Return the close future when serving 404
Return the close future when serving 404

BUG=https://github.com/dart-lang/appengine/issues/20
R=kustermann@google.com

Review URL: https://codereview.chromium.org//755983002

exitlive/appengine,dart-lang/appengine,exitlive/appengine,kevmoo/appengine,kevmoo/appengine,dart-lang/appengine,exitlive/appengine,kevmoo/appengine,kevmoo/appengine