get(key) {
    return new Promise((resolve, reject) => {
      this._client.get(key, (error, json) => {
        if (error) {
          reject(error);
          return;
        }
        try {
          const value = JSON.parse(json || null);
          resolve(value);
        } catch (_error) {
          reject(_error);
        }
      });
    });
  }
@param {!string} key
@return {!Promise}
@override