readFromFile method
Reads a block of data of the given length from the current position.
Implementation
@override
Future<Readbuffer> readFromFile(int length) {
Uint8List cont = content.sublist(_lastOffset, _lastOffset + length);
assert(cont.length == length);
Readbuffer result = Readbuffer(cont, _lastOffset);
_lastOffset += length;
return Future.value(result);
}