OneChildRenderObjectWrapper and Scaffold were ignoring the return value of syncChild().

This would cause them to forget what the most recent child they'd been given was, with disastrous effects later.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1173683003

examples/stocks2/lib/stock_app.dart

@@ -158,7 +158,7 @@ class StocksApp extends App {
     return new ToolBar(
         left: new IconButton(
           icon: 'navigation/menu_white',
-          onGestureTap: _drawerController.toggle),
+          onGestureTap: (_) => _drawerController.toggle()),
         center: new Text('Stocks'),
         right: [
           new IconButton(