1.报错：

Unable to RTCPeerConnection::setRemoteDescription: Failed to set remote answer sdp: Called with SDP without SDES crypto.

解决方案：DtlsSrtpKeyAgreement 的值改为true。

final Map<String, dynamic> loopback_constraints = {
  "mandatory": {},
  "optional": [
    {"DtlsSrtpKeyAgreement": true},
  ],
};

参考文档：https://stackoverflow.com/questions/24062340/failed-to-set-remote-offer-sdp-called-with-sdp-without-sdes-crypto