1. override
void replaceChildAt(Bitmap child, int index)

Source

@override
void replaceChildAt(Bitmap child, int index) {
  if (index < 0 || index >= _children.length) {
    throw new ArgumentError("The supplied index is out of bounds.");
  } else if (child.parent == this) {
    if (_children.indexOf(child) == index) return;
    throw new ArgumentError("The bitmap is already a child of this container.");
  } else {
    var oldChild = _children[index];
    var newChild = child;
    newChild.removeFromParent();
    oldChild._parent = null;
    newChild._parent = this;
    _children[index] = newChild;
  }
}