Add @override annotation to known overriden methods

sky/sdk/example/rendering/touch_demo.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky' as sky;
 
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:sky/rendering/paragraph.dart';
 import 'package:sky/rendering/sky_binding.dart';
@@ skipping 25 matching lines @@
   void paint(PaintingCanvas canvas, Offset offset) {
     canvas.drawCircle(position + offset, radius, _paint);
   }
 }
 
 class RenderTouchDemo extends RenderBox {
   Map<int, Dot> dots = new Map();
 
   RenderTouchDemo();
 
+  @override
   void handleEvent(sky.Event event, BoxHitTestEntry entry) {
     if (event is sky.PointerEvent) {
       switch (event.type) {
         case 'pointerdown':
           Color color = colors[event.pointer.remainder(colors.length)];
           dots[event.pointer] = new Dot(color: color)..update(event);
           break;
         case 'pointerup':
           dots.remove(event.pointer);
           break;
         case 'pointercancel':
           dots = new Map();
           break;
         case 'pointermove':
           dots[event.pointer].update(event);
           break;
       }
     }
     markNeedsPaint();
   }
 
+  @override
   void performLayout() {
     size = constraints.biggest;
   }
 
+  @override
   void paint(PaintingCanvas canvas, Offset offset) {
     Paint white = new Paint()
         ..color = const Color(0xFFFFFFFF);
     canvas.drawRect(offset & size, white);
     for (Dot dot in dots.values)
       dot.paint(canvas, offset);
   }
 }
 
 void main() {
   var paragraph = new RenderParagraph(new InlineText("Touch me!"));
   var stack = new RenderStack(children: [
     new RenderTouchDemo(),
     paragraph,
   ]);
   // Prevent the RenderParagraph from filling the whole screen so
   // that it doesn't eat events.
   paragraph.parentData..top = 40.0
                       ..left = 20.0;
   new SkyBinding(root: stack);
 }