Make EventHandler return type mixed
Make EventHandler return type mixed

Summary: Closes #1242

Reviewed By: gabelevi

Differential Revision: D2801830

fb-gh-sync-id: e14b04f7e8c4d6f470e5c6243db3d095343d2713

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