code: function getAsset(key) {
    return new Promise((resolve, reject) => {
      return instance.assets.then(db => {
        const req = db.transaction(db.name)
          .objectStore(db.name).get(key);
        req.onsuccess = event => {
          const undef = event.target.result === undefined;
          const value = undef ? undefined : event.target.result.value;
          resolve(value);
        };
        req.onerror = event => reject(event);
      });
    });
  }
docs: Get asset by a key
  @param {string} key - key
  @return {array} asset object (if not found, resolve with undefined)