你需要的是屈服和等待
检查这个

import 'dart:async';

void main() {

  final folder1 = Future.value({"a","b","c","d"});
  final folder2 = Future.value({"a","b","c","d",folder1});
  final folder3 = {"a","b","c","d",folder1,folder2}; //what you get initially

  Stream printElement(list) async*{

     for(dynamic element in list){

      if(element is String){
        yield element;
       }else{
        yield* printElement(
        await(element));  //make your http call here for your nested folder
        }

     };
  }

  printElement(folder3).listen((element){
      print(element);
  });

}

您也可以使用更多类型安全代码与Either来自dartz package

import 'dart:async';
import 'package:dartz/dartz.dart';

class MyFile{

  final String fileName ;

  MyFile(this.fileName);

  @override
  String toString() {
    return this.fileName;
  }
}

class MYFolder{

  final String folderName;
  final List<Either<Future<MYFolder>,MyFile>> files; //files can be Either file or Future of folder from HTTP

  MYFolder(this.folderName, this.files);

  @override
  String toString() {
    return this.folderName;
  }
}

void main() {

  //leaf folders

  final foldera = MYFolder("foldera",[Right(MyFile("a")),Right(MyFile("a")),Right(MyFile("a")),Right(MyFile("a"))]);
  final folderb = MYFolder("folderb",[Right(MyFile("b")),Right(MyFile("b")),Right(MyFile("b")),Right(MyFile("b"))]);
  final folderc = MYFolder("folderc",[Right(MyFile("c")),Right(MyFile("c")),Right(MyFile("c")),Right(MyFile("c"))]);
  final folderd = MYFolder("folderd",[Right(MyFile("d")),Right(MyFile("d")),Right(MyFile("d")),Right(MyFile("d"))]);

  //                    ...foldera
  //                   - folderb
  //        -  folder1
  //
  //folder3 -  folder2
  //                  ... folderc
  //                  - folderd
  //
  // --triple dot notation for folding elements in lists ,it is just a shortcut for populating lists

  final  folder1 = Future.value(MYFolder("folder1", [...foldera.files,Left(Future.value(folderb))]));
  final folder2 = Future.value(MYFolder("folder2", [...folderc.files, Left(Future.value(folderd))]));
  final folder3 = MYFolder("folder3",[Left(Future.value(folder1)),Left(Future.value(folder2))]); //what you get initially

  Stream printFilesIn(MYFolder folder) async*{

    print("Current folder is $folder");

     for(Either file in folder.files){
      if(file is Right){
        yield file.value;
      }else{
        yield* printFilesIn(
        await(file as Left).value); //make your http call here for your nested folder
        }
      };
  }

  printFilesIn(folder3).listen(print);

}

虽然这个答案对你来说可能太晚了（3年后），但它仍然可以帮助其他人在未来遇到类似的问题。
Saed可能误解了您的意图，他提供的答案是将Iterator转换为Stream，而不是迭代Stream（可能相当于将Stream转换为Iterator）。
据我所知，这件事可能办不到。但是有一个非常相似的解决方案--使用StreamIterator。它具有与Iterator类似的接口。不同的是它的方法是异步的。例如，您应该使用await streamIterator.moveNext()而不是iterator.moveNext()。
将Stream转换为StreamIterator非常简单，只需使用final streamIterator = StreamIterator(yourStream);即可。
Iterator和StreamIterator之间可能很难共享相同的功能，因为Iterator具有同步方法。目前，我通过将Iterable转换为Stream，然后将其转换为StreamIterator来实现这一点：
final streamIterator = StreamIterator(Stream.fromIterable(iterable));
请让我知道，如果你有任何更优雅的解决方案或任何意见，我的答案。