[PR] Make event handlers on Worker nullable
[PR] Make event handlers on Worker nullable

Summary:
Seems like the `onmessage`, `onerror`, and `onmessageerror` event handlers on `Worker` should be able to be set to null.

Apologies for the hassle If this is not something flow is inclined to allow.
Closes https://github.com/facebook/flow/pull/6193

Reviewed By: gabelevi

Differential Revision: D7765082

Pulled By: mrkev

fbshipit-source-id: b4a0523d9101ae29540594f73dbe23adfec28493

gabelevi/flow,popham/flow,claudiopro/flow,facebook/flow,samwgoldman/flow,gabelevi/flow,mroch/flow,nmote/flow,popham/flow,samwgoldman/flow,samwgoldman/flow,samwgoldman/flow,JonathanUsername/flow,nmote/flow,claudiopro/flow,mroch/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,jamesgpearce/flow,jamesgpearce/flow,popham/flow,JonathanUsername/flow,JonathanUsername/flow,AgentME/flow,AgentME/flow,AgentME/flow,nmote/flow,gabelevi/flow,gabelevi/flow,nmote/flow,jamesgpearce/flow,claudiopro/flow,jamesgpearce/flow,popham/flow,samwgoldman/flow,mroch/flow,facebook/flow,jamesgpearce/flow,JonathanUsername/flow,AgentME/flow,nmote/flow,facebook/flow,mroch/flow,nmote/flow,JonathanUsername/flow,jamesgpearce/flow,popham/flow,mroch/flow,samwgoldman/flow,jamesgpearce/flow,facebook/flow,gabelevi/flow,mroch/flow,gabelevi/flow,nmote/flow,popham/flow,facebook/flow,JonathanUsername/flow,JonathanUsername/flow,mroch/flow,facebook/flow,AgentME/flow,claudiopro/flow,claudiopro/flow,popham/flow