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