code: function buildUpdateFn(element, className, attrs) {
      return function updateAttrValue(fallback) {
        if (!needsInterpolation(fallback)) {
          // Do not modify the element's attribute value; so
          // uses '<ui-layout layout="/api/sidebar.html" />' will not
          // be affected. Just update the attrs value.
          attrs[attrs.$normalize(className)] = fallback;
        }
      };
    }
docs: Replace current attribute value with fallback value