Project file must include the .NET Framework assembly 'WindowsBase, PresentationCore, PresentationFramework' in the reference list.

   I was adding a splash screen to my Silverlight project and ran into a sudden compile problem with the ASP.NET web site. The compile error was: Project file must include the .NET Framework assembly 'WindowsBase, PresentationCore, PresentationFramework' in the reference list. I verified that the loose Silverlight xaml file was set to just content, with no build action. However, I kept getting