code: public void unregisterPropertyOnType(String propertyName, JSType type) {
      // TODO(bashir): typesIndexedByProperty should also be updated!
      Map<String, ObjectType> typeSet =
          eachRefTypeIndexedByProperty.get(propertyName);
      if (typeSet != null) {
        typeSet.remove(type.toObjectType().getReferenceName());
      }
    }
docs: Removes the index's reference to a property on the given type (if it is
  currently registered). If the property is not registered on the type yet,
  this method will not change internal state.
  
  @param propertyName the name of the property to unregister
  @param type the type to unregister the property on.