code: function(extnds) {
        var prototype = this.findBasePrototype(extnds);
        if (!prototype) {
          // create a prototype based on tag-name extension
          var prototype = HTMLElement.getPrototypeForTag(extnds);
          // insert base api in inheritance chain (if needed)
          prototype = this.ensureBaseApi(prototype);
          // memoize this base
          memoizedBases[extnds] = prototype;
        }
        return prototype;
      }
docs: build prototype combining extendee, Polymer base, and named api