Upgrade null safety example (#748)

null_safety/null_unsafe_app/lib/main.dart

@@ -0,0 +1,70 @@
+// Copyright 2020, the Flutter project authors. Please see the AUTHORS file
+// for details. 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';
+
+void main() {
+  runApp(MyApp());
+}
+
+// This app simulates possible null errors. Try running it and see if it fails.
+// You can then try to hot reload a few times; you should see it occasionally
+// failing and occasionally succeeding.
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    // Get data from services. Note: in a real application,
+    // these would be async calls, but we’re using sync calls
+    // for simplicity.
+    final localizedAppName = Config.getAppName();
+    final temperatures = WeatherService.getTemperatures();
+
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(title: Text(localizedAppName)),
+        body: Padding(
+          padding: const EdgeInsets.all(8.0),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children: [
+              Text('Temperature next 3 days:'),
+              for (final t in temperatures) Text(t.round().toString()),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+class Config {
+  static String getAppName() {
+    // Imagine this looks up a localized version of the app name. We're using
+    // the current time to simulate a variance in responses.
+    if (DateTime.now().second.isEven) {
+      return 'Weather forecast';
+    } else {
+      // Oops, we don't have a localization.
+      return null;
+    }
+  }
+}
+
+class WeatherService {
+  static List<double> getTemperatures() {
+    // Imagine this makes a network call to get the current temperature.
+    // We're using the current time to simulate a variance in responses.
+    if (DateTime.now().millisecond.isEven) {
+      return [32.2, 34.5, 31.0];
+    } else {
+      if ((DateTime.now().second / 10).round().isEven) {
+        // Oops, we couldn't get any temperatures.
+        return null;
+      } else {
+        // Oops, we couldn't get one of the temperatures.
+        return [32.2, 34.5, null, 31.0];
+      }
+    }
+  }
+}