in lib/src/game/game_core.dart [58:72]
void setFlag(int x, int y, bool value) {
_ensureStarted();
final currentSS = _states.get(x, y);
if (value) {
assert(currentSS == SquareState.hidden);
_states.set(x, y, SquareState.flagged);
_bombsLeft--;
} else {
assert(currentSS == SquareState.flagged);
_states.set(x, y, SquareState.hidden);
_bombsLeft++;
}
_update();
}