Get the ID of a parent path.

Creates the directory if it doesn't exist yet.

@param {string} path - Full path of a directory or file
@returns {Promise} Resolves with ID of the parent directory.

@private
function (path) {
    const foldername = parentPath(path);

    return this._getFileId(foldername).then((parentId) => {
      if (parentId) {
        return Promise.resolve(parentId);
      } else {
        return this._createFolder(foldername);
      }
    });
  }