如何在ionic 2中将Alert Direction改为RTL？

<button ion-button dir="rtl" (click)="presentAlert()">显示Alert</button>

import { Component } from '@angular/core';
import { AlertController } from 'ionic-angular';

@Component({
  selector: 'page-home',
  templateUrl: 'home.html'
})
export class HomePage {

  constructor(public alertCtrl: AlertController) {}

  presentAlert() {
    let alert = this.alertCtrl.create({
      title: 'Alert',
      subTitle: '这是一个RTL方向的Alert',
      buttons: ['确定']
    });
    alert.present();
  }

}