code: Iterable<Object> getHardReferences() {
          final List<Object> references = new ArrayList<Object>(hardReferences.size());
          for (final HardReference reference : hardReferences) {
              references.add(reference.referenceToKeep);
          }
          return references;
      }
docs: The list of all references that are currently kept.
  
  <p>
  This is only intended to be used by tests.
  </p>
  
  @return All references that are currently held by this instance.