.__templatizeOwner) {
          throw new Error('A <template> can only be templatized once');
        }
        template.__templatizeOwner = owner;
        let templateInfo = owner.constructor._parseTemplate(template);
        // Get memoized base class for the prototypical template, which
        // includes property effects for binding template & forwarding
        let baseClass = templateInfo.templatizeInstanceClass;
        if (!baseClass) {
          baseClass = createTemplatizerClass(template, templat