Progress indicator demo should look good in landscape (#6088)

This patch fixes the issue in two ways:

1. The content area is now scrollable, which means you can scroll to reveal any
   parts that don't fit on screen.
2. The content area is now small enough to fit on screen in landscape on a
   Nexus 5.

Fixes #6054

examples/flutter_gallery/lib/demo/progress_indicator_demo.dart

@@ -63,26 +63,36 @@ class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo> with Sing
 
   Widget _buildIndicators(BuildContext context, Widget child) {
     List<Widget> indicators = <Widget>[
-        new SizedBox(
-          width: 200.0,
-          child: new LinearProgressIndicator()
-        ),
-        new LinearProgressIndicator(),
-        new LinearProgressIndicator(),
-        new LinearProgressIndicator(value: _animation.value),
-        new CircularProgressIndicator(),
-        new SizedBox(
-            width: 20.0,
+      new SizedBox(
+        width: 200.0,
+        child: new LinearProgressIndicator()
+      ),
+      new LinearProgressIndicator(),
+      new LinearProgressIndicator(),
+      new LinearProgressIndicator(value: _animation.value),
+      new Row(
+        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+        children: <Widget>[
+          new CircularProgressIndicator(),
+          new SizedBox(
+              width: 20.0,
+              height: 20.0,
+              child: new CircularProgressIndicator(value: _animation.value)
+          ),
+          new SizedBox(
+            width: 100.0,
             height: 20.0,
-            child: new CircularProgressIndicator(value: _animation.value)
-        ),
-        new Text('${(_animation.value * 100.0).toStringAsFixed(1)}%${ _controller.isAnimating ? "" : " (paused)" }')
+            child: new Text('${(_animation.value * 100.0).toStringAsFixed(1)}%',
+              textAlign: TextAlign.right
+            ),
+          ),
+        ],
+      ),
     ];
     return new Column(
       children: indicators
         .map((Widget c) => new Container(child: c, margin: const EdgeInsets.symmetric(vertical: 15.0, horizontal: 20.0)))
         .toList(),
-      mainAxisAlignment: MainAxisAlignment.center
     );
   }
 
@@ -90,16 +100,20 @@ class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo> with Sing
   Widget build(BuildContext context) {
     return new Scaffold(
       appBar: new AppBar(title: new Text('Progress indicators')),
-      body: new DefaultTextStyle(
-        style: Theme.of(context).textTheme.title,
-        child: new GestureDetector(
-          onTap: _handleTap,
-          behavior: HitTestBehavior.opaque,
-          child: new Container(
-            padding: const EdgeInsets.symmetric(vertical: 12.0, horizontal: 8.0),
-            child: new AnimatedBuilder(
-              animation: _animation,
-              builder: _buildIndicators
+      body: new Center(
+        child: new ScrollableViewport(
+          child: new DefaultTextStyle(
+            style: Theme.of(context).textTheme.title,
+            child: new GestureDetector(
+              onTap: _handleTap,
+              behavior: HitTestBehavior.opaque,
+              child: new Container(
+                padding: const EdgeInsets.symmetric(vertical: 12.0, horizontal: 8.0),
+                child: new AnimatedBuilder(
+                  animation: _animation,
+                  builder: _buildIndicators
+                )
+              )
             )
           )
         )