visitChildren method Null safety
Implementation
void visitChildren(void Function(int, ArgDone) f, [int offset = 0]) {
  int n = 0;
  for (final c in _next) {
    f(offset + n++, c);
  }
  final nd = _nextOnDot;
  if (nd != null) {
    final da = nd.child as DigitArg;
    da.visitChildren(f, n);
  }
}