I extracted agslib-3.0b1-2011-12-16.swc from arcgis_api_for_flex_3_0.zip and placed it into the libs folder of ArcGIS-ArcGISViewerForFlex-flexviewer-3.0Prerelease-src-0-g16b7ba8.zip
This resulted in a reduced number of compile errors (25), but not zero.
[ATTACH=CONFIG]13355[/ATTACH]