Fix system chrome colors on gallery and definition in iOS (#18735)

examples/flutter_gallery/lib/demo/contacts_demo.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 
 class _ContactCategory extends StatelessWidget {
   const _ContactCategory({ Key key, this.icon, this.children }) : super(key: key);
@@ -182,7 +183,9 @@ class ContactsDemoState extends State<ContactsDemo> {
             ),
             new SliverList(
               delegate: new SliverChildListDelegate(<Widget>[
-                new _ContactCategory(
+                new AnnotatedRegion<SystemUiOverlayStyle>(
+                  value: SystemUiOverlayStyle.light,
+                  child: new _ContactCategory(
                     icon: Icons.call,
                     children: <Widget>[
                       new _ContactItem(
@@ -226,6 +229,7 @@ class ContactsDemoState extends State<ContactsDemo> {
                       ),
                     ],
                   ),
+                ),
                 new _ContactCategory(
                   icon: Icons.contact_mail,
                   children: <Widget>[

examples/flutter_gallery/lib/gallery/home.dart

@@ -7,6 +7,7 @@ import 'dart:developer';
 import 'dart:math' as math;
 
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 
 import 'backdrop.dart';
 import 'demos.dart';
@@ -399,6 +400,10 @@ class _GalleryHomeState extends State<GalleryHome> with SingleTickerProviderStat
         ]
       );
     }
+    home = new AnnotatedRegion<SystemUiOverlayStyle>(
+      child: home,
+      value: SystemUiOverlayStyle.dark
+    );
 
     return home;
   }

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

@@ -105,7 +105,7 @@ class SystemUiOverlayStyle {
     statusBarColor: null,
     systemNavigationBarIconBrightness: Brightness.dark,
     statusBarIconBrightness: Brightness.dark,
-    statusBarBrightness: Brightness.dark,
+    statusBarBrightness: Brightness.light,
   );
 
   /// System overlays should be drawn with a dark color. Intended for
@@ -116,7 +116,7 @@ class SystemUiOverlayStyle {
     statusBarColor: null,
     systemNavigationBarIconBrightness: Brightness.light,
     statusBarIconBrightness: Brightness.light,
-    statusBarBrightness: Brightness.light,
+    statusBarBrightness: Brightness.dark,
   );
 
   /// Creates a new [SystemUiOverlayStyle].
@@ -151,7 +151,7 @@ class SystemUiOverlayStyle {
 
   /// The brightness of top status bar.
   /// 
-  /// Only honored in iOS .
+  /// Only honored in iOS.
   final Brightness statusBarBrightness;
 
   /// The brightness of the top status bar icons.