Undefined class error however class is imported

Question

I am developing using Flutter/Dart. At one point I generated Code using the freezed package. The generated code is a 'part of' declarative, however it do not 'see' the package import of its parent:

import 'package:strawberry_keekz/src/domain/keekz/value_objects.dart';

part 'keekz.freezed.dart';

@freezed
abstract class Keekz implements _$Keekz {
  const Keekz._();

  const factory Keekz({
    required UniqueId id,
    required KeekzBody body,
    required KeekzColor color,
  }) = _Keekz;

  factory Keekz.empty() => Keekz(
        id: UniqueId(),
        body: KeekzBody(''),
        color: KeekzColor(KeekzColor.predefinedColors[0]),
      );
}

and the generated code:

// coverage:ignore-file
// GENERATED CODE - DO NOT MODIFY BY HAND
// ignore_for_file: unused_element, deprecated_member_use, deprecated_member_use_from_same_package, use_function_type_syntax_for_parameters, unnecessary_const, avoid_init_to_null, invalid_override_different_default_values_named, prefer_expression_function_bodies, annotate_overrides, invalid_annotation_target

part of 'keekz.dart';

// **************************************************************************
// FreezedGenerator
// **************************************************************************

T _$identity<T>(T value) => value;

final _privateConstructorUsedError = UnsupportedError(
    'It seems like you constructed your class using `MyClass._()`. This constructor is only meant to be used by freezed and you are not supposed to need it nor use it.\nPlease check the documentation here for more informations: https://github.com/rrousselGit/freezed#custom-getters-and-methods');

/// @nodoc
class _$KeekzTearOff {
  const _$KeekzTearOff();

  _Keekz call(
      {required UniqueId id,       <--- Error: Undefined class 'UniqueId'
      required KeekzBody body,  // <--- Error: Undefined class 'KeekzBody'
      required KeekzColor color}) { <--- Error: Undefined class 'KeekzColor'
    return _Keekz(
      id: id,
      body: body,
      color: color,
    );
  }
}

However the first line of the first snippet imported the necessary class. When I add this line manually to the generated code this solves the issue. Why doe it not work as the 'part of'?

Code of value_objects.dart:

import 'dart:ui';

import 'package:dartz/dartz.dart';
import 'package:strawberry_keekz/src/domain/core/failures.dart';
import 'package:strawberry_keekz/src/domain/core/value_objects.dart';
import 'package:strawberry_keekz/src/domain/core/value_transformers.dart';
import 'package:strawberry_keekz/src/domain/core/value_validators.dart';

class KeekzBody extends ValueObject<String> {
  @override
  final Either<ValueFailure<String>, String> value;

  static const maxLength = 1000;

  factory KeekzBody(String input) {
    return KeekzBody._(
      validateMaxStringLength(input, maxLength).flatMap(validateStringNotEmpty),
    );
  }

  const KeekzBody._(this.value);
}

class KeekzColor extends ValueObject<Color> {
  static const List<Color> predefinedColors = [
    Color(0xfffafafa), // canvas
    Color(0xfffa8072), // salmon
    Color(0xfffedc56), // mustard
    Color(0xffd0f0c0), // tea
    Color(0xfffca3b7), // flamingo
    Color(0xff997950), // tortilla
    Color(0xfffffdd0), // cream
  ];

  @override
  final Either<ValueFailure<Color>, Color> value;

  factory KeekzColor(Color input) {
    return KeekzColor._(
      right(makeColorOpaque(input)),
    );
  }

  const KeekzColor._(this.value);
}