Also, Not sure if it would be correct to implement a python dict using an ES6 map. If the keys are all strings, probably better to stick to a JS object.
Check out the performance difference:
http://jsperf.com/es6-map-vs-object-properties/73
For-of only works for iterable sources, it would not be a complete replacement of for...in, also it seems a bit slower that for...in as of now.. see https://kpdecker.github.io/six-speed/