Fix Obserable sink.closed for ES3
Fix Obserable sink.closed for ES3

Summary: Not all environments and tools can support a `get closed()` method. Using Object.defineProperty() also passes tests while avoiding the getter syntax.

Reviewed By: asiandrummer

Differential Revision: D5752644

fbshipit-source-id: d6e365499fb9a472106645ea7ba4038f13279b3a

voideanvalue/relay,wincent/relay,kassens/relay,dbslone/relay,iamchenxin/relay,kassens/relay,benjaminogles/relay,wincent/relay,cpojer/relay,cpojer/relay,apalm/relay,atxwebs/relay,facebook/relay,facebook/relay,cpojer/relay,kassens/relay,xuorig/relay,voideanvalue/relay,kassens/relay,xuorig/relay,atxwebs/relay,freiksenet/relay,wincent/relay,xuorig/relay,xuorig/relay,freiksenet/relay,yungsters/relay,cpojer/relay,josephsavona/relay,josephsavona/relay,iamchenxin/relay,voideanvalue/relay,voideanvalue/relay,yungsters/relay,facebook/relay,benjaminogles/relay,facebook/relay,atxwebs/relay,benjaminogles/relay,apalm/relay,voideanvalue/relay,yungsters/relay,voideanvalue/relay,zpao/relay,facebook/relay,kassens/relay,wincent/relay,dbslone/relay,zpao/relay,iamchenxin/relay,freiksenet/relay,apalm/relay,wincent/relay,xuorig/relay,facebook/relay,kassens/relay,xuorig/relay,josephsavona/relay,atxwebs/relay,iamchenxin/relay,wincent/relay,zpao/relay