Make various images in gallery accessible (#14467)

Discovered during an a11y review of the gallery.

examples/flutter_gallery/lib/demo/colors_demo.dart

@@ -60,20 +60,23 @@ class ColorItem extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new Container(
-      height: kColorItemHeight,
-      padding: const EdgeInsets.symmetric(horizontal: 16.0),
-      color: color,
-      child: new SafeArea(
-        top: false,
-        bottom: false,
-        child: new Row(
-          mainAxisAlignment: MainAxisAlignment.spaceBetween,
-          crossAxisAlignment: CrossAxisAlignment.center,
-          children: <Widget>[
-            new Text('$prefix$index'),
-            new Text(colorString()),
-          ],
+    return new Semantics(
+      container: true,
+      child: new Container(
+        height: kColorItemHeight,
+        padding: const EdgeInsets.symmetric(horizontal: 16.0),
+        color: color,
+        child: new SafeArea(
+          top: false,
+          bottom: false,
+          child: new Row(
+            mainAxisAlignment: MainAxisAlignment.spaceBetween,
+            crossAxisAlignment: CrossAxisAlignment.center,
+            children: <Widget>[
+              new Text('$prefix$index'),
+              new Text(colorString()),
+            ],
+          ),
         ),
       ),
     );

examples/flutter_gallery/lib/demo/cupertino/cupertino_navigation_demo.dart

@@ -218,12 +218,18 @@ class Tab1RowItem extends StatelessWidget {
               ),
               new CupertinoButton(
                 padding: EdgeInsets.zero,
-                child: const Icon(CupertinoIcons.plus_circled, color: CupertinoColors.activeBlue),
+                child: const Icon(CupertinoIcons.plus_circled,
+                  color: CupertinoColors.activeBlue,
+                  semanticLabel: 'Add',
+                ),
                 onPressed: () { },
               ),
               new CupertinoButton(
                 padding: EdgeInsets.zero,
-                child: const Icon(CupertinoIcons.share, color: CupertinoColors.activeBlue),
+                child: const Icon(CupertinoIcons.share,
+                  color: CupertinoColors.activeBlue,
+                  semanticLabel: 'Share',
+                ),
                 onPressed: () { },
               ),
             ],

examples/flutter_gallery/lib/demo/images_demo.dart

@@ -14,13 +14,19 @@ class ImagesDemo extends StatelessWidget {
           tabName: 'ANIMATED WEBP',
           description: '',
           exampleCodeTag: 'animated_image',
-          demoWidget: new Image.asset('packages/flutter_gallery_assets/animated_flutter_stickers.webp'),
+          demoWidget: new Semantics(
+            label: 'Example of animated WEBP',
+            child: new Image.asset('packages/flutter_gallery_assets/animated_flutter_stickers.webp'),
+          ),
         ),
         new ComponentDemoTabData(
           tabName: 'ANIMATED GIF',
           description: '',
           exampleCodeTag: 'animated_image',
-          demoWidget: new Image.asset('packages/flutter_gallery_assets/animated_flutter_lgtm.gif'),
+          demoWidget: new Semantics(
+            label: 'Example of animated GIF',
+            child:new Image.asset('packages/flutter_gallery_assets/animated_flutter_lgtm.gif'),
+          ),
         ),
       ]
     );

examples/flutter_gallery/lib/demo/material/bottom_navigation_demo.dart

@@ -7,14 +7,15 @@ import 'package:flutter/material.dart';
 class NavigationIconView {
   NavigationIconView({
     Widget icon,
-    Widget title,
+    String title,
     Color color,
     TickerProvider vsync,
   }) : _icon = icon,
        _color = color,
+       _title = title,
        item = new BottomNavigationBarItem(
          icon: icon,
-         title: title,
+         title: new Text(title),
          backgroundColor: color,
        ),
        controller = new AnimationController(
@@ -29,6 +30,7 @@ class NavigationIconView {
 
