Refactor stateful parts of Component into StatefulComponent

sky/sdk/example/mine_digger/lib/main.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 'dart:math';
 
 import 'package:sky/rendering/flex.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/scaffold.dart';
@@ skipping 167 matching lines @@
       toolbar: buildToolBar(),
       body: new Container(
         child: new Center(child: board),
         decoration: new BoxDecoration(backgroundColor: colors.Grey[50])
       )
     );
   }
 
   void handleBannerPointerDown(sky.PointerEvent event) {
     initialize();
-    app.setState((){});
+    app.scheduleBuild();
   }
 
   // User action. The user uncovers the cell which can cause losing the game.
   void probe(int x, int y) {
     if (!alive)
       return;
     if (uiState[y][x] == flaggedCell)
       return;
     // Allowed to probe.
     if (cells[y][x]) {
       // Probed on a mine --> dead!!
       uiState[y][x] = explodedCell;
       alive = false;
     } else {
       // No mine, uncover nearby if possible.
       cull(x, y);
     }
-    app.setState((){});
+    app.scheduleBuild();
   }
 
   // User action. The user is sure a mine is at this location.
   void flag(int x, int y) {
     if (uiState[y][x] == flaggedCell) {
       uiState[y][x] = coveredCell;
       --detectedCount;
     } else {
       uiState[y][x] = flaggedCell;
       ++detectedCount;
     }
-    app.setState((){});
+    app.scheduleBuild();
   }
 
   // Recursively uncovers cells whose totalMineCount is zero.
   void cull(int x, int y) {
     if ((x < 0) || (x > rows - 1))
       return;
     if ((y < 0) || (y > cols - 1))
       return;
 
     if (uiState[y][x] == clearedCell)
@@ skipping 134 matching lines @@
   }
 
   Widget build() {
     return game.buildUI();
   }
 }
 
 void main() {
   runApp(new MineDiggerApp());
 }