title: Alert before leaving page (navigate back) with Ionic v2
body: <p>How do you show an <code>alert</code>, that the user must close, before going back to the previous page? I'm using the standard <code>&lt;ion-navbar *navbar&gt;</code> <em>arrow button</em>.</p>
  
  <p>I tried hooking into the <code>NavController</code> event <code>ionViewWillLeave</code> like this, but it doesn't work:</p>
  
  <pre><code>ionViewWillLeave() {
    let alert = Alert.create({
      title: 'Bye',
      subTitle: 'Now leaving',
      buttons: ['OK']
    });
    this.nav.present(alert);
  }
  </code></pre>
  
  <p>This shows the alert, but doesn't go back once it is closed. If I comment it out, the back button works fine.</p>