Fix Sky issue where RenderNode was not painting automatically in interactive situations

R=abarth@chromium.org, abarth

Review URL: https://codereview.chromium.org/1168623002

sdk/lib/framework/rendering/node.dart

@@ -98,6 +98,7 @@ abstract class RenderNode extends AbstractNode {
       parent.markNeedsLayout();
     } else {
       _nodesNeedingLayout.add(this);
+      scheduler.ensureVisualUpdate();
     }
   }
   static void flushLayout() {