Fix `Module_js.file_exists` on Windows
Fix `Module_js.file_exists` on Windows

Summary: The current test for detecting the root path does not on Windows.
Now, we detect the root path by testing : `path = dirname path`.
Closes https://github.com/facebook/flow/pull/1096

Reviewed By: svcscm

Differential Revision: D2652661

Pulled By: mroch

fb-gh-sync-id: 41cc02d139547ed342e2e2c12e6a6211cce8b292

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