Add a new null safety example (#562)

experimental/null_safety/null_safe_app/lib/services.dart

@@ -0,0 +1,34 @@
+// 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.
+
+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];
+      }
+    }
+  }
+}