The unique identifier for this node.
The root node has an id of zero. Other nodes are given a unique id when they are attached to a SemanticsOwner. If they are detached, their ids are invalid and should not be used.
In rare circumstances, id may change if this node is detached and re-attached to the SemanticsOwner. This should only happen when the application has generated too many semantics nodes.
int get id => _id;