Ensure that Dart SDK dirs have appropriate permissions (#26315)
Ensure that Dart SDK dirs have appropriate permissions (#26315)

This is a partial re-application of #24669

https://github.com/flutter/flutter/issues/24413
flutter/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter