Allow dead code in completion_metrics script.

Change-Id: I16751bcbb25f09c3a897de82c06ab424abcfcf1b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/293005
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
diff --git a/pkg/analysis_server/tool/code_completion/completion_metrics.dart b/pkg/analysis_server/tool/code_completion/completion_metrics.dart
index e93c38e..c737b84 100644
--- a/pkg/analysis_server/tool/code_completion/completion_metrics.dart
+++ b/pkg/analysis_server/tool/code_completion/completion_metrics.dart
@@ -2,6 +2,9 @@
 // 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.
 
+// This file has commented out code that uses code that otherwise unreachable.
+// ignore_for_file: unreachable_from_main
+
 import 'dart:convert';
 import 'dart:developer';
 import 'dart:io' show stdout;