Gallery demos: reordered and retitled (#17220)

examples/flutter_gallery/lib/gallery/home.dart

@@ -176,6 +176,25 @@ class _DemoItem extends StatelessWidget {
     final bool isDark = theme.brightness == Brightness.dark;
     final double textScaleFactor = MediaQuery.of(context)?.textScaleFactor ?? 1.0;
 
+    final List<Widget> titleChildren = <Widget>[
+      new Text(
+        demo.title,
+        style: theme.textTheme.subhead.copyWith(
+          color: isDark ? Colors.white : const Color(0xFF202124),
+        ),
+      ),
+    ];
+    if (demo.subtitle != null) {
+      titleChildren.add(
+        new Text(
+          demo.subtitle,
+          style: theme.textTheme.body1.copyWith(
+            color: isDark ? Colors.white : const Color(0xFF60646B)
+          ),
+        ),
+      );
+    }
+
     return new RawMaterialButton(
       padding: EdgeInsets.zero,
       splashColor: theme.primaryColor.withOpacity(0.12),
@@ -201,19 +220,7 @@ class _DemoItem extends StatelessWidget {
               child: new Column(
                 mainAxisAlignment: MainAxisAlignment.center,
                 crossAxisAlignment: CrossAxisAlignment.stretch,
-                children: <Widget>[
-                  new Text(
-                    demo.title,
-                    style: theme.textTheme.subhead.copyWith(
-                      color: isDark ? Colors.white : const Color(0xFF202124),
-                    ),
-                  ),
-                  new Text(
-                    demo.subtitle,
-                    style: theme.textTheme.body1.copyWith(
-                      color: isDark ? Colors.white : const Color(0xFF60646B)),
-                  ),
-                ],
+                children: titleChildren,
               ),
             ),
             const SizedBox(width: 44.0),