FlutterCodecs.mm uses NSJSONSerialization to encode/decode platform messages.

If the Dart side sends invalid UTF8 sequences, NSJSONSerialization fails to decode or encode (and we ignore the error). We then assert that we successfully encoded/decoded, and die.

We should not crash in this case. Ideally we'd do what Android does and add replacement characters.

We should make sure the framework doesn't encode bad data if possible, but we should also make the platform side more resillient.

/cc @GaryQian @jason-simmons from discussion offline.

See also #30851