groupBy method
Implementation
Map<String, List<V>> groupBy(String Function(V item) funcGroupKeyResolver) {
Map<String, List<V>> odRet = Map<String, List<V>>();
for (V item in this.values) {
String key = funcGroupKeyResolver(item);
odRet.putIfAbsent(key, () => [ ]);
odRet[key]!.add(item);
}
return odRet;
}