improve shutdown routine

packages/flutter_mozilla_components/lib/src/domain/services/gecko_browser.dart

@@ -64,4 +64,8 @@ class GeckoBrowserService {
   Future<void> requestDefaultBrowser() {
     return _api.requestDefaultBrowser();
   }
+
+  Future<void> shutdown() {
+    return _api.shutdown();
+  }
 }

packages/flutter_mozilla_components/lib/src/pigeons/gecko.g.dart

@@ -5805,6 +5805,28 @@ class GeckoBrowserApi {
       return;
     }
   }
+
+  Future<void> shutdown() async {
+    final pigeonVar_channelName = 'dev.flutter.pigeon.flutter_mozilla_components.GeckoBrowserApi.shutdown$pigeonVar_messageChannelSuffix';
+    final pigeonVar_channel = BasicMessageChannel<Object?>(
+      pigeonVar_channelName,
+      pigeonChannelCodec,
+      binaryMessenger: pigeonVar_binaryMessenger,
+    );
+    final Future<Object?> pigeonVar_sendFuture = pigeonVar_channel.send(null);
+    final pigeonVar_replyList = await pigeonVar_sendFuture as List<Object?>?;
+    if (pigeonVar_replyList == null) {
+      throw _createConnectionError(pigeonVar_channelName);
+    } else if (pigeonVar_replyList.length > 1) {
+      throw PlatformException(
+        code: pigeonVar_replyList[0]! as String,
+        message: pigeonVar_replyList[1] as String?,
+        details: pigeonVar_replyList[2],
+      );
+    } else {
+      return;
+    }
+  }
 }
 
 class GeckoSyncApi {