async addFileSystem(cwd, mainFileName) {
    let promise = this._cwds.get(cwd);
    if (!promise) {
      async function innerAdd() {
        const fileSystemManager = Persistence.isolatedFileSystemManager;
        const fs = await Ndb.FileSystem.create(fileSystemManager, cwd);
        fileSystemManager.addPlatformFileSystem(cwd, fs);
        return fs;
      }
      promise = innerAdd();
      this._cwds.set(cwd, promise);
    }
    if (mainFileName)
      await (await promise).forceFileLoad(mainFileName);
    await promise;
  }
@param {string} cwd
@param {string=} mainFileName
@return {!Promise}