code: static <K, V> ImmutableMapEntry<K, V> entryOf(K key, V value) {
      return new ImmutableMapEntry<K, V>(key, value);
    }
docs: Verifies that {@code key} and {@code value} are non-null, and returns a new
  immutable entry with those values.
  
  <p>A call to {@link Map.Entry#setValue} on the returned entry will always
  throw {@link UnsupportedOperationException}.