Add some dartdoc comments

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/widgets/theme.dart';
 
 import 'basic.dart';
 import 'toggleable.dart';
 export 'toggleable.dart' show ValueChanged;
 
 const double _kMidpoint = 0.5;
 const sky.Color _kLightUncheckedColor = const sky.Color(0x8A000000);
 const sky.Color _kDarkUncheckedColor = const sky.Color(0xB2FFFFFF);
 const double _kEdgeSize = 20.0;
 const double _kEdgeRadius = 1.0;
 
+/// A material design checkbox
+///
+/// The checkbox itself does not maintain any state. Instead, when the state of
+/// the checkbox changes, the component calls the `onChange` callback. Most
+/// components that use a checkbox will listen for the `onChange` callback and
+/// rebuild the checkbox with a new `value` to update the visual appearance of
+/// the checkbox.
+///
+/// <https://www.google.com/design/spec/components/lists-controls.html#lists-controls-types-of-list-controls>
 class Checkbox extends Toggleable {
 
+  /// Constructs a checkbox
+  ///
+  /// * `value` determines whether the checkbox is checked.
+  /// * `onChanged` is called whenever the state of the checkbox should change.
   Checkbox({
     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) {
     ThemeData themeData = Theme.of(this);
@@ skipping 47 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);
     }
   }
 }