Re-work MaterialComponent

sky/framework/components/radio.dart

Index: sky/framework/components/radio.dart
diff --git a/sky/framework/components/radio.dart b/sky/framework/components/radio.dart
index 11e7e1c9428a0d4d1c129fcefe59532205269852..221ffcfcdb16f49a74a65f9202163df27a3bf4a5 100644
--- a/sky/framework/components/radio.dart
+++ b/sky/framework/components/radio.dart
@@ -4,6 +4,7 @@
 
 import '../fn.dart';
 import 'button_base.dart';
+import 'material.dart';
 
 typedef void ValueChanged(value);
 
@@ -54,10 +55,9 @@ class Radio extends ButtonBase {
   }
 
   Node build() {
-    return new Container(
+    return new Material(
       style: highlight ? _highlightStyle : _style,
-      children: value == groupValue ?
-          [super.build(), new Container( style : _dotStyle )] : [super.build()]
+      children: value == groupValue ? [new Container( style : _dotStyle )] : []
     )
   }