The getter 'future' isn't defined for the class 'firebase_storage::StorageUploadTask'

Question

In the my code below, I am getting the error of future not being defined when I try to build it on iOS. However, this set of code works when I build on Android.

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

Error Message:

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 output:

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)