没有为类“ firebase_storage :: StorageUploadTask”定义吸气剂“ future”

Question

在下面的我的代码中，当我尝试在iOS上构建它时，出现了未定义的将来错误。但是，当我在Android上构建时，这组代码有效。

StorageReference ref = FirebaseStorage.instance.ref().child("$requestId.jpg");
StorageUploadTask uploadTask = ref.put(imageFile);
Uri downloadUrl = (await uploadTask.future).downloadUrl ;

错误消息：

The getter 'future' isn't defined for the class 'firebase_storage::StorageUploadTask'. 
compiler message: Try correcting the name to the name of an existing getter, or defining a getter or field named 'future'.

Flutter Doctor输出：

Flutter (Channel beta, v0.7.3, on Mac OS X 10.13.6 17G65, locale en-SG)
iOS toolchain - develop for iOS devices (Xcode 9.4.1)