code: function() {
          const prefix = keyEndToEndDevicesForUser('');
          const devices = {};
          for (let i = 0; i < this.store.length; ++i) {
              const key = this.store.key(i);
              const userId = key.substr(prefix.length);
              if (key.startsWith(prefix)) devices[userId] = getJsonItem(this.store, key);
          }
          return devices;
      }
docs: Retrieves the known devices for all users.
  @return {object} A map from user ID to map of device ID to keys for the device.