Bound data getting cleared on clone  #20

      <sortable-js id="master">
        <template is="dom-repeat" items={{externals}}>
          <div class="list-item">
            <paper-icon-button icon="[[_generateIcon(item.type)]]"></paper-icon-button>
            <span>{{item.key}}</span>
          </div>
        </template>
      </sortable-js>

      attached: function() {
        this.$.master.group = {
          name: this.uuid,
          pull: 'clone'
        };
      },