Add library tag to common_usage_sorter.dart.

pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart b/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
index 05c6fb48e6cb50cfd8dc83369a514e14632ccf4d..a515bedff80d8ef36b8d5f0502ead1da4257204a 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
@@ -2,6 +2,8 @@
 // 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.
 
+library services.completion.dart.sorter.common;
+
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;