code: function putAsset(key, content) {
    return new Promise((resolve, reject) => {
      return instance.assets.then(db => {
        const obj = db.transaction(db.name, 'readwrite')
          .objectStore(db.name);
        const req = obj.put({key: key, value: content});
        req.onerror = event => reject(event);
        req.onsuccess = () => resolve();
      });
    });
  }
docs: Put asset object with a key
  @param {string} key - key
  @param {string} content - asset to store