Added original SDK code for Alien Swarm.

public/ifilelist.h

@@ -0,0 +1,28 @@
+//========= Copyright <20> 1996-2005, Valve Corporation, All rights reserved. ============//
+//
+// Purpose: 
+//
+// $NoKeywords: $
+//=============================================================================//
+
+#ifndef IFILELIST_H
+#define IFILELIST_H
+#ifdef _WIN32
+#pragma once
+#endif
+
+
+// This class represents a group of files. Internally, it can represent whole folders of files
+// that are in or out of the group. So you can't iterate the list, but you can ask the
+// class if a particular filename is in the list.
+class IFileList
+{
+public:
+	virtual bool	IsFileInList( const char *pFilename ) = 0;
+	virtual void	Release() = 0;
+};
+
+
+#endif // IFILELIST_H
+
+