Set derived dir Flutter.framework directory readonly (#8748)
Set derived dir Flutter.framework directory readonly (#8748)

Provides a strong hint to developers that editing Flutter framework
headers isn't supported.
flutter/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,turnidge/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,turnidge/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,turnidge/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter