DartでmapIndexedなど ref: https://qiita.com/kurogelee/items/7a244b9ef7305fc727df

map_indexed.dart

Iterable<R> mapIndexed<T,R>(Iterable<T> list, R f(int index, T val)) sync*{
  int i = 0;
  for(T val in list){
    yield f(i++, val);
  }
}

map_notnull.dart

Iterable<R> mapNotNull<T,R>(Iterable<T> list, R f(T val)) sync*{
  for(T val in list){
    R result = f(val);
    if(result != null){
      yield result;
    }
  }
}

range.dart

Iterable<int> range(int size) sync* {
  for(int i = 0; i < size; i++){
    yield i;
  }
}