Move README sample to a separate file

Author: aednlaxer

packages/google_maps_flutter/google_maps_flutter/README.md

@@ -121,7 +121,7 @@ in an unbounded widget will cause the application to throw a Flutter exception.
 [Advanced Markers](https://developers.google.com/maps/documentation/javascript/advanced-markers/overview) are map markers that offer extra customization options. [Map Id](https://developers.google.com/maps/documentation/get-map-id) is required in order to use Advanced Markers:
 
 
-<?code-excerpt "readme_sample.dart (AdvancedMarkersSample)"?>
+<?code-excerpt "readme_sample_advanced_markers.dart (AdvancedMarkersSample)"?>
 ```dart
 GoogleMap(
   // Set your Map Id

packages/google_maps_flutter/google_maps_flutter/example/lib/readme_sample.dart

@@ -8,7 +8,6 @@ import 'dart:async';
 
 import 'package:flutter/material.dart';
 import 'package:google_maps_flutter/google_maps_flutter.dart';
-import 'package:google_maps_flutter_platform_interface/google_maps_flutter_platform_interface.dart';
 
 void main() => runApp(const MyApp());
 
@@ -71,28 +70,3 @@ class MapSampleState extends State<MapSample> {
   }
 }
 // #enddocregion MapSample
-
-// #docregion AdvancedMarkersSample
-class AdvancedMarkersSample extends StatelessWidget {
-  const AdvancedMarkersSample({super.key});
-
-  static const CameraPosition _kGooglePlex = CameraPosition(
-    target: LatLng(37.42796133580664, -122.085749655962),
-    zoom: 14.4746,
-  );
-
-  @override
-  Widget build(BuildContext context) {
-    return const Scaffold(
-      body: GoogleMap(
-        // Set your Map Id
-        mapId: 'my-map-id',
-
-        // Let map know that you're using Advanced Markers
-        markerType: MarkerType.advancedMarker,
-        initialCameraPosition: _kGooglePlex,
-      ),
-    );
-  }
-}
-// #enddocregion AdvancedMarkersSample

packages/google_maps_flutter/google_maps_flutter/example/lib/readme_sample_advanced_markers.dart

@@ -0,0 +1,48 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: public_member_api_docs
+
+import 'package:flutter/material.dart';
+import 'package:google_maps_flutter/google_maps_flutter.dart';
+import 'package:google_maps_flutter_platform_interface/google_maps_flutter_platform_interface.dart';
+
+void main() => runApp(const MyApp());
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      title: 'Flutter Google Maps Demo',
+      home: AdvancedMarkersSample(),
+    );
+  }
+}
+
+// #docregion AdvancedMarkersSample
+class AdvancedMarkersSample extends StatelessWidget {
+  const AdvancedMarkersSample({super.key});
+
+  static const CameraPosition _kGooglePlex = CameraPosition(
+    target: LatLng(37.42796133580664, -122.085749655962),
+    zoom: 14.4746,
+  );
+
+  @override
+  Widget build(BuildContext context) {
+    return const Scaffold(
+      body: GoogleMap(
+        // Set your Map Id
+        mapId: 'my-map-id',
+
+        // Let map know that you're using Advanced Markers
+        markerType: MarkerType.advancedMarker,
+        initialCameraPosition: _kGooglePlex,
+      ),
+    );
+  }
+}
+// #enddocregion AdvancedMarkersSample