readAt method
Implementation
Future<int> readAt(int offset, ReadBuf dst, {CancelToken? cancel}) async {
_checkReadable();
cancel?.throwIfCancelled(path: path);
if (offset < 0) throw RangeError.value(offset, 'offset');
if (!dst.hasRemaining) return 0;
return _track(_readAtImpl(offset, dst));
}