Fix up the javadoc for Event by removing mentions of AssertionError, which we
don't throw anymore.

patch by: ajr
review by: ecc
pointed out by: colesbury



git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4425 8db76d5a-ed1c-0410-87a9-c151d255dfc7