1.8 is a legacy/production branch (DotNet 1.1)

2.0 is a current release (DotNet 2.0)

The latest sources are on SourceForge at: http://builds.flexwiki.com/download/flexwikicore-20 then select the highest version number and find the source code in the files FlexWikiCore-2.0.0.<x>-src.zip