Port Toggleable to the new animation system

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/theme/shadows.dart';
 
 import '../painting/shadows.dart';
 import '../rendering/box.dart';
@@ skipping 43 matching lines @@
     canvas.drawRRect(rrect, paint);
 
     // Draw the raised thumb with a shadow
     paint.color = thumbColor;
     var builder = new ShadowDrawLooperBuilder();
     for (BoxShadow boxShadow in shadows[1])
       builder.addShadow(boxShadow.offset, boxShadow.color, boxShadow.blur);
     paint.setDrawLooper(builder.build());
 
     // The thumb contracts slightly during the animation
-    double inset = 2.0 - (toggleAnimation.value - 0.5).abs() * 2.0;
+    double inset = 2.0 - (position.value - 0.5).abs() * 2.0;
     Point thumbPos = new Point(
-      _kTrackRadius + toggleAnimation.value * (_kTrackWidth - _kTrackRadius * 2),
+      _kTrackRadius + position.value * (_kTrackWidth - _kTrackRadius * 2),
       _kSwitchHeight / 2.0
     );
     canvas.drawCircle(thumbPos, _kThumbRadius - inset, paint);
   }
 }