Make Demos in flutter_gallery more accessible (#10832)

Remaining known issues are #10831 and #10830.

examples/flutter_gallery/lib/demo/contacts_demo.dart

@@ -70,12 +70,14 @@ class _ContactItem extends StatelessWidget {
         )
       ));
     }
-    return new Padding(
+    return new MergeSemantics(
+      child: new Padding(
         padding: const EdgeInsets.symmetric(vertical: 16.0),
         child: new Row(
           mainAxisAlignment: MainAxisAlignment.spaceBetween,
           children: rowChildren
         )
+      ),
     );
   }
 }

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -246,7 +246,8 @@ class RecipeCard extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new GestureDetector(
+    return new MergeSemantics(
+      child: new GestureDetector(
         onTap: onTap,
         child: new Card(
           child: new Column(
@@ -283,6 +284,7 @@ class RecipeCard extends StatelessWidget {
             ],
           ),
         ),
+      ),
     );
   }
 }

examples/flutter_gallery/lib/demo/shrine/shrine_home.dart

@@ -252,7 +252,8 @@ class _Heading extends StatelessWidget {
   Widget build(BuildContext context) {
     final Size screenSize = MediaQuery.of(context).size;
     final ShrineTheme theme = ShrineTheme.of(context);
-    return new SizedBox(
+    return new MergeSemantics(
+      child: new SizedBox(
         height: screenSize.width > screenSize.height
           ? (screenSize.height - kToolbarHeight) * 0.85
           : (screenSize.height - kToolbarHeight) * 0.70,
@@ -287,6 +288,7 @@ class _Heading extends StatelessWidget {
             ],
           ),
         ),
+      ),
     );
   }
 }
@@ -303,7 +305,8 @@ class _ProductItem extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new Card(
+    return new MergeSemantics(
+      child: new Card(
         child: new Stack(
           children: <Widget>[
             new Column(
@@ -333,6 +336,7 @@ class _ProductItem extends StatelessWidget {
             ),
           ],
         ),
+      ),
     );
   }
 }