dropChild method
Called by subclasses when they decide a render object is no longer a child.
Only for use by subclasses when changing their child lists. Calling this in other cases will lead to an inconsistent tree and probably cause crashes.
Implementation
@override
void dropChild(RenderObject child) {
_firstSortedChild = null;
_lastSortedChild = null;
super.dropChild(child);
}