BoxDecoration API changes: backgroundColor -> color et al (#9648)

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage
diff --git a/examples/layers/widgets/spinning_square.dart b/examples/layers/widgets/spinning_square.dart
index e9a4f7b..be59f1c 100644
--- a/examples/layers/widgets/spinning_square.dart
+++ b/examples/layers/widgets/spinning_square.dart
@@ -37,9 +37,7 @@
       child: new Container(
         width: 200.0,
         height: 200.0,
-        decoration: const BoxDecoration(
-          backgroundColor: const Color(0xFF00FF00)
-        )
+        color: const Color(0xFF00FF00),
       )
     );
   }