fix `exports` reference in flow.js
fix `exports` reference in flow.js

Summary:
as written, js_of_ocaml can rewrite it to

```
var _x_ = exports;
var _y_ = "undefined";
var _z_ = typeof _x_ !== _y_;
```

which doesn't work, because line 1 is a reference error when it's extracted from
the `typeof`.

we use this new inline pattern in the parser and it works well.

Reviewed By: samwgoldman

Differential Revision: D6140214

fbshipit-source-id: 9861d2343de6dc98b97f364da5ec6a5cb65362be

ylu1317/flow,popham/flow,facebook/flow,gabelevi/flow,popham/flow,nmote/flow,samwgoldman/flow,JonathanUsername/flow,samwgoldman/flow,JonathanUsername/flow,AgentME/flow,AgentME/flow,nmote/flow,TiddoLangerak/flow,JonathanUsername/flow,mroch/flow,gabelevi/flow,jamesgpearce/flow,samwgoldman/flow,claudiopro/flow,nmote/flow,claudiopro/flow,samwgoldman/flow,claudiopro/flow,popham/flow,TiddoLangerak/flow,ylu1317/flow,AgentME/flow,samwgoldman/flow,popham/flow,jamesgpearce/flow,facebook/flow,facebook/flow,ylu1317/flow,gabelevi/flow,mroch/flow,samwgoldman/flow,AgentME/flow,jamesgpearce/flow,TiddoLangerak/flow,jamesgpearce/flow,facebook/flow,JonathanUsername/flow,nmote/flow,mroch/flow,TiddoLangerak/flow,claudiopro/flow,gabelevi/flow,popham/flow,mroch/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,TiddoLangerak/flow,AgentME/flow,nmote/flow,JonathanUsername/flow,jamesgpearce/flow,facebook/flow,nmote/flow,JonathanUsername/flow,jamesgpearce/flow,facebook/flow,gabelevi/flow,ylu1317/flow,mroch/flow,mroch/flow,gabelevi/flow,claudiopro/flow,nmote/flow,ylu1317/flow,mroch/flow,samwgoldman/flow,JonathanUsername/flow,facebook/flow,popham/flow