Fix a synchronous event bug in futureStream. R=rnystrom@google.com TBR Review URL: https://codereview.chromium.org//68713004 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30182 260f80e4-7a28-3924-810f-c04153c831b5