protected A param(String key, String val) {
    // Enforce singleton parameter semantics for most API surfaces
    params.put(key, new ArrayList<String>());
    return paramAddToList(key, val);
  }
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