build_runner.dart

main(List<String> arguments) {
  var myBuilder = new MyBuilder();
  var buildActions = [new BuildAction(myBuilder, 'some package', inputs: ['.dart'], excludes: excludes, isOptional: isOptional)]
  final buildResult = await build_runner.build(buildActions, //....);
}
                                               
                                               
class MyBuilder implements Builder {

  MyBuilder();

  @override
  Future build(BuildStep buildStep) async {
    final input = buildStep.inputId;
		// что-то делаем с ассетами
  }

  @override
  Map<String, List<String>> get buildExtensions => {
        '.dart': ['.dart'] // преобразовываем из дарт в дарт
      };
}