Make the StockList actually work in stocks2

RenderScaffold needs to attach and detach its children when asked.

R=eseidel@chromium.org, ianh@google.com

Review URL: https://codereview.chromium.org/1149893025

examples/stocks2/lib/stock_app.dart

@@ -204,7 +204,7 @@ class StocksApp extends App {
     List<UINode> overlays = [
       new Scaffold(
         toolbar: _isSearching ? buildSearchBar() : buildToolBar(),
-        // body: new Stocklist(stocks: _stocks, query: _searchQuery),
+        body: new Stocklist(stocks: _stocks, query: _searchQuery),
         // floatingActionButton: new FloatingActionButton(
         //   content: new Icon(type: 'content/add_white', size: 24),
         //   level: 3),

sdk/lib/framework/components2/scaffold.dart

@@ -46,6 +46,22 @@ class RenderScaffold extends RenderBox {
     markNeedsLayout();
   }
 
+  void attachChildren() {
+    for (ScaffoldSlots slot in ScaffoldSlots.values) {
+      RenderBox box = _slots[slot];
+      if (box != null)
+        box.attach();
+    }
+  }
+
+  void detachChildren() {
+    for (ScaffoldSlots slot in ScaffoldSlots.values) {
+      RenderBox box = _slots[slot];
+      if (box != null)
+        box.detach();
+    }
+  }
+
   ScaffoldSlots remove(RenderBox child) {
     assert(child != null);
     for (ScaffoldSlots slot in ScaffoldSlots.values) {