This addresses a problem with the recent patch, in that it is giving
the warning for JUnitShell.  Since you had asked about customizing the
message anyway, I just went ahead with a solution that allows each
subclass of DevModeBase to define its own warning behavior.

Review at http://gwt-code-reviews.appspot.com/134805/show

Patch by: jat
Review by: rdayal


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7492 8db76d5a-ed1c-0410-87a9-c151d255dfc7