example_main.dart

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

import 'package:flutter/services.dart';
import 'package:flutter_alert_demo/flutter_alert_demo.dart';

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

class MyApp extends StatefulWidget {
 @override
 _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {
 String _platformVersion = 'Unknown';

 @override
 void initState() {
   super.initState();
   initPlatformState();
 }

 // Platform messages are asynchronous, so we initialize in an async method.
 Future<void> initPlatformState() async {
   String platformVersion;
   // Platform messages may fail, so we use a try/catch PlatformException.
   try {
     platformVersion = await FlutterAlertDemo.platformVersion;
     FlutterAlertDemo.showAleartDialog();
   } on PlatformException {
     platformVersion = 'Failed to get platform version.';
   }

   // If the widget was removed from the tree while the asynchronous platform
   // message was in flight, we want to discard the reply rather than calling
   // setState to update our non-existent appearance.
   if (!mounted) return;

   setState(() {
     _platformVersion = platformVersion;
   });
 }

 @override
 Widget build(BuildContext context) {
   return new MaterialApp(
     home: new Scaffold(
         appBar: new AppBar(
           title: const Text('Plugin example app'),
         ),
         body:  Center(
           child:  MaterialButton(onPressed: (){
             FlutterAlertDemo.showAleartDialog;
           }, child: Text("Show Aleart",style: TextStyle(color: Colors.white),),color: Colors.blue,),)
     ),
   );
 }
}

mispelling
showAleartDialog should be
showAlertDialog