removeRef_(ref, featureUid) {
    if (!ref) {
      throw new Error('This feature has no ref: ' + featureUid);
    }
    this.refToFeature_.delete(ref);
    this.uidToRef_.delete(featureUid);
    this.freeGlobalRef_.push(ref);
  }
Return a ref to the pool of available refs.
@param {number} ref the ref to return
@param {string} featureUid the feature uid
@private