Upgrade null safety example (#748)

null_safety/null_safe_app/test/widget_test.dart

@@ -0,0 +1,14 @@
+// This is a basic widget test that tests just the good app code.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:null_safe_app/goodapp.dart';
+
+void main() {
+  testWidgets('App smoke test', (WidgetTester tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(GoodMyApp());
+
+    // Verify that we have a forecast, or a managed failure.
+    expect(find.textContaining('Temperature'), findsOneWidget);
+  });
+}