Given a dependency tree of objects, output it in DFS order.

def output_tree(self, t, seen):
        """Given a dependency tree of objects, output it in DFS order."""
        if not t:
            return
        for c in t.children:
            self.output_tree(c, seen)
        if t.name in seen:
            return
        seen[t.name] = True
        if t.resource_name:
            output_class(t.name, t.props, self.override, t.resource_name)
        else:
            output_class(t.name, t.props, self.override)