Use '&mdash; Select &mdash;' instead of '-- Select --' for nav menus widget. Looks nicer.

Partially merges [28205] to the 3.9 branch.

see #27878

git-svn-id: https://develop.svn.wordpress.org/branches/3.9@28206 602fd350-edb4-49c9-b593-d223f7449a82