default.dart

String get platform => throw UnimplementedError();

io.dart

String get platform => 'io';

main.dart

import 'platform.dart';

main() {
  print(platform);
}

platform.dart

// see https://github.com/dart-lang/sdk/issues/34177 for an explanation about these ignores

// ignore: URI_DOES_NOT_EXIST
export 'default.dart'
    // ignore: URI_DOES_NOT_EXIST
    if (dart.library.io) 'io.dart'
    // ignore: URI_DOES_NOT_EXIST
    if (dart.library.html) 'web.dart';

web.dart

String get platform => 'web';