Package entry point is not ECMAScript

`FS.common.js` is not valid ES, and Node throws:

```none
/.../node_modules/react-native-fs/FS.common.js:30
var normalizeFilePath = (path: string) => (path.startsWith('file://') ? path.slice(7) : path);
^

SyntaxError: Unexpected token ':'
```

Of all the liberties package maintainers take, serving a straight-up different language at the entry point is one of the bigger head-scratchers I've seen.

Also, Flow sucks, and you should just use TS.