mobx: 3.1.12 breaks webpack build

Working for 3.1.11, broken for 3.1.12

Webpack does not emit any errors at build time, but the code will not run:

mobx.module.js:3335 Uncaught ReferenceError: exports is not defined
    at Object.<anonymous> (mobx.module.js:3335)
    at Object.defineProperty.value (mobx.module.js:3335)
    at __webpack_require__ (bootstrap 81606c5…:19)
    at Object.defineProperty.value (gui.tsx:19) (require mobx)
    at __webpack_require__ (bootstrap 81606c5…:19)
    at Object.<anonymous> (main.tsx:5)
    at Object.setPrototypeOf.__proto__ (main.tsx:109)
    at __webpack_require__ (bootstrap 81606c5…:19)
    at module.exports (bootstrap 81606c5…:65)
    at bootstrap 81606c5…:65

The emitted code looks like this:

export { extras, Reaction, untracked, IDerivationState, Atom, BaseAtom, useStrict, isStrictModeEnabled, spy, asReference, asFlat, asStructure, asMap, isModifierDescriptor, isObservableObject, isObservableValue as isBoxedObservable, isObservableArray, ObservableMap, isObservableMap, map, transaction, observable, IObservableFactories, computed, isObservable, isComputed, extendObservable, extendShallowObservable, observe, intercept, autorun, autorunAsync, when, reaction, action, isAction, runInAction, expr, toJS, createTransformer, whyRun, isArrayLike };export default exports;

exports is not defined.

Webpack configuration excerpt:

            loaders: [
                {
                    test: /\.jsx?$/,
                    exclude: path => path.split("/").filter(x => x === "node_modules").length >= 2,
                    loader: 'babel-loader',
                    query: {
                        presets: ['es2015', 'react'],
                        plugins: ['transform-decorators-legacy', 'transform-class-properties']
                    }
                },
                {
                    test: /\.tsx?$/,
                    exclude: path => path.split("/").filter(x => x === "node_modules").length >= 2,
                    loader: 'ts-loader',
                    options: {
                        transpileOnly: true
                    }
                },

All our code is contained in a folder called node_modules, not sure if that’s relevant. Edit: probably not

I’m guessing this is caused by https://github.com/mobxjs/mobx/commit/823b713f2f44078b4e2cd3bea94ce3082f61f011

(https://github.com/mobxjs/mobx/pull/1027) @rossipedia