constructor(mapId, map, opt_context) {
    'use strict';
    /**
     * The id for this map.
     * @type {number}
     */
    this.mapId = mapId;

    /**
     * The map itself.
     * @type {!Object|!goog.collections.maps.MapLike}
     */
    this.map = map;

    /**
     * The context for the map.
     * @type {Object}
     */
    this.context = opt_context || null;
  }
@param {number} mapId The id for this map.
@param {!Object|!goog.collections.maps.MapLike} map The map itself.
@param {!Object=} opt_context The context associated with the map.