guava/src/com/google/common/collect/RegularImmutableMap.java
static <K, V> ImmutableMapEntry<K, V> makeImmutable(Entry<K, V> entry) {
    return makeImmutable(entry, entry.getKey(), entry.getValue());
  }
static <K, V> ImmutableMapEntry<K, V> makeImmutable(Entry<K, V> entry) {
    return makeImmutable(entry, entry.getKey(), entry.getValue());
  }
Makes an entry usable internally by a new ImmutableMap.