Associates the key with the given value.
If the key was already in the map, its associated value is changed. Otherwise the key/value pair is added to the map.
void operator []=(K key, V value) {
  if (key == null) throw ArgumentError(key);
  // Splay on the key to move the last node on the search path for
  // the key to the root of the tree.
  int comp = _splay(key);
  if (comp == 0) {
    _root.value = value;
    return;
  }
  _addNewRoot(_SplayTreeMapNode(key, value), comp);
}
    © 2012 the Dart project authors
Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
    https://api.dart.dev/stable/2.5.0/dart-collection/SplayTreeMap/operator_put.html