Adding listener count
Adding listener count

Summary:https://nodejs.org/api/events.html#events_eventemitter_listenercount_emitter_event
Closes https://github.com/facebook/flow/pull/1563

Reviewed By: samwgoldman

Differential Revision: D3121016

Pulled By: gabelevi

fb-gh-sync-id: 38c5d139ab6e74c5529b238432da1f2b831c11c2
fbshipit-source-id: 38c5d139ab6e74c5529b238432da1f2b831c11c2

jamesgpearce/flow,JonathanUsername/flow,popham/flow,nmote/flow,d14b3l/flow,facebook/flow,cesarandreu/flow,samwgoldman/flow,gabro/flow,claudiopro/flow,fletcherw/flow,jdelStrother/flow,d14b3l/flow,jgrund/flow,mroch/flow,cesarandreu/flow,popham/flow,popham/flow,gabro/flow,fletcherw/flow,samwgoldman/flow,MichaelDeBoey/flow,AprilArcus/flow,cesarandreu/flow,gabelevi/flow,jeffmo/flow,TiddoLangerak/flow,ylu1317/flow,JonathanUsername/flow,chenglou/flow,JonathanUsername/flow,gabro/flow,ForbesLindesay/flow,AprilArcus/flow,chenglou/flow,AgentME/flow,mroch/flow,popham/flow,jeffmo/flow,ForbesLindesay/flow,AprilArcus/flow,gabelevi/flow,fletcherw/flow,AprilArcus/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,jgrund/flow,jdelStrother/flow,chenglou/flow,samwgoldman/flow,nmote/flow,jdelStrother/flow,nmote/flow,AgentME/flow,popham/flow,gabro/flow,claudiopro/flow,facebook/flow,facebook/flow,samwgoldman/flow,nmote/flow,mroch/flow,AgentME/flow,MichaelDeBoey/flow,AgentME/flow,ForbesLindesay/flow,jeffmo/flow,JonathanUsername/flow,nmote/flow,JonathanUsername/flow,ylu1317/flow,samwgoldman/flow,d14b3l/flow,jgrund/flow,fletcherw/flow,jeffmo/flow,JonathanUsername/flow,fletcherw/flow,MichaelDeBoey/flow,mroch/flow,facebook/flow,d14b3l/flow,mroch/flow,gabelevi/flow,ylu1317/flow,ylu1317/flow,gabro/flow,MichaelDeBoey/flow,AgentME/flow,gabelevi/flow,mroch/flow,jamesgpearce/flow,gabelevi/flow,chenglou/flow,jeffmo/flow,d14b3l/flow,claudiopro/flow,TiddoLangerak/flow,nmote/flow,TiddoLangerak/flow,facebook/flow,AprilArcus/flow,jgrund/flow,TiddoLangerak/flow,facebook/flow,ylu1317/flow,facebook/flow,AprilArcus/flow,chenglou/flow,popham/flow,gabro/flow,jamesgpearce/flow,samwgoldman/flow,jgrund/flow,nmote/flow,ForbesLindesay/flow,jeffmo/flow,jamesgpearce/flow,claudiopro/flow,ForbesLindesay/flow,jamesgpearce/flow,chenglou/flow,JonathanUsername/flow,jamesgpearce/flow,d14b3l/flow,jdelStrother/flow,MichaelDeBoey/flow,TiddoLangerak/flow,mroch/flow,fletcherw/flow,claudiopro/flow,jdelStrother/flow,cesarandreu/flow,gabelevi/flow,jdelStrother/flow,samwgoldman/flow,jgrund/flow,ForbesLindesay/flow