code: function () {
        var a$ = this._instanceAttributes;
        for (var k in a$) {
          if (!this.hasAttribute(k)) {
            this.setAttribute(k, a$[k]);
          }
        }
      }
docs: copy attributes defined in the element declaration to the instance e.g. <polymer-element name="x-foo" tabIndex="0"> tabIndex is copied to the element instance here.