Adds (defaults to ___) when using flutter config --help (flutter#172329)

Closes flutter#81665.

Author: matanlurey

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

@@ -67,7 +67,12 @@ class ConfigCommand extends FlutterCommand {
       if (configSetting == null) {
         continue;
       }
-      argParser.addFlag(configSetting, help: feature.generateHelpMessage());
+      final String channel = globals.flutterVersion.channel;
+      argParser.addFlag(
+        configSetting,
+        help: feature.generateHelpMessage(),
+        defaultsTo: feature.getSettingForChannel(channel).enabledByDefault,
+      );
     }
     argParser.addFlag(
       'clear-features',

packages/flutter_tools/test/commands.shard/hermetic/config_test.dart

@@ -59,6 +59,13 @@ void main() {
       );
     });
 
+    testUsingContext('prints default values with --help', () async {
+      final configCommand = ConfigCommand();
+      final CommandRunner<void> commandRunner = createTestCommandRunner(configCommand);
+      await commandRunner.run(<String>['config', '--help']);
+      expect(testLogger.statusText, contains('(defaults to on)'));
+    });
+
     testUsingContext('throws error on excess arguments', () {
       final configCommand = ConfigCommand();
       final CommandRunner<void> commandRunner = createTestCommandRunner(configCommand);