Send DC info for syncState (#88)

* dc private methods

* impl

* pub

* id

* fix test

test/mock/peerconnection_mock.dart

@@ -172,7 +172,7 @@ class MockPeerConnection extends RTCPeerConnection {
   @override
   Future<RTCDataChannel> createDataChannel(
       String label, RTCDataChannelInit dataChannelDict) async {
-    return MockDataChannel(label);
+    return MockDataChannel(dataChannelDict.id, label);
   }
 
   @override