how to split dart class in flutter?

Dart doesn't support partial classes. part and part of are to split a library into multiple files, not a class.

Private (identifiers starting with _) in Dart is per library which is usually a *.dart file.

main.dart

part 'part.dart';

class Test {
  /// When someone tries to create an instance of this class
  /// Create an instance of _Test instead
  factory Test() = _Test;

  /// private constructor that can only be accessed within the same library
  Test._(); 

  static const   a = 10;
  final b = 20;
  final c = a+1;
}

part.dart

part of 'main.dart';
class _Test extends Test {
  /// private constructor can only be called from within the same library
  /// Call the private constructor of the super class
  _Test() : super._();

  /// static members of other classes need to be prefixed with
  /// the class name, even when it is the super class
  final d = Test.a +1;   //<---undefined name 'a'
} 

A similar pattern is used in many code-generation scenarios in Dart like in

• https://pub.dartlang.org/packages/built_value
• https://pub.dartlang.org/packages/built_redux
• https://pub.dartlang.org/packages/json_serializable

and many others.

I am faced with same problem. My variant based on extensions:

page.dart

part 'section.dart';

class _PageState extends State<Page> {
    build(BuildContext context) {
        // ...
        _buildSection(context);
        // ...
    }
}

section.dart

part of 'page.dart';

extension Section on _PageState {
    _buildSection(BuildContext context) {
        // ...
    }
}