function warn(format) {
    {
      for (var _len = arguments.length, args = new Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
        args[_key - 1] = arguments[_key];
      }

      printWarning('warn', format, args);
    }
  }
Used by act() to track whether you're inside an act() scope.