Remove one more use of mirrors: Components now have to explicitly sync their fields.

sky/sdk/lib/framework/components2/checkbox.dart

Index: sky/sdk/lib/framework/components2/checkbox.dart
diff --git a/sky/sdk/lib/framework/components2/checkbox.dart b/sky/sdk/lib/framework/components2/checkbox.dart
index 706dc6020c09aaaa529d390a217a650ef106707b..2a1df9be1277067ad32a30bbca689891534c704f 100644
--- a/sky/sdk/lib/framework/components2/checkbox.dart
+++ b/sky/sdk/lib/framework/components2/checkbox.dart
@@ -19,6 +19,12 @@ class Checkbox extends ButtonBase {
   bool checked;
   ValueChanged onChanged;
 
+  void syncFields(Checkbox source) {
+    checked = source.checked;
+    onChanged = source.onChanged;
+    super.syncFields(source);
+  }
+
   void _handleClick(sky.Event e) {
     onChanged(!checked);
   }