How do I test JS prototypes (non-modules) in Mocha/Chai?

I want to set up tests for the project I'm building. In the examples I can find, they all say including the relevant code to test is done by a require statement: require('foo');. However my project isn't built in modules, but in ES6 classes that are then translated to ES5 prototypes.

I'm wondering how to include the file/s?

So for instance the ES6 class:

class Foo {
  constructor() {
    // do things
  }
}

Translates roughly to:

// ES5 compatible code is here (_classCallCheck etc).
var Foo = (function () {    
  function Foo() {
    _classCallCheck(this, Foo);

   // do things
  }
}

And I'd like to know how to include it in my test file:

var expect   = require('chai').expect;
// how to require or whatever else here?

describe('Foo', function() {
  it('creates an Foo object', function() {
    var foo = new Foo({});
  });
});