Object is not a hash
Currently example fails with usernames such as constructor.
We solve this partially with https://github.com/gaearon/flux-react-router-example/commit/487745ffe9eddc06cf37001ecd6c9735c0d640b2 but the same needs to be done for RepoStore and UserStore. The easiest way to do this is to create a Bag class and use it instead of mergeIntoBag, isInBag helpers. Inside Bag, we will use prefixes for IDs.
Could ES6 Maps be used for this instead?
Yes but they're not really there yet. I don't know much about the polyfill though.
Kangax's ES6 compat table shows that Babel passes all the Map tests. The polyfill is a part of core-js.