调用textContent时$mdDialog返回错误

Question

$mdDialog.show(
    $mdDialog.alert()
    .title('Confirm delete?')
    .clickOutsideToClose(true)
    .textContent('Hello world')
    .ok('Yes')
    .targetEvent(ev)
  );

我的浏览器记录这个错误，如下所示。$mdDialog.alert(...).title(...).clickOutsideToClose(...).textContent is not a function