Fix the sector demo.

Turns out it's ok to not detach all the RenderObjects from the widget
tree from each other, since they're not reused... but the
WidgetToRenderBoxAdapter's renderBox _is_ reused, so we better detach
that one.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1191053002.

sdk/lib/widgets/basic.dart

@@ -429,4 +429,9 @@ class WidgetToRenderBoxAdapter extends RenderObjectWrapper {
     // definition, it is the transition out of the Widget world.
   }
 
+  void remove() {
+    parent.detachChildRoot(this);
+    super.remove();
+  }
+
 }