webpack 1 : cheap-module-eval-source-map causes Reference undefined

1-data-projector.js

// A simple module. One function uses a function defined in the same file.
// Easy as pie
function project(functions, path, statsParams, mapParams) {
  // Undefined reference: loadDataset is not defined
  return loadDataset(path, functions, statsParams).then(dataset => {
    // etc
    // return mapDataset(functions, mapParams, dataset);
  });
}

function loadDataset(path, functions, statsParams) {
  return new Promise();  // implementation goes here
}

module.exports = {
  project,
  loadDataset  
};

2-use_with_node.js

// using this with node on the commandline works without any problem

const dp = require('data-projector');

// module exports everything correctly
console.log(dp);

// yes, of course it's defined
console.log(dp.loadDataset);

// yep, it works
dp
  .project(
    {},
    '/Users/crucial/code/idmx/playsplom/app/vendor/datasets/seals.csv',
    {},
  {
    lat: {
      fn: (s, f, v) => v * 2
    },
    long: {
      fn: (s, f, v) => v * 2
    }
  }
  )
  .then(console.log, console.error);

3-use-in-webpacked-app.js

const dp = require('data-projector');
console.log(dp);
// yes, its defined ...
console.log(dp.loadDataset);

// but when loadDataset is called by the function project it is undefined within that function !
dp
  .project(
    {},
    '/Users/crucial/code/idmx/playsplom/app/vendor/datasets/seals.csv',
    {},
  {
    lat: {
      fn: (s, f, v) => v * 2
    },
    long: {
      fn: (s, f, v) => v * 2
    }
  }
  )
  .then(console.log, e => {
    //  Undefined reference. loadDataset is not defined
    console.error('dp failed', e);
  });

4-webpack.config.js

// this is causing the problem:
config.devtool = 'cheap-module-eval-source-map';

// This works fine
// config.devtool = 'source-map';

/**
I haven't upgraded this project to webpack 2,
so maybe this is long ago solved.

    "webpack": "^1.13.2",
    "source-map-support": "^0.4.5",
    "webpack-dev-middleware": "^1.8.4",
    "webpack-hot-middleware": "^2.13.0"
**/

5-debugger-shows.md

Strangely enough the Chrome/Electron devtools debugger shows loadDataset as defined inside the function project:

It is not in the function locals, but it is in the parent closure: