Bump to Dart SDK `1.23.0-dev.11.6`. (#9285)
Bump to Dart SDK `1.23.0-dev.11.6`. (#9285)

Pulls in latest 1.23. cherry picks, including @abarth’s fix to restrict `@immutable` checks to instance fields.
flutter/flutter,turnidge/flutter,turnidge/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,turnidge/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,turnidge/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter