Add level field to the "showMessage" daemon event. (#9889)

Spawned from a discussion in https://github.com/flutter/flutter-intellij/pull/1003

packages/flutter_tools/lib/src/commands/daemon.dart

@@ -543,9 +543,12 @@ class DeviceDomain extends Domain {
       return;
 
     if (!discoverer.canListAnything) {
+      // This event will affect the client UI. Coordinate changes here
+      // with the Flutter IntelliJ team.
       sendEvent(
         'daemon.showMessage',
         <String, String>{
+          'level': 'warning',
           'title': 'Unable to list devices',
           'message':
               'Unable to discover ${discoverer.name}. Please run '

packages/flutter_tools/test/daemon_test.dart

@@ -216,6 +216,7 @@ void main() {
       final Map<String, dynamic> response = await responses.stream.first;
       expect(response['event'], 'daemon.showMessage');
       expect(response['params'], isMap);
+      expect(response['params'], containsPair('level', 'warning'));
       expect(response['params'], containsPair('title', 'Unable to list devices'));
       expect(response['params'], containsPair('message', contains('Unable to discover Android devices')));
     }, overrides: <Type, Generator>{