Add an example of listening to app lifecycle events (#4623)

Fixes #4553

examples/layers/services/lifecycle.dart

+// Copyright 2016 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.
+
+import 'package:flutter/widgets.dart';
+
+class LifecycleWatcher extends StatefulWidget {
+  @override
+  _LifecycleWatcherState createState() => new _LifecycleWatcherState();
+}
+
+class _LifecycleWatcherState extends State<LifecycleWatcher>
+                             with WidgetsBindingObserver {
+  AppLifecycleState _lastLifecyleState;
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    setState(() {
+      _lastLifecyleState = state;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (_lastLifecyleState == null)
+      return new Text('This widget has not observed any lifecycle changes.');
+    return new Text('The most recent lifecycle state this widget observed was: $_lastLifecyleState.');
+  }
+}
+
+
+void main() {
+  runApp(new Center(
+    child: new LifecycleWatcher()
+  ));
+}

packages/flutter_tools/lib/src/run.dart

@@ -394,9 +394,9 @@ String getMissingPackageHintForPlatform(TargetPlatform platform) {
   switch (platform) {
     case TargetPlatform.android_arm:
     case TargetPlatform.android_x64:
-      return 'Is your project missing an android/AndroidManifest.xml?';
+      return 'Is your project missing an android/AndroidManifest.xml?\nConsider running "flutter create ." to create one.';
     case TargetPlatform.ios:
-      return 'Is your project missing an ios/Info.plist?';
+      return 'Is your project missing an ios/Info.plist?\nConsider running "flutter create ." to create one.';
     default:
       return null;
   }