Add @override annotation to known overriden methods

sky/sdk/example/rendering/solid_color_box.dart

Index: sky/sdk/example/rendering/solid_color_box.dart
diff --git a/sky/sdk/example/rendering/solid_color_box.dart b/sky/sdk/example/rendering/solid_color_box.dart
index b6624c666fbb353de51c00d3be1098271d912447..3ce9ef3e6f0fae1a51cfea0808201314a9e88162 100644
--- a/sky/sdk/example/rendering/solid_color_box.dart
+++ b/sky/sdk/example/rendering/solid_color_box.dart
@@ -14,34 +14,40 @@ class RenderSolidColorBox extends RenderDecoratedBox {
       : backgroundColor = backgroundColor,
         super(decoration: new BoxDecoration(backgroundColor: backgroundColor));
 
+  @override
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     return constraints.constrainHeight(
       this.desiredSize == Size.infinite ? 0.0 : desiredSize.width
     );
   }
 
+  @override
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
     return constraints.constrainWidth(
       this.desiredSize == Size.infinite ? 0.0 : desiredSize.width
     );
   }
 
+  @override
   double getMinIntrinsicHeight(BoxConstraints constraints) {
     return constraints.constrainHeight(
       this.desiredSize == Size.infinite ? 0.0 : desiredSize.height
     );
   }
 
+  @override
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
     return constraints.constrainHeight(
       this.desiredSize == Size.infinite ? 0.0 : desiredSize.height
     );
   }
 
+  @override
   void performLayout() {
     size = constraints.constrain(desiredSize);
   }
 
+  @override
   void handleEvent(sky.Event event, BoxHitTestEntry entry) {
     if (event.type == 'pointerdown')
       decoration = new BoxDecoration(backgroundColor: const Color(0xFFFF0000));