export directive is not implemented

[jmesserly]

something I noticed a while back (in one of my aborted attempts to switch to ES6 modules): we don't implement the "export" directive.

[jmesserly]

in one of my aborted attempts to switch to ES6 modules

(clarification for any readers: ES6 modules are still very important... I just aborted my previous attempt when realized it required a bit more analysis to generate correctly, and we likely need to support several ES5 lowering modes, similar to what BabelJS provides)

[jmesserly]

https://codereview.chromium.org/1263583005/