@wincent I was wondering why Flow silently ignored missing export. So I discovered that a few files missing @flow directive.
And lexer.js had /* @flow / since the first commit, not sure how Flow is parsing such comments.
I also unify @flow directives to always be in a first line, so you can use simple shell script to find all files without it.
After enabling flow checks on more files two new issue appeared which I fixed in the separate commit.

Thanks @IvanGoncharov.

FWIW I don't really like the hoisting up of the @flow annotation to the top; it smells of special-casing to me and @flow shouldn't be more special than any other annotation that we might use (and we do use quite a few at FB). Having said that, consistency is good, and there is a precedent for this style, so... ¯\(ツ)/¯