anchor.diff:    }
   
     invoke2(arg1, arg2) {
  -    return guardAsync(
  +    return _guardAsync(
           () {
             if (shouldCallBack()) {
               return callback(arg1, arg2);
  
positive.code:   }
  
    invoke1(arg1) {
      return guardAsync(
          () {
            if (shouldCallBack()) {
              return callback(arg1);
  
negative.code:  *
   * The value returned by [tryBody] (if any) is returned by [guardAsync].
   */
  guardAsync(Function tryBody) {
    return _guardAsync(tryBody, null, _currentTest);
  }
  
  _guardAsync(Function tryBody, Function finallyBody, int testNum) {
    assert(testNum >= 0);
    try {
      return tryBody();
    } catch (e, trace) {
  