The problem is that Storage objects will most probably change entity's
dictionary so that values are correctly set before the source's
corresponding method (e.g. entity_added()) is called.

For instance, the BFSFileStorage will change the original binary
data and replace it with the destination file path in order to store
the file path in the database. This change must be local
to the source in order not to impact other hooks or attribute access
during the transaction, the whole idea being that the same
application code should work exactly the same whether or not a
BFSStorage is used or not.