code: function userReqFromBundle(mId) {
    const possibleIds = nodejsIds(mId);
    const bundleName = Object.keys(_bundles).find(bn => {
      const {nameSpace, user} = _bundles[bn];
      return possibleIds.some(d => {
        if (nameSpace) {
          const parsed = parse(d);
          if (parsed.bareId.slice(0, nameSpace.length + 1) === nameSpace + '/') {
            d = parsed.prefix + parsed.bareId.slice(nameSpace.length + 1);
          }
        }
  
        if (user.hasOwnProperty(d)) return true;
        const p = parse(d);
        // For module with unknown plugin prefix, try bareId.
        // This relies on dumber bundler's default behaviour, it write in bundle config
        // both 'foo.html' and 'text!foo.html'.
        if (p.prefix) return user.hasOwnProperty(p.bareId);
      });
    });
  
    if (bundleName) {
      return loadBundle(bundleName)
      .then(() => {
        if (userSpace.has(mId)) return userSpace.req(mId);
        // mId is not directly defined in the bundle, it could be
        // behind a customised plugin or ext plugin.
        const tried = tryPlugin(mId, userSpace);
        if (tried) return tried;
        throw new Error(`module "${mId}" is missing from bundle "${bundleName}"`);
      });
    }
  }
docs: incoming id is already mapped return a promise to load additional bundle or return undefined.