addPolygons(polygonIds) {
    polygonIds.forEach((function(polygonId) {
                         this.map.data.loadGeoJson(
                             'static/polygons/' + polygonId + '.json');
                       }).bind(this));
    this.map.data.setStyle(function(feature) {
      return {fillColor: 'white', strokeColor: 'white', strokeWeight: 3};
    });
  }
Adds the polygons with the passed-in IDs to the map.
@param {Array<string>} polygonIds The IDs of the polygons to show on the
    map. For example ['poland', 'moldova'].