Add missing 'help' option to DevTools server arg parser.

This option existed in the previous impl of devtools server: https://github.com/flutter/devtools/blob/v2.10.0/packages/devtools_server/lib/src/server.dart#L507-L516

Change-Id: Icdc2f1b29e9128531a51fbc471c7b52e761e8f31
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232122
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Ben Konyi <[email protected]>
Commit-Queue: Kenzie Davisson <[email protected]>

Author: kenzieschmoll

pkg/dartdev/lib/src/commands/devtools.dart

@@ -14,7 +14,10 @@ class DevToolsCommand extends DartdevCommand {
   DevToolsCommand({
     this.customDevToolsPath,
     bool verbose = false,
-  })  : _argParser = DevToolsServer.buildArgParser(verbose: verbose),
+  })  : _argParser = DevToolsServer.buildArgParser(
+          verbose: verbose,
+          includeHelpOption: false,
+        ),
         super(
           'devtools',
           DevToolsServer.commandDescription,

pkg/dds/lib/devtools_server.dart

@@ -47,8 +47,24 @@ class DevToolsServer {
   late ClientManager clientManager;
   final bool _isChromeOS = File('/dev/.cros_milestone').existsSync();
 
-  static ArgParser buildArgParser({bool verbose = false}) {
+  /// Builds an arg parser for the DevTools server.
+  ///
+  /// [includeHelpOption] should be set to false if this arg parser will be used
+  /// in a Command subclass.
+  static ArgParser buildArgParser({
+    bool verbose = false,
+    bool includeHelpOption = true,
+  }) {
     final argParser = ArgParser();
+
+    if (includeHelpOption) {
+      argParser.addFlag(
+        argHelp,
+        negatable: false,
+        abbr: 'h',
+        help: 'Prints help output.',
+      );
+    }
     argParser
       ..addFlag(
         argVersion,