Make UINode's key a String

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';
@@ skipping 11 matching lines @@
 const double _kThumbRadius = 10.0;
 const double _kSwitchHeight = _kThumbRadius * 2.0;
 const double _kTrackHeight = 14.0;
 const double _kTrackRadius = _kTrackHeight / 2.0;
 const double _kTrackWidth = _kSwitchWidth - (_kThumbRadius - _kTrackRadius) * 2.0;
 
 class Switch extends Toggleable {
   // TODO(jackson): Hit-test the switch so that it can respond to both taps and swipe gestures
 
   Switch({
-    Object key,
+    String key,
     bool value,
     ValueChanged onChanged
   }) : super(key: key, value: value, onChanged: onChanged);
 
   Size get size => const Size(_kSwitchWidth + 2.0, _kSwitchHeight + 2.0);
 
   void customPaintCallback(sky.Canvas canvas, Size size) {
     sky.Color thumbColor = value ? _kThumbOnColor : _kThumbOffColor;
     sky.Color trackColor = value ? _kTrackOnColor : _kTrackOffColor;
 
@@ skipping 18 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);
   }
 }