fromExistingSession function Null safety

Implementation

WebDriver fromExistingSession(String sessionId,
    SyncRequestClient Function(Uri prefix) createRequestClient,
    {Uri? uri,
    WebDriverSpec spec = WebDriverSpec.Auto,
    Map<String, dynamic>? capabilities}) {
  uri ??= defaultUri;

  var session = SessionInfo(sessionId, spec, capabilities);

  // Update session info if not all is provided.
  if (spec == WebDriverSpec.Auto || capabilities == null) {
    // This client's prefix at root, it has no session prefix in it.
    final client = createRequestClient(uri);

    final handler = getHandler(spec);

    session = client.send(handler.session.buildInfoRequest(sessionId),
        (response) => handler.session.parseInfoResponse(response, sessionId));
  }

  if (session.spec != WebDriverSpec.JsonWire &&
      session.spec != WebDriverSpec.W3c) {
    throw 'Unexpected spec: ${session.spec}';
  }

  return WebDriver(
      uri,
      session.id,
      UnmodifiableMapView(session.capabilities ?? {}),
      createRequestClient(uri.resolve('session/${session.id}/')),
      session.spec);
}