decreaseKey(node, key) {
        if (node.index === undefined) {
            node.key = key;
            this.insertNode(node);
        } else {
            node.key = key;
            this.moveUp_(node.index);
        }
    }
Move the node up in hierarchy
@param {Node} node the node
@param {K} key new ley, must be smaller than current key