code: @Nullable
    public String getServiceId() throws AppEngineException {
      String serviceId = getText(getNode(document, "appengine-web-app", "service"));
      if (serviceId != null) {
        return serviceId;
      }
      return getText(getNode(document, "appengine-web-app", "module"));
    }
docs: Returns service ID from the &lt;service&gt; element of the appengine-web.xml, or null if it is
  missing. Will also look at module ID.