Add a new null safety example (#562)

experimental/null_safety/null_safe_app/lib/goodapp.dart

@@ -0,0 +1,54 @@
+// 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';
+
+import 'services.dart';
+
+class GoodMyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final localizedAppName = Config.getAppName();
+    final temperatures = WeatherService.getTemperatures();
+
+    List<Widget> tempWidgets;
+    if (temperatures == null) {
+      tempWidgets = [Text('Temperature: Failed getting forecast :-(')];
+    } else {
+      // Null safety uses flow analysis. The code checked for null in the branch
+      // above, so in this branch it known that temperatures cannot be null.
+      // Notice how we access temperatures without getting an analysis error.
+      tempWidgets = [
+        Text('Temperature next 3 days:'),
+        for (final t in temperatures)
+          // We first use the conditional member access operator to only call
+          // round() and toString() if t isn't null. We then test if that
+          // succeeded, and if not use a null conditional operator to supply a
+          // value handling the error case.
+          Text(t?.round().toString() ?? 'no forecast'),
+      ];
+    }
+
+    return MaterialApp(
+      home: Scaffold(
+        // Here we again use the null conditional operator.
+        appBar: AppBar(title: Text(localizedAppName ?? 'Weather')),
+        body: Padding(
+          padding: const EdgeInsets.all(8.0),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            // Notice how we're *not* getting an error saying that `tempWidgets`
+            // is null here despite tempWidgets being declared in line 9 without
+            // an initial value. This is because of null safety's definite
+            // assignment feature: We know that in all branches of the code
+            // (both the if- and else- branches above), the variable was
+            // assigned a value, and thus isn't null. Try removing the assigment
+            // from the if-branch and notice the immediate error.
+            children: tempWidgets,
+          ),
+        ),
+      ),
+    );
+  }
+}