Apply media padding in Cupertino Navigation demo (#13585)

Adds safe areas around:
1. The body of the colour swatch view (tab 1)
2. The chat header (tab 2)
3. The chat bubbles (tab 2)
4. The 'Sign In' launcher button (tab 3)

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

@@ -132,8 +132,13 @@ class CupertinoDemoTab1 extends StatelessWidget {
             trailing: const ExitButton(),
           ),
           new SliverPadding(
-            // Top media query padding already consumed by CupertinoSliverNavigationBar.
-            padding: MediaQuery.of(context).removePadding(removeTop: true).padding,
+            // Top media padding consumed by CupertinoSliverNavigationBar.
+            // Left/Right media padding consumed by Tab1RowItem.
+            padding: MediaQuery.of(context).removePadding(
+              removeTop: true,
+              removeLeft: true,
+              removeRight: true,
+            ).padding,
             sliver: new SliverList(
               delegate: new SliverChildBuilderDelegate(
                 (BuildContext context, int index) {
@@ -175,6 +180,9 @@ class Tab1RowItem extends StatelessWidget {
           ),
         ));
       },
+      child: new SafeArea(
+        top: false,
+        bottom: false,
         child: new Padding(
           padding: const EdgeInsets.only(left: 16.0, top: 8.0, bottom: 8.0, right: 8.0),
           child: new Row(
@@ -220,6 +228,7 @@ class Tab1RowItem extends StatelessWidget {
             ],
           ),
         ),
+      ),
     );
 
     if (lastItem) {
@@ -273,6 +282,9 @@ class Tab1ItemPageState extends State<Tab1ItemPage> {
         middle: new Text(widget.colorName),
         trailing: const ExitButton(),
       ),
+      child: new SafeArea(
+        top: false,
+        bottom: false,
         child: new ListView(
           children: <Widget>[
             const Padding(padding: const EdgeInsets.only(top: 16.0)),
@@ -386,6 +398,7 @@ class Tab1ItemPageState extends State<Tab1ItemPage> {
             ),
           ],
         ),
+      ),
     );
   }
 }
@@ -412,6 +425,9 @@ class Tab2Header extends StatelessWidget {
   Widget build(BuildContext context) {
     return new Padding(
       padding: const EdgeInsets.all(16.0),
+      child: new SafeArea(
+        top: false,
+        bottom: false,
         child: new ClipRRect(
           borderRadius: const BorderRadius.all(const Radius.circular(16.0)),
           child: new Column(
@@ -520,6 +536,7 @@ class Tab2Header extends StatelessWidget {
             ],
           ),
         ),
+      ),
     );
   }
 }
@@ -623,11 +640,13 @@ class Tab2ConversationRow extends StatelessWidget {
           : Tab2ConversationBubbleColor.gray,
       ),
     );
-    return new Row(
+    return new SafeArea(
+      child: new Row(
         mainAxisAlignment: isSelf ? MainAxisAlignment.end : MainAxisAlignment.start,
         mainAxisSize: MainAxisSize.min,
         crossAxisAlignment: isSelf ? CrossAxisAlignment.center : CrossAxisAlignment.end,
         children: children,
+      ),
     );
   }
 }
@@ -703,6 +722,9 @@ class CupertinoDemoTab3 extends StatelessWidget {
                 height: 44.0,
                 child: new Padding(
                   padding: const EdgeInsets.symmetric(horizontal: 16.0, vertical: 8.0),
+                  child: new SafeArea(
+                    top: false,
+                    bottom: false,
                       child: new Row(children: <Widget>[ const Text(
                       'Sign in',
                       style: const TextStyle(color: CupertinoColors.activeBlue),
@@ -710,6 +732,7 @@ class CupertinoDemoTab3 extends StatelessWidget {
                   ),
                 ),
               ),
+            ),
           ],
         ),
       ),