*<Null safety>*
Map<K, G> groupFoldBy <K, G>(K keyOf(T element), G combine(G? previous, T element))
Map<K, G> groupFoldBy<K, G>( K Function(T element) keyOf, G Function(G? previous, T element) combine) { var result = <K, G>{}; for (var element in this) { var key = keyOf(element); result[key] = combine(result[key], element); } return result; }