Heh, I always get those two crates names confused, even now, rereading comments I again thought it was "clap" and not "cpal" (because "clap" like in "clapping").
Dart being a GC'd runtime makes it quite hard to use for low latency audio applications, probably not as critical for audio input as output, but still potential issue, for more details see: https://github.com/dart-lang/sdk/issues/46943
I hope you didn't use a command line parser to stream audio...