Fix assertion message.

Fixes issue 4534.

Change-Id: If17d1c835a05df4409e242bd27c52fbc673ccd90
Review-Link: https://gwt-review.googlesource.com/#/c/1730/

Review by: goktug@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11454 8db76d5a-ed1c-0410-87a9-c151d255dfc7