Code snippet：

......
  (dio.httpClientAdapter as DefaultHttpClientAdapter).onHttpClientCreate =
      (client) {
    SecurityContext securityContext = SecurityContext(withTrustedRoots: true);
    // securityContext.setTrustedCertificatesBytes(certBytes);
    securityContext.setTrustedCertificates('assets/ca-cert.crt');
    HttpClient httpClient = HttpClient(context: securityContext);
    httpClient.badCertificateCallback =
        (X509Certificate cert, String host, int port) {
      return true;
    };
  };
......

Error prompt:

Failed to load "...dio_ca_trust_test.dart":
DioError [DioErrorType.other]: TlsException: Failure trusting builtin roots (OS Error: 
    CERT_ALREADY_IN_HASH_TABLE(../../third_party/boringssl/src/crypto/x509/x509_lu.c:357), errno = 184549481)

reference document:
3.2.flutter-https验证
Flutter 自签名证书