Fix the sector demo.
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.

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