Add library tag to common_usage_sorter.dart.

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

 // Copyright (c) 2015, 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.
 
+library services.completion.dart.sorter.common;
+
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analysis_server/src/protocol_server.dart'
     show CompletionSuggestion, CompletionSuggestionKind;
 import 'package:analysis_server/src/provisional/completion/completion_core.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/services/completion/dart/contribution_sorter.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 101 matching lines @@
   DartType visitNode(AstNode node) {
     return null;
   }
 
   DartType visitPrefixedIdentifier(PrefixedIdentifier node) =>
       node.identifier == entity ? node.prefix?.bestType : null;
 
   DartType visitPropertyAccess(PropertyAccess node) =>
       node.propertyName == entity ? node.realTarget?.bestType : null;
 }