Cause Flutter to write an error to stderr, for example by adding invalid toolArgs:
[12:51:40 PM] [DAP] [Info] <== {"seq":8,"type":"event","body":{"message":"<== [Flutter] [stderr] [91, 32, 32, 43, 49, 52, 32, 109, 115, 93, 32, 67, 111, 117, 108, 100, 32, 110, 111, 116, 32, 102, 105, 110, 100, 32, 97, 110, 32, 111, 112, 116, 105, 111, 110, 32, 110, 97, 109, 101, 100, 32, 34, 105, 110, 118, 97, 108, 105, 100, 34, 46, 10, 10, 91, 32, 32, 32, 32, 32, 32, 32, 32, 93, 32, 82, 117, 110, 32, 39, 102, 108, 117, 116, 116, 101, 114, 32, 45, 104, 39, 32, 40, 111, 114, 32, 39, 102, 108, 117, 116, 116, 101, 114, 32, 60, 99, 111, 109, 109, 97, 110, 100, 62, 32, 45, 104, 39, 41, 32, 102, 111, 114, 32, 97, 118, 97, 105, 108, 97, 98, 108, 101, 32, 102, 108, 117, 116, 116, 101, 114, 32, 99, 111, 109, 109, 97, 110, 100, 115, 32, 97, 110, 100, 32, 111, 112, 116, 105, 111, 110, 115, 46, 10]"},"event":"dart.log"}
[12:51:40 PM] [General] [Info] [flutter_bad_stderr (Windows)] <== [Flutter] [stderr] [91, 32, 32, 43, 49, 52, 32, 109, 115, 93, 32, 67, 111, 117, 108, 100, 32, 110, 111, 116, 32, 102, 105, 110, 100, 32, 97, 110, 32, 111, 112, 116, 105, 111, 110, 32, 110, 97, 109, 101, 100, 32, 34, 105, 110, 118, 97, 108, 105, 100, 34, 46, 10, 10, 91, 32, 32, 32, 32, 32, 32, 32, 32, 93, 32, 82, 117, 110, 32, 39, 102, 108, 117, 116, 116, 101, 114, 32, 45, 104, 39, 32, 40, 111, 114, 32, 39, 102, 108, 117, 116, 116, 101, 114, 32, 60, 99, 111, 109, 109, 97, 110, 100, 62, 32, 45, 104, 39, 41, 32, 102, 111, 114, 32, 97, 118, 97, 105, 108, 97, 98, 108, 101, 32, 102, 108, 117, 116, 116, 101, 114, 32, 99, 111, 109, 109, 97, 110, 100, 115, 32, 97, 110, 100, 32, 111, 112, 116, 105, 111, 110, 115, 46, 10]
[12:51:40 PM] [DAP] [Info] <== {"seq":9,"type":"event","body":{"category":"stderr","output":"[ +14 ms] Could not find an option named \"invalid\".\n"},"event":"output"}
The last line is decoded correctly and what the user sees, however the dart.log events are a result of calling toString() on a List<int>. They only contain the same data so it's not critical, but if only looking at the log events it makes it an extra step to decode.