Fixed BOO-515 (Expression resolver thinks " is String.Empty).
Fixed BOO-346 (Addin does not reference Boo.Lang.Useful by default)
ParserFoldingStrategy now uses both the RecentCompilationUnit and ValidCompilationUnit for folding. This makes folding more stable in VB and Boo.
git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@584 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61