delete(key) {
      var index = getIndex(this, key);
      if (index != null && this._mapData[index]) {
        setIndex(this, key, undefined);
        this._mapData[index] = undefined;
        if (__DEV__) {
          this[SECRET_SIZE_PROP] -= 1;
        } else {
          this.size -= 1;
        }
        return true;
      } else {
        return false;
      }
    }
23.1.3.3
Delete a key/value from the collection.

@param {*} key
@return {boolean} Whether the key was found and deleted.