Marshall ``flavor`` and ``data`` into a standard event.

def make_event_data(self, flavor, data):
        """
        Marshall ``flavor`` and ``data`` into a standard event.
        """
        if not flavor.startswith('_'):
            raise ValueError('Event flavor must start with _underscore')

        d = {'source': {'space': self._event_space, 'id': self._source_id}}
        d.update(data)
        return {flavor: d}