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

sky/sdk/lib/widgets/toggleable.dart

Index: sky/sdk/lib/widgets/toggleable.dart
diff --git a/sky/sdk/lib/widgets/toggleable.dart b/sky/sdk/lib/widgets/toggleable.dart
index 5d5a945e2d7e9552972569fa111e210c97f472e9..c970d70ae36c35966603658ecbfc7402eb0dec2c 100644
--- a/sky/sdk/lib/widgets/toggleable.dart
+++ b/sky/sdk/lib/widgets/toggleable.dart
@@ -58,11 +58,10 @@ abstract class Toggleable extends AnimatedComponent {
     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;