Fix stocks app usage of checkbox

sky/sdk/lib/widgets/switch.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/theme2/colors.dart' as colors;
 import 'package:sky/theme2/shadows.dart';
 
 import '../painting/shadows.dart';
 import '../rendering/box.dart';
 import 'basic.dart';
 import 'toggleable.dart';
+export 'toggleable.dart' show ValueChanged;
 
 // TODO(jackson): This should change colors with the theme
 sky.Color _kThumbOnColor = colors.Purple[500];
 const sky.Color _kThumbOffColor = const sky.Color(0xFFFAFAFA);
 sky.Color _kTrackOnColor = new sky.Color(_kThumbOnColor.value & (0x80 << 24));
 const sky.Color _kTrackOffColor = const sky.Color(0x42000000);
 const double _kSwitchWidth = 35.0;
 const double _kThumbRadius = 10.0;
 const double _kSwitchHeight = _kThumbRadius * 2.0;
 const double _kTrackHeight = 14.0;
@@ skipping 36 matching lines @@
 
     // The thumb contracts slightly during the animation
     double inset = 2.0 - (toggleAnimation.value - 0.5).abs() * 2.0;
     Point thumbPos = new Point(
       _kTrackRadius + toggleAnimation.value * (_kTrackWidth - _kTrackRadius * 2),
       _kSwitchHeight / 2.0
     );
     canvas.drawCircle(thumbPos.x, thumbPos.y, _kThumbRadius - inset, paint);
   }
 }