systemjs/lib/extension-bundles.js

/*
  System bundles

  Allows a bundle module to be specified which will be dynamically 
  loaded before trying to load a given module.

  For example:
  System.bundles['mybundle'] = ['jquery', 'bootstrap/js/bootstrap']

  Will result in a load to "mybundle" whenever a load to "jquery"
  or "bootstrap/js/bootstrap" is made.

  In this way, the bundle becomes the request that provides the module
*/

function bundles(loader) {
  if (typeof indexOf == 'undefined')
    indexOf = Array.prototype.indexOf;

  // bundles support (just like RequireJS)
  // bundle name is module name of bundle itself
  // bundle is array of modules defined by the bundle
  // when a module in the bundle is requested, the bundle is loaded instead
  // of the form System.bundles['mybundle'] = ['jquery', 'bootstrap/js/bootstrap']
  loader.bundles = loader.bundles || {};

  var loaderFetch = loader.fetch;
  loader.fetch = function(load) {
    if (!loader.bundles)
      loader.bundles = {};

    // if this module is in a bundle, load the bundle first then
    for (var b in loader.bundles) {
      if (indexOf.call(loader.bundles[b], load.name) == -1)
        continue;
      // we do manual normalization in case the bundle is mapped
      // this is so we can still know the normalized name is a bundle
      return Promise.resolve(loader.normalize(b))
      .then(function(normalized) {
        loader.bundles[normalized] = loader.bundles[normalized] || loader.bundles[b];
        return loader.load(normalized);
      })
      .then(function() {
        return '';
      });
    }
    return loaderFetch.apply(this, arguments);
  }
}