Refactor stateful parts of Component into StatefulComponent

sky/sdk/example/mine_digger/lib/main.dart

Index: sky/sdk/example/mine_digger/lib/main.dart
diff --git a/sky/sdk/example/mine_digger/lib/main.dart b/sky/sdk/example/mine_digger/lib/main.dart
index c52cf6691ea32f5a020d8eb09b38776ce80c18e4..bc5fea6d6b721137ac2190a4c515f6d9f72c8035 100644
--- a/sky/sdk/example/mine_digger/lib/main.dart
+++ b/sky/sdk/example/mine_digger/lib/main.dart
@@ -185,7 +185,7 @@ class Game {
 
   void handleBannerPointerDown(sky.PointerEvent event) {
     initialize();
-    app.setState((){});
+    app.scheduleBuild();
   }
 
   // User action. The user uncovers the cell which can cause losing the game.
@@ -203,7 +203,7 @@ class Game {
       // 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.
@@ -215,7 +215,7 @@ class Game {
       uiState[y][x] = flaggedCell;
       ++detectedCount;
     }
-    app.setState((){});
+    app.scheduleBuild();
   }
 
   // Recursively uncovers cells whose totalMineCount is zero.