Correct grammar in shrine demo (#4381)

I was also going to implement sorting and emptying the cart but the
current data model doesn't make that easy, so I gave up on that. That's
why the TODOs are moved around though.

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

@@ -162,7 +162,10 @@ class _OrderPageState extends State<OrderPage> {
       floatingActionButton: new FloatingActionButton(
         onPressed: () {
           updateOrder(inCart: true);
-          showSnackBarMessage('There are ${currentOrder.quantity} items in the shopping cart');
+          final int n = currentOrder.quantity;
+          showSnackBarMessage(
+            'There ${ n == 1 ? "is one item" : "are $n items" } in the shopping cart.'
+          );
         },
         backgroundColor: const Color(0xFF16F0F0),
         child: new Icon(

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

@@ -29,11 +29,13 @@ class ShrinePage extends StatelessWidget {
           child: new Text('SHRINE', style: ShrineTheme.of(context).appBarTitleStyle)
         ),
         backgroundColor: Theme.of(context).canvasColor,
-        actions: <Widget>[  // TODO(hansmuller): implement the actions.
+        actions: <Widget>[
           new IconButton(
             icon: Icons.shopping_cart,
             tooltip: 'Shopping cart',
-            onPressed: () { /* activate the button for now */ }
+            onPressed: () {
+              // TODO(hansmuller): implement the action.
+            }
           ),
           new PopupMenuButton<ShrineAction>(
             itemBuilder: (BuildContext context) => <PopupMenuItem<ShrineAction>>[
@@ -49,7 +51,20 @@ class ShrinePage extends StatelessWidget {
                 value: ShrineAction.emptyCart,
                 child: new Text('Empty shopping cart')
               )
-            ]
+            ],
+            onSelected: (ShrineAction action) {
+              switch (action) {
+                case ShrineAction.sortByPrice:
+                  // TODO(hansmuller): implement the action.
+                  break;
+                case ShrineAction.sortByProduct:
+                  // TODO(hansmuller): implement the action.
+                  break;
+                case ShrineAction.emptyCart:
+                  // TODO(hansmuller): implement the action.
+                  break;
+              }
+            }
           )
         ]
       ),