foobar.js

export function Foo() {};

Foo.prototype.test = function() {
  console.log("fail")
};

export function Bar() {};

Bar.prototype = Object.create(Foo.prototype);

Bar.prototype.test = function() {
  console.log("success");
};

index.js

import {Foo, Bar} from "./foobar";

(new Foo);
(new Bar).test();

package.json

{
  "scripts": {
    "test": "rollup -- index.js | node"
  },
  "dependencies": {
    "rollup": "^0.8.3"
  }
}