can't find System.Web.Security namespace on codeplex

Question

Does anyone know why I cant find System.Web.Security namespace on codeplex? http://aspnetwebstack.codeplex.com/SourceControl/latest

I am looking for the membership provider class that is part of this namespace.

http://msdn.microsoft.com/en-us/library/system.web.security.membershipprovider(v=vs.110).aspx