code: static <K, V> ImmutableMapEntry<K, V> makeImmutable(Entry<K, V> entry, K key, V value) {
      boolean reusable =
          entry instanceof ImmutableMapEntry && ((ImmutableMapEntry<K, V>) entry).isReusable();
      return reusable ? (ImmutableMapEntry<K, V>) entry : new ImmutableMapEntry<K, V>(key, value);
    }
docs: Makes an entry usable internally by a new ImmutableMap without rereading its contents.