protected A paramAddToList(String key, String val) {
    // Multiple parameter values required to support Static Maps API paths and markers.
    if (params.get(key) == null) {
      params.put(key, new ArrayList<String>());
    }
    params.get(key).add(val);
    return getInstance();
  }
Sets the value for the HTTP header field name {@link HttpHeaders#X_GOOG_MAPS_EXPERIENCE_ID}.
Passing null to this method will unset the experienceId header field.

@param experienceIds The experience IDs
src/main/java/com/google/maps/PendingResultBase.java