Inheriting class via a external extended export

[sgwilym]

I think I've found an issue somewhat similar to #150, where classes don't have their superclasses defined.

The problem is happening with classes extending Mutation from react-relay/classic:

import { Mutation } from "react-relay/classic";

export default class MyCoolMutation extends Mutation {
  ...
}

This is bundled by Rollup into something like the following:

var MyCoolMutation = function (_Mutation) {
  _inherits(MyCoolMutation, _Mutation);

 ...
}

_Mutation being an argument of this function, you end up with this error when you try to use this export:

TypeError: Super expression must either be null or a function, not undefined

I looked into react-relay/classic to see how the exports were structured:

module.exports = require('./lib/ReactRelayClassicExports.js');

Which leads to where I suspect the source of the trouble is coming from:

'use strict';

var _extends3 = _interopRequireDefault(require('babel-runtime/helpers/extends'));

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { 'default': obj }; }

// As early as possible, check for the existence of the JavaScript globals which
// React Relay relies upon, and produce a clear message if they do not exist.
if (process.env.NODE_ENV !== 'production') {
  if (typeof Map !== 'function' || typeof Set !== 'function' || typeof Promise !== 'function' || typeof Object.assign !== 'function' || typeof Array.prototype.find !== 'function') {
    throw new Error('react-relay requires Map, Set, Promise, Object.assign, and Array#find ' + 'to exist. Use a polyfill to provide these for older browsers.');
  }
}

// By default, assume that GraphQL is served at `/graphql` on the same domain.
// To override, use `Relay.injectNetworkLayer`.
require('./RelayStore').injectDefaultNetworkLayer(new (require('./RelayDefaultNetworkLayer'))('/graphql'));

module.exports = (0, _extends3['default'])({}, require('./RelayPublic'), {
  // Expose the default network layer to allow convenient re-configuration.
  DefaultNetworkLayer: require('./RelayDefaultNetworkLayer')
});

I'd be grateful to know if there's some option or plugin that I'm not using properly or at all, or whether this is indeed something weird going on here. Sorry if this is in the wrong repo, I wasn't sure where the problem, if any, could be.