Add some dartdoc comments

sky/sdk/lib/widgets/dialog.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 '../theme/colors.dart' as colors;
 import 'basic.dart';
 import 'default_text_style.dart';
 import 'material.dart';
 import "theme.dart";
 
+/// A material design dialog
+///
+/// <https://www.google.com/design/spec/components/dialogs.html>
 class Dialog extends Component {
   Dialog({
     String key,
     this.title,
     this.content,
     this.actions,
     this.onDismiss
   }) : super(key: key);
 
+  /// The (optional) title of the dialog is displayed in a large font at the top

[Hixie, 2015/07/08 15:10:57]

This is an interesting convention (putting the documentation on the fields,
rather than on the constructor). What does the output look like? Is it clear
that these really refer to the constructor arguments?

If we document these, we're gonna have to reconsider making the (mutable) fields
public on StatefulComponents.

[abarth-chromium, 2015/07/08 15:29:29]

I was just experimenting with different approaches.  I'm not sure happy with how
any of them look.  I'll show you in person what the output looks like.

+  /// of the dialog.
   final Widget title;
+
+  /// The (optional) content of the dialog is displayed in the center of the
+  /// dialog in a lighter font.
   final Widget content;
+
+  /// The (optional) set of actions that are displayed at the bottom of the
+  /// dialog.
   final List<Widget> actions;
+
+  /// An (optional) callback that is called when the dialog is dismissed.
   final Function onDismiss;
 
-  Color get color {
+  Color get _color {
     switch (Theme.of(this).brightness) {
       case ThemeBrightness.light:
         return colors.White;
       case ThemeBrightness.dark:
         return colors.Grey[800];
     }
   }
 
   Widget build() {
     Container mask = new Container(
@@ skipping 30 matching lines @@
         child: mask,
         onGestureTap: (_) => onDismiss()
       ),
       new Center(
         child: new Container(
           margin: new EdgeDims.symmetric(horizontal: 40.0, vertical: 24.0),
           child: new ConstrainedBox(
             constraints: new BoxConstraints(minWidth: 280.0),
             child: new Material(
               level: 4,
-              color: color,
+              color: _color,
               child: new ShrinkWrapWidth(
                 child: new Block(children)
               )
             )
           )
         )
       )
     ]);
   }
 }