Create an explicit lambda for onExit.

This is needed because completer.complete() now takes an optional
argument. A function with an optional argument isn't a subtype of
the same function without the argument being optional.

Review URL: https://codereview.chromium.org//11794051

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@16817 260f80e4-7a28-3924-810f-c04153c831b5