title: Dismiss old alert and present latest alert - Ionic3
body: <p>I am using the alerts of Ionic 3 and I am facing an issue of alerts stacking up.
  I am using the network plugin to check if the user is connected to a network(WiFi/2G/3G etc.) or not and the idea is to trigger an alert every time the user goes offline or comes online.</p>
  
  <pre><code>this.connectSubscription = this.network.onConnect().subscribe(() =&gt; {
    console.log('network connected!');
    let connectedToInternet = this.alertController.create({
      subTitle: 'Connected to Internet',
      buttons: ['OK']
    });
    connectedToInternet.present();
  });
  
  this.disconnectSubscription = this.network.onDisconnect().subscribe(() =&gt; {
    let noInternetAlert = this.alertController.create({
      subTitle: 'No Internet Connection. Please enable Wifi or Mobile data to continue.',
      buttons: ['OK']
    });
    noInternetAlert.present();
  });
  </code></pre>
  
  <p><strong>Current behaviour:</strong> If the user disconnects and reconnects multiple times, for every instance of change in the network, an alert is presented and the alerts stack up on the view until the user manually dismisses the alerts.</p>
  
  <p><strong>Required behaviour:</strong> If the user disconnects and reconnects multiple times, for every instance of change in network, an alert should be presented while the older alert automatically gets dismissed so that at any given point in time, the user is not presented with more than one instance of any alert on the view.</p>