Fix gallery demo body centering (#6841)

examples/flutter_gallery/lib/demo/bottom_navigation_demo.dart

@@ -119,7 +119,7 @@ class _BottomNavigationDemoState extends State<BottomNavigationDemo>
     });
   }
 
-  Widget _buildBody() {
+  Widget _buildTransitionsStack() {
     final List<FadeTransition> transitions = <FadeTransition>[];
 
     for (NavigationIconView view in _navigationViews)
@@ -175,7 +175,9 @@ class _BottomNavigationDemoState extends State<BottomNavigationDemo>
           )
         ],
       ),
-      body: _buildBody(),
+      body: new Center(
+        child: _buildTransitionsStack()
+      ),
       bottomNavigationBar: botNavBar,
     );
   }

examples/flutter_gallery/lib/demo/date_picker_demo.dart

@@ -36,7 +36,9 @@ class _DatePickerDemoState extends State<DatePickerDemo> {
     return
       new Scaffold(
       appBar: new AppBar(title: new Text('Date picker')),
-      body: new Column(
+      body: new Center(
+        child: new Column(
+          mainAxisAlignment: MainAxisAlignment.center,
           children: <Widget>[
             new Text(new DateFormat.yMMMd().format(_selectedDate)),
             new SizedBox(height: 20.0),
@@ -45,7 +47,7 @@ class _DatePickerDemoState extends State<DatePickerDemo> {
               child: new Text('SELECT DATE')
             ),
           ],
-        mainAxisAlignment: MainAxisAlignment.center
+        ),
       )
     );
   }

examples/flutter_gallery/lib/demo/slider_demo.dart

@@ -19,7 +19,8 @@ class _SliderDemoState extends State<SliderDemo> {
   Widget build(BuildContext context) {
     return new Scaffold(
       appBar: new AppBar(title: new Text('Sliders')),
-      body: new Column(
+      body: new Center(
+        child: new Column(
           mainAxisAlignment: MainAxisAlignment.spaceAround,
           children: <Widget>[
             new Column(
@@ -61,10 +62,11 @@ class _SliderDemoState extends State<SliderDemo> {
                   }
                 ),
                 new Text('Discrete'),
-            ]
+              ],
+            ),
+          ],
+        ),
       ),
-        ]
-      )
     );
   }
 }

examples/flutter_gallery/lib/demo/time_picker_demo.dart

@@ -32,7 +32,8 @@ class _TimePickerDemoState extends State<TimePickerDemo> {
   Widget build(BuildContext context) {
     return new Scaffold(
       appBar: new AppBar(title: new Text('Time picker')),
-      body: new Column(
+      body: new Center(
+        child: new Column(
           mainAxisAlignment: MainAxisAlignment.center,
           children: <Widget>[
             new Text('$_selectedTime'),
@@ -41,8 +42,9 @@ class _TimePickerDemoState extends State<TimePickerDemo> {
               onPressed: _handleSelectTime,
               child: new Text('SELECT TIME')
             ),
-        ]
-      )
+          ],
+        ),
+      ),
     );
   }
 }