Implement radio button for Sky’s fn2

sky/sdk/lib/framework/rendering/box.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:math' as math;
 import 'dart:sky' as sky;
 import 'dart:typed_data';
 import 'object.dart';
 import 'package:vector_math/vector_math.dart';
 import 'package:sky/framework/net/image_cache.dart' as image_cache;
@@ skipping 640 matching lines @@
     return paint;
   }
 
   void paint(RenderObjectDisplayList canvas) {
     sky.Paint paint = _createShadowPaint(_shadow);
     canvas.drawRect(new sky.Rect.fromLTRB(0.0, 0.0, size.width, size.height), paint);
     super.paint(canvas);
   }
 }
 
+typedef void CustomPaintCallback(RenderObjectDisplayList canvas);
+
+class RenderCustomPaint extends RenderProxyBox {
+
+  RenderCustomPaint({
+    CustomPaintCallback callback,
+    RenderBox child
+  }) : super(child) {
+    assert(callback != null);
+    _callback = callback;
+  }
+
+  CustomPaintCallback _callback;
+  void set callback (CustomPaintCallback value) {
+    assert(value != null);
+    if (_callback == value)
+      return;
+    _callback = value;
+    markNeedsPaint();
+  }
+
+  void paint(RenderObjectDisplayList canvas) {
+    _callback(canvas);
+    super.paint(canvas);
+  }
+}
+
 // RENDER VIEW LAYOUT MANAGER
 
 class ViewConstraints {
 
   const ViewConstraints({
     this.width: 0.0, this.height: 0.0, this.orientation: null
   });
 
   final double width;
   final double height;
@@ skipping 87 matching lines @@
 
   void defaultPaint(RenderObjectDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }