nebula.js/commands/build/lib/systemjs.js

const systemjsBehaviours = {
  getExternal: ({ config: cfg }) => {
    const defaultExternal = ['@nebula.js/stardust', 'picasso.js', 'picasso-plugin-q', 'react', 'react-dom'];
    const { external } = cfg.systemjs || {};
    return Array.isArray(external) ? external : defaultExternal;
  },
  getOutputFile: ({ pkg }) => pkg.systemjs,
  getOutputName: () => undefined,
  enabled: ({ pkg }) => !!pkg.systemjs,
};

module.exports = systemjsBehaviours;