Add @override annotation to known overriden methods

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";
 
@@ skipping 26 matching lines @@
 
   Color get _color {
     switch (Theme.of(this).brightness) {
       case ThemeBrightness.light:
         return colors.White;
       case ThemeBrightness.dark:
         return colors.Grey[800];
     }
   }
 
+  @override
   Widget build() {
     Container mask = new Container(
       decoration: const BoxDecoration(
         backgroundColor: const Color(0x7F000000)));
 
     List<Widget> children = new List<Widget>();
 
     if (title != null) {
       children.add(new Padding(
         padding: new EdgeDims(24.0, 24.0, content == null ? 20.0 : 0.0, 24.0),
@@ skipping 33 matching lines @@
               child: new ShrinkWrapWidth(
                 child: new Block(children)
               )
             )
           )
         )
       )
     ]);
   }
 }