Issue #189 details the current dilemma of being unable to add or
monkeypatch methods on a GObject interface. At this time developers
must add an additional line of code to their applications to use
Gio.File.prototype. This is a temporary fix faking it until such a
fix has been made for that issue.

Example:
promisify(Gio._LocalFilePrototype, load_contents_async, load_contents_finish);