AdMob Banner on one screen

AdMobBannerOneScreen.dart

import 'package:flutter/material.dart';
import 'package:firebase_admob/firebase_admob.dart';

void main() => runApp(new MyApp());

BannerAd myBanner = new BannerAd(
  adUnitId: BannerAd.testAdUnitId,
  size: AdSize.smartBanner,
  listener: (MobileAdEvent event) {
    print("BannerAd event is $event");
  },
);

BannerAd _bannerAd = myBanner..load();

banner() {
  print("load and show banner");
  myBanner
    ..load
    ..show(
      anchorType: AnchorType.bottom,
    );
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  void initState() {
    _bannerAd..show();
    super.initState();
  }

  @override
  void dispose() {
    try {
      _bannerAd?.dispose();
    } catch (e) {}
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    _bannerAd..load();
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new RaisedButton(
              onPressed: () {
                Navigator.push(context,
                    MaterialPageRoute(builder: (context) => new TestScreen()));
              },
              child: new Text("test screen"),
            )
          ],
        ),
      ),
    );
  }
}

class TestScreen extends StatefulWidget {
  @override
  TestScreenState createState() => new TestScreenState();
}

class TestScreenState extends State<TestScreen> {
  @override
  void initState() {
    try {
      _bannerAd?.dispose();
    } catch (e) {}
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Test Screen"),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new RaisedButton(
              onPressed: () {
                print("goBack");
                Navigator.pop(context);
              },
              child: new Text("go back"),
            )
          ],
        ),
      ),
    );
  }
}