Fixes a build break from rev 3073.  Wrapper types that are not used in a program are
not present, so don't rely on being able to look up all wrapper types.

Review by: scottb (postmortem)


git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.5@3076 8db76d5a-ed1c-0410-87a9-c151d255dfc7