Assets defined in overridden blocks of embedded Twig templates are not loaded

Using the Twig [`embed`](http://twig.sensiolabs.org/doc/tags/embed.html) tag, I am including a template and extending/overriding blocks within it. This is the code in question:

````twig
{% block footer %}
    {% embed 'Bundle::footer.html.twig' %}
        {% block sponsors %}
            {{ parent() }}
            <li>
                <a href="http://website.org">
                    {% image '@Bundle/Resources/public/images/logo.png' %}
                    <img src="{{ asset_url }}" alt="Organization Logo" />
                    {% endimage %}
                </a>
            </li>
        {% endblock %}
    {% endembed %}
{% endblock %}
````

However, the blocks inside the embed tag are never scanned for instances of AsseticNode, because the embedded template is not a child but rather it's an attribute on the node.