repro.dart

import 'dart:async';
import 'dart:io';

Future<void> main() async {
  File('script.dart').writeAsStringSync('''
import 'dart:io';

void main() {
  stderr.writeln("*" * ((1 << 16) + 1));
}
''');

  var process = await Process.start(
      Platform.executable, [...Platform.executableArguments, 'script.dart']);

  var controller = StreamController<List<int>>();
  process.stderr.pipe(controller.sink);
  print("exit code: ${await process.exitCode}");
  print(
      await controller.stream.fold<int>(0, (sum, chunk) => sum + chunk.length));
}