Undefined class 'StorageReference'. Try changing the name to the name of an existing class, or creating a class with the name 'StorageReference'. code example

Example: Undefined class 'StorageReference'

FirebaseStorage storage = FirebaseStorage.instance;
Reference ref = storage.ref().child("image1" + DateTime.now().toString());
UploadTask uploadTask = ref.putFile(_image1);
uploadTask.then((res) {
   res.ref.getDownloadURL();
});