1. mustCallSuper
void attach(@checked Object owner)

Mark this node as attached to the given owner.

Typically called only from the parent's attach method, and by the owner to mark the root of a tree as attached.

Subclasses with children should attach all their children to the same owner whenever this method is called.

Source 

@mustCallSuper
void attach(@checked Object owner) {
  assert(owner != null);
  assert(_owner == null);
  _owner = owner;
}