Get the last key in the map that is strictly smaller than key. Returns
null if no key was not found.
Source
K lastKeyBefore(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.left;
if (node == null) return null;
while (node.right != null) {
node = node.right;
}
return node.key;
}