code: @GwtCompatible(serializable = true)
    public static <K, V> Entry<K, V> immutableEntry(@Nullable K key, @Nullable V value) {
      return new ImmutableEntry<K, V>(key, value);
    }
docs: Returns an immutable map entry with the specified key and value. The {@link
  Entry#setValue} operation throws an {@link UnsupportedOperationException}.
  
  <p>The returned entry is serializable.
  
  @param key the key to be associated with the returned entry
  @param value the value to be associated with the returned entry