finishData method
Implementation
void finishData(int nextState) {
padMAC(_dataCount);
var lengths = Uint8List(16);
pack32(_aadCount, lengths, 0, Endian.little);
pack32(_dataCount, lengths, 8, Endian.little);
poly1305.update(lengths, 0, 16);
poly1305.doFinal(_mac, 0);
_state = nextState;
}