1. override
void removeChildren([int beginIndex, int endIndex ])

Source 

@override
void removeChildren([int beginIndex, int endIndex]) {
  int length = _children.length;
  int i1 = beginIndex is int ? beginIndex : 0;
  int i2 = endIndex is int ? endIndex : length - 1;
  if (i1 > i2) {
    // do nothing
  } else if (i1 < 0 || i1 >= length || i2 < 0 || i2 >= length) {
    throw new ArgumentError("The supplied index is out of bounds.");
  } else {
    for (int i = i1; i <= i2 && i1 < _children.length; i++) {
      removeChildAt(i1);
    }
  }
}