funcunit/site/node_modules/steal/src/system-extension-contextual.js

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function addContextual(loader){
  if (loader._extensions) {
    loader._extensions.push(addContextual);
  }
  loader._contextualModules = {};

  loader.setContextual = function(moduleName, definer){
    this._contextualModules[moduleName] = definer;
  };

  var normalize = loader.normalize;
  loader.normalize = function(name, parentName){
    var loader = this;

    if (parentName) {
      var definer = this._contextualModules[name];

      // See if `name` is a contextual module
      if (definer) {
        name = name + '/' + parentName;

        if(!loader.has(name)) {
          // `definer` could be a function or could be a moduleName
          if (typeof definer === 'string') {
            definer = loader['import'](definer);
          }

          return Promise.resolve(definer)
          .then(function(definer) {
            if (definer['default']) {
              definer = definer['default'];
            }
            loader.set(name, loader.newModule(definer.call(loader, parentName)));
            return name;
          });
        }
        return Promise.resolve(name);
      }
    }

    return normalize.apply(this, arguments);
  };
}

if(typeof System !== "undefined") {
  addContextual(System);
}