[Lyokone]

Builds on #18830, now merged: its Dart fix makes onMessage work without another messaging call first, on Android too.

Description

On Android, foreground messages go from FlutterFirebaseMessagingReceiver to the plugin through a LiveData. That caused two bugs, both reproduced on an emulator (Pixel 9a):

1. Messages lost in bursts. The receiver posts with postValue from a background thread, and postValue keeps only the last of the values posted before the main thread runs. 5 messages handed to the receiver back to back reached onMessage as 1 (the last).

2. Old messages delivered again (#12411, closed by the stale bot, never fixed). LiveData hands its last value to every new observer, so each newly attached engine gets the previous foreground message as a new onMessage. For example: a message arrives, the user leaves with Back (the engine is destroyed, the process is kept), then reopens the app, and the new engine gets the old message again.

FlutterFirebaseRemoteMessageDispatcher replaces the LiveData:

• Every message is delivered once, in order, on the main thread, to the engines whose Dart listens to FirebaseMessaging.onMessage.
• Listening and cancelling tell the plugin (Messaging#startForegroundMessages / Messaging#stopForegroundMessages). The signal is sent from the firebase_messaging package, which ships together with the native code, so a newer native side can't be paired with Dart that never sends it.
• Messages received while no engine listens are kept, for up to a minute and at most 50, and handed to the first engine that listens, never again. Engines that never listen, like the background engine that runs onBackgroundMessage, get none.
• FlutterFirebaseRemoteMessageLiveData is kept for compatibility. It is deprecated, postRemoteMessage forwards to the dispatcher, and its observers are still told about every message.

The main channel buffer is also enlarged (Flutter keeps one message by default), with every message sent to Dart. Engines created with new FlutterEngine(...) register plugins before they run Dart, and Flutter drops messages until then.

Emulator	before	after
5 messages back to back	onMessage gets 1 (the last)	all 5, in order
A second engine starts listening after a message	it also gets the old message	only new messages
3 messages sent before Dart handles the channel	only the last is kept	all 3

Notes:

• App unit tests that mock the plugins.flutter.io/firebase_messaging channel on Android (flutter_test's default platform) now also see Messaging#startForegroundMessages / Messaging#stopForegroundMessages calls.
• Token and FID LiveDatas still replay their last value to new engines (onTokenRefresh with the same token); left as is.
• After a hot restart, messages that arrive before the new Dart listens are not kept (debug only).

Tests

• JUnit (android/src/test, now run by android-unit-tests): bursts, every listener, no replay to later listeners, kept messages delivered once, lifetime and size limits, removal, and the legacy LiveData being notified.
• Dart (firebase_messaging/test/on_message_listening_test.dart): start on the first listen, stop on the last cancel, nothing on iOS.
• Android e2e (example/integration_test/remote_notification_delivery.dart, run by e2e-messaging): a test-only channel in the example's MainActivity.kt hands FCM-shaped intents to the real FlutterFirebaseMessagingReceiver. Each test below fails on the previous code and passes with this change:
  • a burst reaches onMessage once and in order;
  • a second engine started via a callback handle gets only the messages received after it listens;
  • messages sent before Dart handles the channel are all kept.

Related Issues

Fixes #12411. Builds on #18830 (merged).

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• No, this is not a breaking change.

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