       snapshot: {
          type: Object,
          notify: true
        }
      },

      behaviors: [
        Polymer.FirebaseStorageBehavior
      ],

      /**
      * @override
      */
      get zeroValue() {
        return [];
      },

      _updateProperties: function(snapshot) {
        this.state = snapshot.state;
        this.totalBytes = snapshot.totalBytes;
        this.bytesTransferred = snapshot.bytesTransferred;
        this.downloadUrl = snapshot.downloadURL;
        this.metadata = snaps