Webpack use existing source map from previous build step

It turns out an extra webpack "preLoader" called source-map-loader does the trick:

$ npm install source-map-loader --save

Then in webpack.config.js:

module.exports = {
  devtool: 'source-map',
  module:  {
    preLoaders: [
      {
        test:   /\.js$/,
        loader: 'source-map-loader'
      }
    ]
  }
};

Update for Webpack 2+

module.exports = {
  devtool: 'source-map',
  module: {
    rules: [
      {
        test: /\.js$/,
        use: ["source-map-loader"],
        enforce: "pre"
      }
    ]
  }
};