Create an explicit lambda for onExit.

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.

Committed: https://code.google.com/p/dart/source/detail?r=16817

[Bob Nystrom, 2013-01-08 19:00:42]

This fixes a bunch of pub tests. Still more to come.