Dart environment config with defaults

import 'dart:io';

class Config {
  Config._(this.host, this.port, this.debug);
  final String host;
  final int port;
  final bool debug;

  factory Config.fromEnv([Map<String, String>? env]) {
    final e = env ?? Platform.environment;
    final port = int.tryParse(e['PORT'] ?? '8080');
    if (port == null || port < 1 || port > 65535) {
      throw FormatException('PORT must be 1-65535, got ${e['PORT']}');
    }
    final host = e['HOST'] ?? 'localhost';
    if (host.isEmpty) throw StateError('HOST must not be empty');
    return Config._(host, port, e['DEBUG'] == '1');
  }
}