dropdown_menu/templates.xml

<openerp>
    <data>
        <template id="dropdown_menu.assets" name="Eiru Assets" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/dropdown_menu/static/src/main.js" />
            </xpath>
        </template>
    </data>
</openerp>