I would create a more generic Composite widget called HeaderWidgetWithArrow
that contains
- The down arrow image
- Any arbitrary widget (such as a FlexTable)
The way if you want to include the arrow for a disclosure panel and say, a HorizontalPanel, you could just re-use the HeaderWidgetWithArrow
for that.
Either way, I would not recommend extending FlexTable or DisclosurePanel. You should favor composition over inheritance.