is possible have a configuration file in DART?

There are different way.

You could just create a map

my_config.dart

const Map properties = const {
  'CHANNEL': 'sport',
  'VIEW_ELEMENTS': const {
    'LOADER_CLASS': '.loader',
    'SPLASH_CLASS': '.splash'
  }
}

then use it like

main.dart

import 'my_config.dart';

main() {
  print(properties['VIEW_ELEMENTS']['SPLASH_CLASS']);
}

or you can use classes to get proper autocompletion and type checking

my_config.dart

const properties = const Properties('sport', const ViewElements('.loader', '.splash'));

class Properties {
  final String channel;
  final ViewElements viewElements;
  const Properties(this.channel, this.viewElements;
}

class ViewElements {
  final String loaderClass;
  final String splashClass;
  const ViewElements(this.loaderClass, this.splashClass);
}

main.dart

import 'my_config.dart';

main() {
  print(properties.viewElements.splashClass);
}