anchor.diff:              null,
               null,
               null,
  -            null
  -            ) {
  +            null,
  +            mapEntryBuilderInfo) {
       assert(name != null);
       assert(tagNumber != null);
       assert(_isMapField(type));
  
positive.code:   int keyFieldType;
    int valueFieldType;
    CreateBuilderFunc valueCreator;
    // BuilderInfo used when creating a field set for a map field.
    final BuilderInfo _mapEntryBuilderInfo;
  
    MapFieldInfo(String name, int tagNumber, int index, int type,
        this.keyFieldType, this.valueFieldType, BuilderInfo mapEntryBuilderInfo)
        : super._(
              name,
              tagNumber,
              index,
  
negative.code: class FieldInfo<T> {
    FrozenPbList<T> _emptyList;
  
    // BuilderInfo used when creating a field set for a map field.
    final BuilderInfo _mapEntryBuilderInfo;
  
    final String name;
    final int tagNumber;
  