Refactor fn2.dart, since it breached our 1000-line threshold.

sky/sdk/lib/framework/components2/radio.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 'package:sky/framework/theme2/colors.dart' as colors;
-
-import '../fn2.dart';
-import '../rendering/object.dart';
-import 'button_base.dart';
-import 'ink_well.dart';
-import 'dart:sky' as sky;
-
-typedef void ValueChanged(value);
-
-class Radio extends ButtonBase {
-
-  Radio({
-    Object key,
-    this.value,
-    this.groupValue,
-    this.onChanged
-  }) : super(key: key);
-
-  Object value;
-  Object groupValue;
-  ValueChanged onChanged;
-
-  void syncFields(Radio source) {
-    value = source.value;
-    groupValue = source.groupValue;
-    onChanged = source.onChanged;
-    super.syncFields(source);
-  }
-
-  UINode buildContent() {
-    // TODO(jackson): This should change colors with the theme
-    Color color = highlight ? colors.Purple[500] : const Color(0x8A000000);
-    const double kDiameter = 16.0;
-    const double kOuterRadius = kDiameter / 2;
-    const double kInnerRadius = 5.0;
-    return new EventListenerNode(
-      new Container(
-        margin: const EdgeDims.symmetric(horizontal: 5.0),
-        width: kDiameter,
-        height: kDiameter,
-        child: new CustomPaint(
-          callback: (sky.Canvas canvas, Size size) {
-
-            Paint paint = new Paint()..color = color;
-
-            // Draw the outer circle
-            paint.setStyle(sky.PaintingStyle.stroke);
-            paint.strokeWidth = 2.0;
-            canvas.drawCircle(kOuterRadius, kOuterRadius, kOuterRadius, paint);
-
-            // Draw the inner circle
-            if (value == groupValue) {
-              paint.setStyle(sky.PaintingStyle.fill);
-              canvas.drawCircle(kOuterRadius, kOuterRadius, kInnerRadius, paint);
-            }
-          }
-        )
-      ),
-      onGestureTap: _handleClick
-    );
-  }
-
-  void _handleClick(_) {
-    onChanged(value);
-  }
-
-}