Currently all imports exist solely, as far as I can tell, to configure the order in which the files are concatenated (via smash). In a real ES6 environment instead of this

import "iterable"
class Collection extends Iterable {}

you'd do something like this

import { Iterable } from "./iterable";
class Collection extends Iterable {}

Ideally you'd then combine the files using a smarter version of smash, i.e. the (shameless plug) es6-module-transpiler's bundle format.

I'm working on extracting the bundler from es6-module-transpiler into its own project, and I tried using immutable-js as a test project. I've converted some of the files to the style above and the output is essentially the same as when used with smash.