Minor changes to dashboard notifier interface to enable better session-tracking
in notifier implementation. Also added some stuff to
...dev.shell.BrowserChannelServerTest to verify that the integration with the
dashboard is behaving correctly.

Review at http://gwt-code-reviews.appspot.com/1450812

Review by: tobyr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10299 8db76d5a-ed1c-0410-87a9-c151d255dfc7