read & write use signed bytes

Hey!

For me it was counter-intuitive to discover that the posix unistd functions, e.g. `read` and `write` use signed 8-bit values, instead of unsigned.

The rest of dart seems to use unsigned 0-255 values as bytes, e.g.: https://api.dart.dev/dart-io/File/writeAsBytes.html or the codecs in https://api.flutter.dev/flutter/dart-convert/

Is that intentional? If it is, I think it might be worth adding notes about that in the docs, or providing uint8 variants