Fix a race condition in pub serve's exitOnClose test.

It was possible for the web socket to be closed before a response was
sent to the exitOnClose request, causing the test (which expected such
a response) to fail.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//316623005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@36962 260f80e4-7a28-3924-810f-c04153c831b5