Prueba test Flutter positioned into Stack

ads.dart

import 'package:firebase_admob/firebase_admob.dart';
import '../services/globals.dart';

class Ads {
  InterstitialAd myInterstitial;

  MobileAdTargetingInfo targetingInfo = new MobileAdTargetingInfo(
    testDevices: TEST_DEVICES,
    /* keywords: <String>[
      "tarot",
      "horoscopo gratis",
      "horoscopo diario",
      "tarot gratis",
      "videntes buenas",
      "free tarot reading",
      "voyance",
      "free voyance",
      "free Horoscope"
    ],
    contentUrl: "https://tarotgratishd.com/",
    childDirected: false,
    designedForFamilies: true,
    gender: MobileAdGender.unknown,
    */
  );

  InterstitialAd createBannerInter() {
    return new InterstitialAd(
      adUnitId: AD_MOB_INTERSITIAL_ID, //InterstitialAd.testAdUnitId,
      targetingInfo: targetingInfo,
      listener: (MobileAdEvent event) {
        print("BannerAd event is $event");
        if (event == MobileAdEvent.closed) {}
      },
    );
  }

  void showInter() {
    FirebaseAdMob.instance.initialize(appId: AD_MOB_APP_ID);
    myInterstitial = createBannerInter()
      ..load()
      ..show();
  }

  void showPub() {
    print("no hace nada por ahora");
  }
  
  //**********************************************
  //la idea es hacer algo asi para poder llamarlo desde otras pantallas, el load al cargar la pantalla y 
  // el show en cualquier evento que yo quiera. Ads().loadInter(); y Ads().showInter()
  
  showInter(){
     myInterstitial..show();
  }
  
  loadInter(){
      FirebaseAdMob.instance.initialize(appId: AD_MOB_APP_ID);
    myInterstitial = createBannerInter()
      ..load();
  }
}

pruebas.dart

// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can
// be found in the LICENSE file.

// Demonstrates a basic shared element (Hero) animation.

import 'package:flutter/material.dart';

import '../comunes/recursos.dart' as _recursos;

class Pruebas extends StatelessWidget {
  Widget build(BuildContext context) {
    var arregloListaCartas = [
      'm0',
      'm1',
      'm2',
      'm3',
      'm4',
      'm5',
      'm6',
      'm7',
      'm8',
      'm9',
      'm10',
      'm11',
      'm12',
      'm13',
      'm14',
      'm15',
      'm16',
      'm17',
      'm18',
      'm19',
      'm20',
      'm21'
    ];
    var randomItem = (arregloListaCartas..shuffle());

    aleatorio(int _id) {
      print(randomItem[_id]);
    }

    Widget _stacker() {
      return Container(
          height: 196.0,
          margin: const EdgeInsets.only(top: 40.0),
          child: new ListView(
            scrollDirection: Axis.horizontal,
            children: <Widget>[
              new Stack(
                  children: new List.generate(22, (int index) {
                   
                return GestureDetector(
                    onTap: () {
                      aleatorio(index);
                    },
                    child: new Container(
                        // padding: const EdgeInsets.all(4.0),
                        // margin: const EdgeInsets.only(left: 20.0),
                        margin: const EdgeInsets.only(right: 5.0),
                        width: 110.0,
                        height: 196.0,
                        child: new Image(
                            image: new AssetImage('assets/bgcard-a.jpg'),
                            fit: BoxFit.cover)));

              }))
            ],
          ));
    }

    return Scaffold(
      appBar: AppBar(
        title: const Text('Prueba test'),
      ),
      body: new Stack(
        children: <Widget>[
          new Container(
            decoration: _recursos.backgroundImageComun(), 
          ),
          _stacker()
        ],
      ),
    );
  }
}