Add some dartdoc comments

sky/sdk/lib/widgets/dialog.dart

Index: sky/sdk/lib/widgets/dialog.dart
diff --git a/sky/sdk/lib/widgets/dialog.dart b/sky/sdk/lib/widgets/dialog.dart
index fc20694e529578cf9379b6ed5a58237cbcfba605..20e8650cbf3467dea34145d5d483af8f95a7409e 100644
--- a/sky/sdk/lib/widgets/dialog.dart
+++ b/sky/sdk/lib/widgets/dialog.dart
@@ -8,6 +8,9 @@ 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,
@@ -17,12 +20,22 @@ class Dialog extends Component {
     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;
@@ -73,7 +86,7 @@ class Dialog extends Component {
             constraints: new BoxConstraints(minWidth: 280.0),
             child: new Material(
               level: 4,
-              color: color,
+              color: _color,
               child: new ShrinkWrapWidth(
                 child: new Block(children)
               )