Make UINode's key a String

sky/sdk/lib/widgets/checkbox.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 'basic.dart';
 import 'toggleable.dart';
 export 'toggleable.dart' show ValueChanged;
 
 const double _kMidpoint = 0.5;
 const sky.Color _kUncheckedColor = const sky.Color(0x8A000000);
 // TODO(jackson): This should change colors with the theme
 sky.Color _kCheckedColor = colors.Purple[500];
 const double _kEdgeSize = 20.0;
 const double _kEdgeRadius = 1.0;
 
 class Checkbox extends Toggleable {
 
   Checkbox({
-    Object key,
+    String key,
     bool value,
     ValueChanged onChanged
   }) : super(key: key, value: value, onChanged: onChanged);
 
   Size get size => const Size(_kEdgeSize + 2.0, _kEdgeSize + 2.0);
 
   void customPaintCallback(sky.Canvas canvas, Size size) {
     // Choose a color between grey and the theme color
     sky.Paint paint = new sky.Paint()..strokeWidth = 2.0
                                      ..color = _kUncheckedColor;
@@ skipping 42 matching lines @@
         => new Point(p1.x * (1.0 - t) + p2.x * t, p1.y * (1.0 - t) + p2.y * t);
       sky.Point drawStart = lerp(start, mid, 1.0 - t);
       sky.Point drawEnd = lerp(mid, end, t);
       path.moveTo(drawStart.x, drawStart.y);
       path.lineTo(mid.x, mid.y);
       path.lineTo(drawEnd.x, drawEnd.y);
       canvas.drawPath(path, paint);
     }
   }
 }