How Do I Stub webpack's require.ensure?

I use webpack's code splitting feature (require.ensure) to reduce the initial bundle size of my React application by loading components that are not visible on page load from a separate bundle that is loaded asynchronously.

This works perfectly, but I have trouble writing a unit test for it.

My test setup is based on Mocha, Chai and Sinon.

Here is the relevant excerpt from the code I have tried so far:

describe('When I render the component', () => {
    let component,
        mySandbox;
    beforeEach(() => {
        mySandbox = sandbox.create();
        mySandbox.stub(require, 'ensure');
        component = mount(<PageHeader />);
    });
    describe('the rendered component', () =>
        it('contains the SideNav component', () =>
            component.find(SideNav).should.have.length(1)
        )
    );
    afterEach(() => mySandbox.restore());
});

When running the test, I get this error message:

"before each" hook for "contains the SideNav component": Cannot stub non-existent own property ensure

This happens because require.ensure is a method that only exists in a webpack bundle, but I'm not bundling my tests with webpack, nor do I want to, because it would create more overhead and presumably longer test execution times.

So my question is:

Is there a way to stub webpack's require.ensure with Sinon without running the tests through webpack?