merge_maps.dart

void main() {
final firstMap = {"1":[1,2,3], "2": [0,0]};
final secondMap = {"2":[1,1]};

final thirdMap = {
   ...firstMap,
   ...secondMap,
};
  
  print(firstMap.merge(secondMap).values);
  print(firstMap.values);
}


extension Merger<T, V> on Map<T, List<V>> {
  Map<T, List<V>> merge(Map<T, List<V>> second) {
    var copy = {...this};
    
    second.forEach((key, value) {
      if (copy[key] != null) {
        copy[key]!.addAll(value);
      } else {
        copy[key] = [...value];
      }
    });
    return copy;
  }
}