Fixes HTMLPanel to keep it from yanking it out of the DOM whenever a widget is added to it.

Issue: 1741
Patch by: jgw, knorton
Review by: knorton


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2134 8db76d5a-ed1c-0410-87a9-c151d255dfc7