detach method
Mark this node as detached.
Typically called only from the parent's detach, and by the owner to mark the root of a tree as detached.
Subclasses with children should override this method to
detach all their children after calling the inherited method,
as in super.detach()
.
Implementation
@mustCallSuper
void detach() {
assert(_owner != null);
_owner = null;
assert(parent == null || attached == parent!.attached);
}