Let the analyzer tell you if you've failed to override these methods by making them abstract instea…

sky/sdk/lib/widgets/toggleable.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/animation/animation_performance.dart';
 import 'package:sky/animation/curves.dart';
 import 'package:sky/widgets/animated_component.dart';
 import 'package:sky/widgets/basic.dart';
@@ skipping 40 matching lines @@
         performance.reverse();
       }
     }
     super.syncFields(source);
   }
 
   void _handleClick(sky.Event e) {
     onChanged(!value);
   }
 
-  // Override these methods to draw yourself
-  void customPaintCallback(sky.Canvas canvas, Size size) {
-    assert(false);
-  }
-  Size get size => Size.zero;
+  // Override these to draw yourself
+  void customPaintCallback(sky.Canvas canvas, Size size);
+  Size get size;
+
   EdgeDims get margin => const EdgeDims.symmetric(horizontal: 5.0);
   double get duration => 200.0;
   Curve get curveUp => easeIn;
   Curve get curveDown => easeOut;
 
   Widget build() {
     return new Listener(
       child: new Container(
         margin: margin,
         width: size.width,
         height: size.height,
         child: new CustomPaint(
           token: position.value,
           callback: customPaintCallback
         )
       ),
       onGestureTap: _handleClick
     );
   }
 }