. "results.first" is read-only, so I have to make copy of it to make it mutable.

I did this: Map<String, dynamic> userMapMutable = Map.of( results.first);
This works now when I want to write true/false in userMapMutable['isAdmin'].
Hope this helps someone in future :D