When zone errors bubble up through Dart's async machinery the stack trace is in package:stack_trace chain format (with '===== asynchronous gap =====' separators). Flutter's StackFrame parser asserts on those lines. FlutterError.demangleStackTrace strips the chain format back to a plain VM trace before Flutter tries to parse it. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>