set(key, value) {
      var index = getIndex(this, key);

      if (index != null && this._mapData[index]) {
        this._mapData[index][1] = value;
      } else {
        index = this._mapData.push([
          key,
          value
        ]) - 1;
        setIndex(this, key, index);
        if (__DEV__) {
          this[SECRET_SIZE_PROP] += 1;
        } else {
          this.size += 1;
        }
      }

      return this;
    }
23.1.3.9
Adds a key/value pair to the collection.

@param {*} key
@param {*} value
@return {map}