Fixes how we translate between Java and JavaScript source positions in BuildTypeMap.

This fixes issue #3506, which was caused by computing this incorrectly and overflowing the end of file in some cases.

Issue: 3506
Found by: rice
Review by: bobv

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5220 8db76d5a-ed1c-0410-87a9-c151d255dfc7