Fixes a build break caused by r2047.  We were always attempting to generate an instantiate method for a generated field serializer; even if the the class was not default instantiable.  This error was only detected in web mode.

Review by: Bobv


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2050 8db76d5a-ed1c-0410-87a9-c151d255dfc7