code: public BundleList findExtraBundles(BundleList newBundleList, FeatureManager featureManager) {
          List<RuntimeFeatureResource> bundles = new ArrayList<RuntimeFeatureResource>(resources);
          bundles.removeAll(newBundleList.resources);
          resources.removeAll(bundles);
          BundleList result = new BundleList(featureManager);
          result.resources.addAll(bundles);
          stale.set(true);
          return result;
      }
docs: This is like retain all except it returns a list of what was removed.
  
  TODO remove the bundles.
  
  @param newBundleList
  @return