bar.js

module.setters = {};
var __es6__import__ = function(name, setter) {
    require(name);
    var resolvedPath = require.resolve(name);
    require.cache[resolvedPath].dependants.push(module);
    module.setters[resolvedPath] = setter;
};

module.dependants = [];
var __es6__export__ = (function() {
    var values = Object.create(null), proxy = module.exports = Object.create(null);

    return function(identifier, value) {
        values[identifier] = value;
        module.dependants.forEach(function(mod) {
            mod.setters[module.filename](values);
        });
        if (!Object.getOwnPropertyDescriptor(proxy, identifier)) {
            Object.defineProperty(proxy, identifier, {
                enumerable: true,
                get: function() {
                    return values[identifier];
                }
            });
        }
        return value;
    }
})();

// export var bar;
var bar = __es6__export__("bar");

// bar = 99;
bar = __es6__export__("bar", 99);

foo.js

module.setters = {};
var __es6__import__ = function(name, setter) {
    require(name);
    var resolvedPath = require.resolve(name);
    require.cache[resolvedPath].dependants.push(module);
    module.setters[resolvedPath] = setter;
};

module.dependants = [];
var __es6__export__ = (function() {
    var values = Object.create(null), proxy = module.exports = Object.create(null);

    return function(identifier, value) {
        values[identifier] = value;
        module.dependants.forEach(function(mod) {
            mod.setters[module.filename](values);
        });
        if (!Object.getOwnPropertyDescriptor(proxy, identifier)) {
            Object.defineProperty(proxy, identifier, {
                enumerable: true,
                get: function() {
                    return values[identifier];
                }
            });
        }
        return value;
    }
})();

// import { bar } from "./bar";
var bar;
__es6__import__("./bar", function(m) {
    bar = m.bar;
});