sync_mail_notification/views/mail_notification_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <template id="assets_backend" name="sync_mail assets" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/sync_mail_notification/static/src/js/mail_notification.js"></script>
            </xpath>
        </template>
    </data>
</openerp>