[null-safety] migrate hello_world to null safety (#70112)

* [null-safety] migrate flutter gallery to null safety

examples/hello_world/pubspec.yaml

@@ -2,7 +2,7 @@ name: hello_world
 
 environment:
   # The pub client defaults to an <2.0.0 sdk constraint which we need to explicitly overwrite.
-  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+  sdk: ">=2.12.0-0 <3.0.0"
 
 dependencies:
   flutter:

examples/hello_world/test_driver/smoke_web_engine_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.8
 import 'dart:async';
 
 import 'package:flutter_driver/flutter_driver.dart';
@@ -23,9 +24,7 @@ void main() {
 
     // Close the connection to the driver after the tests have completed.
     tearDownAll(() async {
-      if (driver != null) {
-        driver.close();
-      }
+      driver.close();
     });
 
     test('title is correct', () async {