Remove keys from the examples.

sky/examples/widgets/spinning_mixed.dart

Index: sky/examples/widgets/spinning_mixed.dart
diff --git a/sky/examples/widgets/spinning_mixed.dart b/sky/examples/widgets/spinning_mixed.dart
index 1de1fcdf2454ef98e367fd71e01d6613c23df24b..9bd00702f24aa170eb1808f4f252848e6fc6d5b7 100644
--- a/sky/examples/widgets/spinning_mixed.dart
+++ b/sky/examples/widgets/spinning_mixed.dart
@@ -37,7 +37,7 @@ class Rectangle extends Component {
 
 Widget builder() {
   return new Flex([
-      new Rectangle(const Color(0xFF00FFFF), key: 'a'),
+      new Rectangle(const Color(0xFF00FFFF)),
       new Container(
         padding: new EdgeDims.all(10.0),
         margin: new EdgeDims.all(10.0),
@@ -50,7 +50,7 @@ Widget builder() {
           onPressed: () => print("Hello World")
         )
       ),
-      new Rectangle(const Color(0xFFFFFF00), key: 'b'),
+      new Rectangle(const Color(0xFFFFFF00)),
     ],
     direction: FlexDirection.vertical,
     justifyContent: FlexJustifyContent.spaceBetween