Use 'Future<void>' insted of 'Future<Null>' in SystemChrome (#22583)

packages/flutter/lib/src/services/system_chrome.dart

@@ -237,7 +237,7 @@ class SystemChrome {
   /// The `orientation` argument is a list of [DeviceOrientation] enum values.
   /// The empty list causes the application to defer to the operating system
   /// default.
-  static Future<Null> setPreferredOrientations(List<DeviceOrientation> orientations) async {
+  static Future<void> setPreferredOrientations(List<DeviceOrientation> orientations) async {
     await SystemChannels.platform.invokeMethod(
       'SystemChrome.setPreferredOrientations',
       _stringify(orientations),
@@ -249,7 +249,7 @@ class SystemChrome {
   ///
   /// Any part of the description that is unsupported on the current platform
   /// will be ignored.
-  static Future<Null> setApplicationSwitcherDescription(ApplicationSwitcherDescription description) async {
+  static Future<void> setApplicationSwitcherDescription(ApplicationSwitcherDescription description) async {
     await SystemChannels.platform.invokeMethod(
       'SystemChrome.setApplicationSwitcherDescription',
       <String, dynamic>{
@@ -281,7 +281,7 @@ class SystemChrome {
   /// after a delay of 1 second. This can be achieved through [restoreSystemUIOverlays]
   /// or calling this again. Otherwise, the original UI overlay settings will be
   /// automatically restored only when the application loses and regains focus.
-  static Future<Null> setEnabledSystemUIOverlays(List<SystemUiOverlay> overlays) async {
+  static Future<void> setEnabledSystemUIOverlays(List<SystemUiOverlay> overlays) async {
     await SystemChannels.platform.invokeMethod(
       'SystemChrome.setEnabledSystemUIOverlays',
       _stringify(overlays),
@@ -297,7 +297,7 @@ class SystemChrome {
   ///
   /// On Android, the system UI cannot be changed until 1 second after the previous
   /// change. This is to prevent malware from permanently hiding navigation buttons.
-  static Future<Null> restoreSystemUIOverlays() async {
+  static Future<void> restoreSystemUIOverlays() async {
     await SystemChannels.platform.invokeMethod(
       'SystemChrome.restoreSystemUIOverlays',
       null,