Make findElementReferences' "id" result optional.

pkg/analysis_server/lib/src/search/search_domain.dart

 // Copyright (c) 2014, 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 search.domain;
 
-import 'dart:async';
-
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/protocol.dart' as protocol;
 import 'package:analysis_server/src/search/element_references.dart';
 import 'package:analysis_server/src/search/type_hierarchy.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart';
 import 'package:analyzer/src/generated/element.dart';
 
 /**
  * Instances of the class [SearchDomainHandler] implement a [RequestHandler]
@@ skipping 39 matching lines @@
     // schedule search
     String searchId = (_nextSearchId++).toString();
     elements.forEach((Element element) {
       var computer = new ElementReferencesComputer(searchEngine);
       var future = computer.compute(element, params.includePotential);
       future.then((List<protocol.SearchResult> results) {
         bool isLast = identical(element, elements.last);
         _sendSearchNotification(searchId, isLast, results);
       });
     });
+    // respond
     if (elements.isEmpty) {
-      new Future.microtask(() {
-        _sendSearchNotification(searchId, true, []);
-      });
+      return new protocol.SearchFindElementReferencesResult().toResponse(
+          request.id);
+    } else {
+      protocol.Element element = new protocol.Element.fromEngine(elements[0]);
+      return new protocol.SearchFindElementReferencesResult(id: searchId,
+          element: element).toResponse(request.id);
     }
-    // respond
-    protocol.Element element;
-    if (elements.isNotEmpty) {
-      element = new protocol.Element.fromEngine(elements[0]);
-    }
-    return new protocol.SearchFindElementReferencesResult(searchId,
-        element: element).toResponse(request.id);
   }
 
   protocol.Response findMemberDeclarations(protocol.Request request) {
     var params = new protocol.SearchFindMemberDeclarationsParams.fromRequest(request);
     // schedule search
     String searchId = (_nextSearchId++).toString();
     {
       var matchesFuture = searchEngine.searchMemberDeclarations(params.name);
       matchesFuture.then((List<SearchMatch> matches) {
         _sendSearchNotification(searchId, true, matches.map(toResult));
@@ skipping 84 matching lines @@
   void _sendSearchNotification(String searchId, bool isLast,
       Iterable<protocol.SearchResult> results) {
     server.sendNotification(new protocol.SearchResultsParams(searchId,
         results.toList(), isLast).toNotification());
   }
 
   static protocol.SearchResult toResult(SearchMatch match) {
     return new protocol.SearchResult.fromMatch(match);
   }
 }