Add @override annotation to known overriden methods

sky/sdk/lib/widgets/checkbox.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/widgets/theme.dart';
 
 import 'basic.dart';
 import 'toggleable.dart';
@@ skipping 19 matching lines @@
   /// Constructs a checkbox
   ///
   /// * `value` determines whether the checkbox is checked.
   /// * `onChanged` is called whenever the state of the checkbox should change.
   Checkbox({
     String key,
     bool value,
     ValueChanged onChanged
   }) : super(key: key, value: value, onChanged: onChanged);
 
+  @override
   Size get size => const Size(_kEdgeSize + 2.0, _kEdgeSize + 2.0);
 
+  @override
   void customPaintCallback(sky.Canvas canvas, Size size) {
     ThemeData themeData = Theme.of(this);
     Color uncheckedColor = themeData.brightness == ThemeBrightness.light ? _kLightUncheckedColor : _kDarkUncheckedColor;
 
     // Choose a color between grey and the theme color
     sky.Paint paint = new sky.Paint()..strokeWidth = 2.0
                                      ..color = uncheckedColor;
 
     // The rrect contracts slightly during the animation
     double inset = 2.0 - (toggleAnimation.value - _kMidpoint).abs() * 2.0;
@@ skipping 39 matching lines @@
         => new Point(p1.x * (1.0 - t) + p2.x * t, p1.y * (1.0 - t) + p2.y * t);
       sky.Point drawStart = lerp(start, mid, 1.0 - t);
       sky.Point drawEnd = lerp(mid, end, t);
       path.moveTo(drawStart.x, drawStart.y);
       path.lineTo(mid.x, mid.y);
       path.lineTo(drawEnd.x, drawEnd.y);
       canvas.drawPath(path, paint);
     }
   }
 }