startupChannel property
Startup channel. Runs on a background TaskQueue on both platforms, so a startup message never queues behind the main thread during launch — the very window in which every one of these is sent.
Separate from exportChannel on purpose: that queue is serial and carries every Dart span pushed at onEnd, so a burst of span exports would delay a startup message that has only a few milliseconds of headroom before the native span closes.
markTimeToDisplay lives here too, and must: it ends the app.start span on iOS, and ordering is only guaranteed within one channel. Split across two channels, the close could overtake the events it is supposed to follow.
Implementation
@visibleForTesting
final startupChannel = const MethodChannel('vunet_flutter_plugin/startup');