code: public static <K, V> MapEntry<K, V> newDefaultInstance(Descriptor descriptor, WireFormat.FieldType keyType,
              K defaultKey, WireFormat.FieldType valueType, V defaultValue) {
          return new MapEntry<K, V>(descriptor, keyType, defaultKey, valueType, defaultValue);
      }
docs: Create a default MapEntry instance. A default MapEntry instance should be created only once for each map entry
  message type. Generated code should store the created default instance and use it later to create new MapEntry
  messages of the same type.
  
  @param <K> the key type
  @param <V> the value type
  @param descriptor the descriptor
  @param keyType the key type
  @param defaultKey the default key
  @param valueType the value type
  @param defaultValue the default value
  @return the map entry