Adds support for registerElement for web components
Adds support for registerElement for web components

Summary: Just adds the registerElement function as per http://w3c.github.io/webcomponents/spec/custom/#extensions-to-document-interface-to-register
Closes https://github.com/facebook/flow/pull/855

Reviewed By: @jeffmo

Differential Revision: D2492051

Pulled By: @gabelevi

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