Add an integration test to verify that server I/O is asynchronous.

Currently disabled, since server I/O is currently synchronous.

BUG=dartbug.com/21252
R=danrubel@google.com

Review URL: https://codereview.chromium.org//668953002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41246 260f80e4-7a28-3924-810f-c04153c831b5
update pubspec/changelog

R=sigmund@google.com

Review URL: https://codereview.chromium.org//675503002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41247 260f80e4-7a28-3924-810f-c04153c831b5