prefer_void_to_null in flutter_localizations (#22826)

packages/flutter_localizations/test/date_picker_test.dart

@@ -225,7 +225,7 @@ void main() {
   });
 }
 
-Future<Null> _pumpBoilerplate(
+Future<void> _pumpBoilerplate(
   WidgetTester tester,
   Widget child, {
   Locale locale = const Locale('en', 'US'),

packages/flutter_localizations/test/time_picker_test.dart

@@ -47,7 +47,7 @@ Future<Offset> startPicker(WidgetTester tester, ValueChanged<TimeOfDay> onChange
   return tester.getCenter(find.byKey(const Key('time-picker-dial')));
 }
 
-Future<Null> finishPicker(WidgetTester tester) async {
+Future<void> finishPicker(WidgetTester tester) async {
   final MaterialLocalizations materialLocalizations = MaterialLocalizations.of(tester.element(find.byType(RaisedButton)));
   await tester.tap(find.text(materialLocalizations.okButtonLabel));
   await tester.pumpAndSettle(const Duration(seconds: 1));
@@ -130,7 +130,7 @@ void main() {
   const List<String> labels12To11TwoDigit = <String>['12', '01', '02', '03', '04', '05', '06', '07', '08', '09', '10', '11'];
   const List<String> labels00To23 = <String>['00', '13', '14', '15', '16', '17', '18', '19', '20', '21', '22', '23'];
 
-  Future<Null> mediaQueryBoilerplate(WidgetTester tester, bool alwaysUse24HourFormat) async {
+  Future<void> mediaQueryBoilerplate(WidgetTester tester, bool alwaysUse24HourFormat) async {
     await tester.pumpWidget(
       Localizations(
         locale: const Locale('en', 'US'),

packages/flutter_localizations/test/widgets_test.dart

@@ -230,7 +230,7 @@ void main() {
       DefaultWidgetsLocalizations.delegate,
     ];
 
-    Future<Null> pumpTest(Locale locale) async {
+    Future<void> pumpTest(Locale locale) async {
       await tester.pumpWidget(Localizations(
         locale: locale,
         delegates: delegates,