/usr/lib/mono/2.0/System.Web.Extensions.Design.dll is in libmono-system-web2.0-cil 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/2.0/../gac/System.Web.Extensions.Design/3.5.0.0__31bf3856ad364e35/System.Web.Extensions.Design.dll