To pass Rust stack traces to flutter, you need to set
RUST_BACKTRACE in the running application. For that simply add
env::set_var("RUST_BACKTRACE", "1"); before initialising the bridge.
--dart-define will not work, you must use
env::set_var, because the former does not set the "environment variable" in the common sense, but instead a special thing only visible to Dart.