Fix spinning_mixed widget example, plus minor style fix

examples/layers/widgets/spinning_mixed.dart

@@ -33,7 +33,7 @@ class Rectangle extends StatelessWidget {
 
 double value;
 RenderObjectToWidgetElement<RenderBox> element;
-BuildOwner owner;
+BuildOwner owner = new BuildOwner();
 void attachWidgetTreeToRenderTree(RenderProxyBox container) {
   element = new RenderObjectToWidgetAdapter<RenderBox>(
     container: container,

packages/flutter/lib/src/widgets/framework.dart

@@ -807,9 +807,9 @@ abstract class Element implements BuildContext {
   Widget get widget => _widget;
   Widget _widget;
 
-  BuildOwner _owner;
   /// The owner for this node (null if unattached).
   BuildOwner get owner => _owner;
+  BuildOwner _owner;
 
   bool _active = false;