Removes key and its associated value, if present, from the map.
Returns the value associated with key before it was removed.
Returns null if key was not in the map.
Note that values can be null and a returned null value doesn't
always mean that the key was absent.
Source
V remove(Object key) {
if (!_validKey(key)) return null;
_SplayTreeMapNode<K, V> mapRoot = _remove(key as dynamic/*=K*/);
if (mapRoot != null) return mapRoot.value;
return null;
}