   final Widget _icon;
   final Color _color;
+  final String _title;
   final BottomNavigationBarItem item;
   final AnimationController controller;
   CurvedAnimation _animation;
@@ -56,7 +58,10 @@ class NavigationIconView {
             color: iconColor,
             size: 120.0,
           ),
-          child: _icon,
+          child: new Semantics(
+            label: 'Placeholder for $_title tab',
+            child: _icon,
+          ),
         ),
       ),
     );
@@ -95,31 +100,31 @@ class _BottomNavigationDemoState extends State<BottomNavigationDemo>
     _navigationViews = <NavigationIconView>[
       new NavigationIconView(
         icon: const Icon(Icons.access_alarm),
-        title: const Text('Alarm'),
+        title: 'Alarm',
         color: Colors.deepPurple,
         vsync: this,
       ),
       new NavigationIconView(
         icon: new CustomIcon(),
-        title: const Text('Box'),
+        title: 'Box',
         color: Colors.deepOrange,
         vsync: this,
       ),
       new NavigationIconView(
         icon: const Icon(Icons.cloud),
-        title: const Text('Cloud'),
+        title: 'Cloud',
         color: Colors.teal,
         vsync: this,
       ),
       new NavigationIconView(
         icon: const Icon(Icons.favorite),
-        title: const Text('Favorites'),
+        title: 'Favorites',
         color: Colors.indigo,
         vsync: this,
       ),
       new NavigationIconView(
         icon: const Icon(Icons.event_available),
-        title: const Text('Event'),
+        title: 'Event',
         color: Colors.pink,
         vsync: this,
       )

examples/flutter_gallery/lib/demo/material/page_selector_demo.dart

@@ -7,7 +7,7 @@ import 'package:flutter/material.dart';
 class _PageSelector extends StatelessWidget {
   const _PageSelector({ this.icons });
 
-  final List<IconData> icons;
+  final List<Icon> icons;
 
   void _handleArrowButtonPress(BuildContext context, int delta) {
     final TabController controller = DefaultTabController.of(context);
@@ -46,18 +46,23 @@ class _PageSelector extends StatelessWidget {
             )
           ),
           new Expanded(
-            child: new TabBarView(
-              children: icons.map((IconData icon) {
-                return new Container(
-                  key: new ObjectKey(icon),
-                  padding: const EdgeInsets.all(12.0),
-                  child: new Card(
-                    child: new Center(
-                      child: new Icon(icon, size: 128.0, color: color)
+            child: new IconTheme(
+              data: new IconThemeData(
+                size: 128.0,
+                color: color,
+              ),
+              child: new TabBarView(
+                children: icons.map((Icon icon) {
+                  return new Container(
+                    padding: const EdgeInsets.all(12.0),
+                    child: new Card(
+                      child: new Center(
+                        child: icon,
+                      ),
                     ),
-                  ),
-                );
-              }).toList()
+                  );
+                }).toList()
+              ),
             ),
           ),
         ],
@@ -68,13 +73,13 @@ class _PageSelector extends StatelessWidget {
 
 class PageSelectorDemo extends StatelessWidget {
   static const String routeName = '/material/page-selector';
-  static final List<IconData> icons = <IconData>[
-    Icons.event,
-    Icons.home,
-    Icons.android,
-    Icons.alarm,
-    Icons.face,
-    Icons.language,
+  static final List<Icon> icons = <Icon>[
+    const Icon(Icons.event, semanticLabel: 'Event'),
+    const Icon(Icons.home, semanticLabel: 'Home'),
+    const Icon(Icons.android, semanticLabel: 'Android'),
+    const Icon(Icons.alarm, semanticLabel: 'Alarm'),
+    const Icon(Icons.face, semanticLabel: 'Face'),
+    const Icon(Icons.language, semanticLabel: 'Language'),
   ];
 
   @override

examples/flutter_gallery/lib/demo/material/scrollable_tabs_demo.dart

@@ -117,6 +117,7 @@ class ScrollableTabsDemoState extends State<ScrollableTabsDemo> with SingleTicke
                     page.icon,
                     color: iconColor,
                     size: 128.0,
+                    semanticLabel: 'Placeholder for ${page.text} tab',
                   ),
                 ),
               ),