code: function mdlPublish(pubScope) {
    let cacheTtl = null;
    let src = null;
    let dest = null;
  
    if (pubScope === 'staging') {
      // Set staging specific vars here.
      cacheTtl = 0;
      src = 'dist/*';
      dest = bucketStaging;
    } else if (pubScope === 'prod') {
      // Set prod specific vars here.
      cacheTtl = 60;
      src = 'dist/*';
      dest = bucketProd;
    } else if (pubScope === 'promote') {
      // Set promote (essentially prod) specific vars here.
      cacheTtl = 60;
      src = `${bucketStaging}/*`;
      dest = bucketProd;
    }
  
    let infoMsg = `Publishing ${pubScope}/${pkg.version} to GCS (${dest})`;
    if (src) {
      infoMsg += ` from ${src}`;
    }
    console.log(infoMsg);
  
    // Build gsutil commands:
    // The gsutil -h option is used to set metadata headers.
    // The gsutil -m option requests parallel copies.
    // The gsutil -R option is used for recursive file copy.
    const cacheControl = `-h "Cache-Control:public,max-age=${cacheTtl}"`;
    const gsutilCacheCmd = `gsutil -m setmeta ${cacheControl} ${dest}/**`;
    const gsutilCpCmd = `gsutil -m cp -r -z html,css,js,svg ${src} ${dest}`;
  
    gulp.src('').pipe($.shell([gsutilCpCmd, gsutilCacheCmd]));
  }
docs: Function to publish staging or prod version from local tree,
  or to promote staging to prod, per passed arg.
  @param {string} pubScope the scope to publish to.