Comment 14 for bug 748861

These icons were intentionally removed from the ubuntu mono set. They don't appear in Unity and including them causes problems with other applications because of improper naming.

if they are to be re-implemented, they should use the -symbolic namespace to ensure they don't show up in improper locations. Either way, this is a problem with a non-default setup and I would encourage third party application developers to ship those icons with their application rather then re-add them to ubuntu-mono.