Well, the main evidence for me can be found in the leaked "Dart/Dash" memo, where GWT is referenced a few times:

"The JSPrime effort was begun to unify and be a (single!) successor to GWT and Closure/JSCompiler"

"What is the future of the JSCompiler and GWT? JSCompiler and GWT were already on a merger path. This effort gives us a direction for that unification around the Dash language. We will actively support teams for a long time on the current generation of JSCompiler and GWT and provide fantastic co-existence and migration tools to Dash."

See: http://markmail.org/message/uro3jtoitlmq6x7t

(My emphasis added. I read that as "maintenance mode".)

(Edit: thanks for the heads-up on the FF6 version of the plugin - but I guess I have now missed the two-week window where both FF6 and the compatible version of the plugin where available to download!)