read method
A fresh copy of the payload at [offset, length], so a replay that
hands it to a backend cannot alias the file.
Implementation
ByteData read(Object? where) {
final pair = (where! as List<Object?>).cast<int>();
return ByteData.sublistView(
Uint8List.fromList(_blob.sublist(pair[0], pair[0] + pair[1])),
);
}