How to integrate with google maps in flutter?

I started to develop with flutter and I want to use google_maps_flutter package.

I was going through medium post in order to add map to my app (https://medium.com/flutter-io/google-maps-and-flutter-cfb330f9a245).

I added this code in AppDelegate.m

#include "AppDelegate.h"
#include "GeneratedPluginRegistrant.h"
#import "GoogleMaps/GoogleMaps.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application
    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  [GMSServices provideAPIKey:@"******"];
  [GeneratedPluginRegistrant registerWithRegistry:self];
  return [super application:application didFinishLaunchingWithOptions:launchOptions];
}
@end

and this to Info.plist

<key>io.flutter.embedded_views_preview</key>
<true/>

and this is the widget code:

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

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

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

class _MyAppState extends State<MyApp> {
  GoogleMapController mapController;

  final LatLng _center = const LatLng(45.521563, -122.677433);

  void _onMapCreated(GoogleMapController controller) {
    mapController = controller;
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Maps Sample App'),
          backgroundColor: Colors.green[700],
        ),
        body: GoogleMap(
          onMapCreated: _onMapCreated,
          options: GoogleMapOptions(
            cameraPosition: CameraPosition(
              target: _center,
              zoom: 11.0,
            ),
          ),
        ),
      ),
    );
  }
}

As the example in the post, but unfortunately I get a screen with the google maps widget but not showing a map. like this: https://i.imgur.com/HGam5ac.jpg (I can't upload an image because low reputation, sorry :( ).

Is there a problem with the sdk? or there is something to change in the code?

Thank you for your help!!