code: function(accountData) {
          return Promise.try(() => {
              const txn = this.db.transaction(["accountData"], "readwrite");
              const store = txn.objectStore("accountData");
              for (let i = 0; i < accountData.length; i++) {
                  store.put(accountData[i]); // put == UPSERT
              }
              return txnAsPromise(txn);
          });
      }
docs: Persist a list of account data events. Events with the same 'type' will
  be replaced.
  @param {Object[]} accountData An array of raw user-scoped account data events
  @return {Promise} Resolves if the events were persisted.