anchor.diff:    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,
  +
  +  MapFieldInfo.map(String name, int tagNumber, int index, int type,
         this.keyFieldType, this.valueFieldType, BuilderInfo mapEntryBuilderInfo)
  -      : super._(
  +      : super._map(
               name,
               tagNumber,
               index,
  
positive.code:             null,
              null,
              null,
              null
              ) {
      assert(name != null);
      assert(tagNumber != null);
      assert(_isMapField(type));
  
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;
  