fire base storage image

var fileName = "fileName.jpeg";
  StorageUploadTask putFile =
      storage.ref().child("folder/$fileName").putFile(_image);
  putFile.future.catchError(onError);

  UploadTaskSnapshot uploadSnapshot = await putFile.future;

  print("image uploaded");

  Map<String, dynamic> pictureData = new Map<String, dynamic>();
  pictureData["url"] = uploadSnapshot.downloadUrl.toString();


  DocumentReference collectionReference =
      Firestore.instance.collection("collection").document(fileName);

  await Firestore.instance.runTransaction((transaction) async {
    await transaction.set(collectionReference, pictureData);
    print("instance created");
  }).catchError(onError);