WasmInstanceBuilder builder({
    WasiConfig? wasiConfig,
    WorkersConfig? workersConfig,
  }) {
    _WASI? wasi;
    if (wasiConfig != null) {
      final stdout = wasiConfig.captureStdout ? WasiStdio() : null;
      final stderr = wasiConfig.captureStderr ? WasiStdio() : null;

      final wasiWeb = WASI(
        wasiConfig.args,
        wasiConfig.env
            .map((e) => '${e.name}=${e.value}')
            .toList(growable: false),
        [
          OpenFile(WasiWebFile(Uint8List(0))), // TODO: stdin
          stdout?.fd ?? OpenFile(WasiWebFile(Uint8List(0))),
          stderr?.fd ?? OpenFile(WasiWebFile(Uint8List(0))),
          ...wasiConfig.webBrowserFileSystem.entries.map(
            (e) => PreopenDirectory(
              e.key,
              js_util.jsify(_mapWasiFiles(e.value.items)) as Object,
            ),
          ),
        ],
      );
      wasi = _WASI(wasiWeb, stdout, stderr);
    }

    return _Builder(this, wasi, workersConfig);
  }