Change hasSubscribers to hasListener.
Change hasSubscribers to hasListener.

Review URL: https://codereview.chromium.org//14103010

git-svn-id: b90127a0f73590c96be848d3e4cb640efa9a7657@21337 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/scheduled_test,dart-archive/scheduled_test