Port Toggleable to the new animation system

sky/sdk/lib/widgets/switch.dart

Index: sky/sdk/lib/widgets/switch.dart
diff --git a/sky/sdk/lib/widgets/switch.dart b/sky/sdk/lib/widgets/switch.dart
index 23e4ce514264138630c9987f719ad7fb0933a47e..80364aaad8d9ae389f61ca86159394ea4bff0235 100644
--- a/sky/sdk/lib/widgets/switch.dart
+++ b/sky/sdk/lib/widgets/switch.dart
@@ -61,9 +61,9 @@ class Switch extends Toggleable {
     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);