Dart file upload: Larger files aren't uploaded completely

I implemented a fileupload with dart. This is what my client does:

Future upload(String url, File file){
    Completer completer = new Completer();
    FileReader fileReader = new FileReader();
    fileReader.onLoad.listen((e){
      HttpRequest request = new HttpRequest();
      Map<String,String> headers = createHeaders();
      request.open('POST',url);
      for(String key in headers.keys){
        request.setRequestHeader(key,headers[key]);
      }
      request.send(fileReader.result);
    });
    fileReader.onLoadEnd.listen((e)=>completer.complete());
    fileReader.readAsArrayBuffer(file);
    return completer.future;
  }

And this is what the server does:

 Future saveFileContent(String fileId,   HttpRequest request)async{
    Completer completer = new Completer();
    File file = new File('${pathResolver.files}/${fileId}');
    await file.create(recursive: true);
    request.listen((data){
      file.writeAsBytesSync(data);
    },onDone: (){
      completer.complete();
    });
    return completer.future;
  }

For small files everything works fine, but when i try to upload a larger file,about 20MB in my testcase, just some MB are saved on the server. Has some one a idea where all those bytes got lost?