Remove StreamSink(replaced by EventSink) and make IOSink extend EventSink.

With this change 'writeBytes' was renamed to 'add'.

BUG=

Review URL: https://codereview.chromium.org//14150002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@21275 260f80e4-7a28-3924-810f-c04153c831b5