Migrate to Theme.brightnessOf method (#163950)

Refactor: Migrate to Theme.brightnessOf method
fixes: #163837

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

dev/integration_tests/flutter_gallery/lib/demo/material/date_and_time_picker_demo.dart

@@ -30,7 +30,7 @@ class _InputDropdown extends StatelessWidget {
             Icon(
               Icons.arrow_drop_down,
               color:
-                  Theme.of(context).brightness == Brightness.light
+                  Theme.brightnessOf(context) == Brightness.light
                       ? Colors.grey.shade700
                       : Colors.white70,
             ),

dev/integration_tests/flutter_gallery/lib/demo/material/menu_demo.dart

@@ -77,7 +77,7 @@ class MenuDemoState extends State<MenuDemo> {
       ),
       body: ListTileTheme(
         iconColor:
-            Theme.of(context).brightness == Brightness.light ? Colors.grey[600] : Colors.grey[500],
+            Theme.brightnessOf(context) == Brightness.light ? Colors.grey[600] : Colors.grey[500],
         child: ListView(
           padding: kMaterialListPadding,
           children: <Widget>[

dev/integration_tests/flutter_gallery/lib/gallery/app.dart

@@ -169,7 +169,7 @@ class _GalleryAppState extends State<GalleryApp> {
               Builder(
                 builder: (BuildContext context) {
                   return CupertinoTheme(
-                    data: CupertinoThemeData(brightness: Theme.of(context).brightness),
+                    data: CupertinoThemeData(brightness: Theme.brightnessOf(context)),
                     child: child!,
                   );
                 },

dev/integration_tests/flutter_gallery/lib/gallery/demo.dart

@@ -187,7 +187,7 @@ class FullScreenCodeDialogState extends State<FullScreenCodeDialog> {
   @override
   Widget build(BuildContext context) {
     final SyntaxHighlighterStyle style =
-        Theme.of(context).brightness == Brightness.dark
+        Theme.brightnessOf(context) == Brightness.dark
             ? SyntaxHighlighterStyle.darkThemeStyle
             : SyntaxHighlighterStyle.lightThemeStyle;
 

dev/integration_tests/flutter_gallery/lib/gallery/options.dart

@@ -131,7 +131,7 @@ class _BooleanItem extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final bool isDark = Theme.of(context).brightness == Brightness.dark;
+    final bool isDark = Theme.brightnessOf(context) == Brightness.dark;
     return _OptionsItem(
       child: Row(
         children: <Widget>[

dev/integration_tests/new_gallery/lib/studies/reply/app.dart

@@ -77,7 +77,7 @@ class _ReplyAppState extends State<ReplyApp> with RestorationMixin {
     final ThemeMode galleryThemeMode = GalleryOptions.of(context).themeMode;
     final bool isDark =
         galleryThemeMode == ThemeMode.system
-            ? Theme.of(context).brightness == Brightness.dark
+            ? Theme.brightnessOf(context) == Brightness.dark
             : galleryThemeMode == ThemeMode.dark;
 
     final ThemeData replyTheme =

dev/integration_tests/new_gallery/lib/studies/reply/mail_card_preview.dart

@@ -288,7 +288,7 @@ class _MailPreviewActionBar extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final bool isDark = Theme.of(context).brightness == Brightness.dark;
+    final bool isDark = Theme.brightnessOf(context) == Brightness.dark;
     final Color color = isDark ? ReplyColors.white50 : ReplyColors.blue600;
     final bool isDesktop = isDisplayDesktop(context);
     final Color starredIconColor = isStarred ? Theme.of(context).colorScheme.secondary : color;