expose public arg_parser.dart library (#209)

This can be imported by the dart tool without pulling in a bunch of extra DEPS.

cc @bkonyi

pkgs/dart_mcp_server/CHANGELOG.md

diff --git a/pkgs/dart_mcp_server/CHANGELOG.md b/pkgs/dart_mcp_server/CHANGELOG.md
index 91854d3..c641328 100644
--- a/pkgs/dart_mcp_server/CHANGELOG.md
+++ b/pkgs/dart_mcp_server/CHANGELOG.md
@@ -42,3 +42,4 @@
 * Add `--log-file` argument to log all protocol traffic to a file.
 * Improve error text for failed DTD connections as well as the tool description.
 * Add support for injecting an `Analytics` instance to track usage.
+* Add `arg_parser.dart` public library with minimal deps to be used by the dart tool.

pkgs/dart_mcp_server/lib/arg_parser.dart

diff --git a/pkgs/dart_mcp_server/lib/arg_parser.dart b/pkgs/dart_mcp_server/lib/arg_parser.dart
new file mode 100644
index 0000000..ccb7844
--- /dev/null
+++ b/pkgs/dart_mcp_server/lib/arg_parser.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+export 'src/arg_parser.dart' show argParser;

pkgs/dart_mcp_server/lib/src/arg_parser.dart

diff --git a/pkgs/dart_mcp_server/lib/src/arg_parser.dart b/pkgs/dart_mcp_server/lib/src/arg_parser.dart
new file mode 100644
index 0000000..4dd55c3
--- /dev/null
+++ b/pkgs/dart_mcp_server/lib/src/arg_parser.dart
@@ -0,0 +1,44 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:args/args.dart';
+
+final argParser =
+    ArgParser(allowTrailingOptions: false)
+      ..addOption(
+        dartSdkOption,
+        help:
+            'The path to the root of the desired Dart SDK. Defaults to the '
+            'DART_SDK environment variable.',
+      )
+      ..addOption(
+        flutterSdkOption,
+        help:
+            'The path to the root of the desired Flutter SDK. Defaults to '
+            'the FLUTTER_SDK environment variable, then searching up from '
+            'the Dart SDK.',
+      )
+      ..addFlag(
+        forceRootsFallbackFlag,
+        negatable: true,
+        defaultsTo: false,
+        help:
+            'Forces a behavior for project roots which uses MCP tools '
+            'instead of the native MCP roots. This can be helpful for '
+            'clients like cursor which claim to have roots support but do '
+            'not actually support it.',
+      )
+      ..addOption(
+        logFileOption,
+        help:
+            'Path to a file to log all MPC protocol traffic to. File will be '
+            'overwritten if it exists.',
+      )
+      ..addFlag(helpFlag, abbr: 'h', help: 'Show usage text');
+
+const dartSdkOption = 'dart-sdk';
+const flutterSdkOption = 'flutter-sdk';
+const forceRootsFallbackFlag = 'force-roots-fallback';
+const helpFlag = 'help';
+const logFileOption = 'log-file';

pkgs/dart_mcp_server/lib/src/server.dart

diff --git a/pkgs/dart_mcp_server/lib/src/server.dart b/pkgs/dart_mcp_server/lib/src/server.dart
index e900dd6..d685e5a 100644
--- a/pkgs/dart_mcp_server/lib/src/server.dart
+++ b/pkgs/dart_mcp_server/lib/src/server.dart
@@ -6,7 +6,6 @@
 import 'dart:convert';
 import 'dart:io' as io;
 
-import 'package:args/args.dart';
 import 'package:async/async.dart';
 import 'package:dart_mcp/server.dart';
 import 'package:file/file.dart';
@@ -16,6 +15,7 @@
 import 'package:stream_channel/stream_channel.dart';
 import 'package:unified_analytics/unified_analytics.dart';
 
+import 'arg_parser.dart';
 import 'mixins/analyzer.dart';
 import 'mixins/dash_cli.dart';
 import 'mixins/dtd.dart';
@@ -200,47 +200,8 @@
       validateArguments: validateArguments,
     );
   }
-
-  static final argParser =
-      ArgParser(allowTrailingOptions: false)
-        ..addOption(
-          dartSdkOption,
-          help:
-              'The path to the root of the desired Dart SDK. Defaults to the '
-              'DART_SDK environment variable.',
-        )
-        ..addOption(
-          flutterSdkOption,
-          help:
-              'The path to the root of the desired Flutter SDK. Defaults to '
-              'the FLUTTER_SDK environment variable, then searching up from '
-              'the Dart SDK.',
-        )
-        ..addFlag(
-          forceRootsFallbackFlag,
-          negatable: true,
-          defaultsTo: false,
-          help:
-              'Forces a behavior for project roots which uses MCP tools '
-              'instead of the native MCP roots. This can be helpful for '
-              'clients like cursor which claim to have roots support but do '
-              'not actually support it.',
-        )
-        ..addOption(
-          logFileOption,
-          help:
-              'Path to a file to log all MPC protocol traffic to. File will be '
-              'overwritten if it exists.',
-        )
-        ..addFlag(helpFlag, abbr: 'h', help: 'Show usage text');
 }
 
-const dartSdkOption = 'dart-sdk';
-const flutterSdkOption = 'flutter-sdk';
-const forceRootsFallbackFlag = 'force-roots-fallback';
-const helpFlag = 'help';
-const logFileOption = 'log-file';
-
 /// Creates a `Sink<String>` for [logFile].
 Sink<String> _createLogSink(io.File logFile) {
   logFile.createSync(recursive: true);