testDoubleCalling2() {
    const ex = assertThrows(/**
                             @suppress {checkTypes} suppression added to enable
                             type checking
                           */
                            function() {
                              Deferred.fail(1).errback(2);
                              neverHappen();
                            });
    assertTrue('double-fail', ex instanceof AlreadyCalledError);
  }
@suppress {checkTypes} suppression added to enable
                             type checking