setTreeIndex(node, treeId) {
    if (treeId === undefined) return; // Don't bother

    if (this.trees[node.id] === undefined) {
      this.trees[node.id] = treeId;
      this.treeIndex = Math.max(treeId, this.treeIndex);
    }
  }
Assign a tree id to a node
@param {Node} node
@param {string|number} treeId