Adds add2app Flutter module and an iOS project to run it. (#126)

experimental/add_to_app/example_module/lib/main.dart

@@ -0,0 +1,103 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+void main() => runApp(MyApp());
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter Module Title',
+      routes: {
+        '/': (context) => FullScreenView(),
+        '/mini': (context) => MiniView(),
+      },
+    );
+  }
+}
+
+class FullScreenView extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('A Full-screen Flutter View'),
+      ),
+      body: const MiniView(showExit: true),
+    );
+  }
+}
+
+class MiniView extends StatefulWidget {
+  final bool showExit;
+
+  const MiniView({this.showExit = false});
+
+  @override
+  _MiniViewState createState() => _MiniViewState();
+}
+
+class _MiniViewState extends State<MiniView> {
+  int count = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    final mediaInfo = MediaQuery.of(context);
+
+    return SizedBox.expand(
+      child: Stack(
+        children: [
+          Positioned.fill(
+            child: DecoratedBox(
+              decoration: BoxDecoration(
+                color: Theme.of(context).scaffoldBackgroundColor,
+              ),
+            ),
+          ),
+          Positioned.fill(
+            child: Opacity(
+              opacity: .25,
+              child: FittedBox(
+                fit: BoxFit.cover,
+                child: FlutterLogo(),
+              ),
+            ),
+          ),
+          Center(
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: [
+                Text(
+                  'Window is ${mediaInfo.size.width} x '
+                  '${mediaInfo.size.height}.',
+                  style: Theme.of(context).textTheme.headline,
+                ),
+                SizedBox(height: 16),
+                Text(
+                  'Taps: $count',
+                  style: Theme.of(context).textTheme.headline,
+                ),
+                SizedBox(height: 16),
+                RaisedButton(
+                  onPressed: () => setState(() => count++),
+                  child: Text('Tap me!'),
+                ),
+                if (widget.showExit) ...[
+                  SizedBox(height: 16),
+                  RaisedButton(
+                    onPressed: () => SystemNavigator.pop(),
+                    child: Text('Exit this screen'),
+                  ),
+                ],
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}