Get the first key in the map that is strictly larger than key. Returns
null if no key was not found.
Source
K firstKeyAfter(K key) {
if (key == null) throw new ArgumentError(key);
if (_root == null) return null;
int comp = _splay(key);
if (comp > 0) return _root.key;
_SplayTreeNode<K> node = _root.right;
if (node == null) return null;
while (node.left != null) {
node = node.left;
}
return node.key;
}