Make it compatible with Flutter 2.0 and Dart >= 2.12.0

[martirius]

Hi,
with the introduction of Flutter 2.0 this library is not compatible anymore nor with null-safety.
I tried to update dependencies and make it null-safety in a fork without success, the generator always fails.

Can you solve?

[martirius]

Ok, I have succeeded in solving build error and updated dependencies, and the generator works perfectly.
I can do a PR if you want