SafeASTVisitor unilaterally avoids visiting error/unreachable local types

This prevents a class of compiler crashes that can occur when unreferenced error types end up in the source path.

http://gwt-code-reviews.appspot.com/682801/show
Review by: spoon


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8376 8db76d5a-ed1c-0410-87a9-c151d255dfc7