Make findElementReferences' "id" result optional.

pkg/analysis_server/test/search/element_references_test.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 test.search.element_references;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_testing/reflective_tests.dart';
@@ skipping 15 matching lines @@
   Future findElementReferences(String search, bool includePotential) {
     int offset = findOffset(search);
     return waitForTasksFinished().then((_) {
       Request request = new SearchFindElementReferencesParams(testFile, offset,
           includePotential).toRequest('0');
       Response response = handleSuccessfulRequest(request);
       var result = new SearchFindElementReferencesResult.fromResponse(response);
       searchId = result.id;
       searchElement = result.element;
       results.clear();
-      return waitForSearchResults();
+      if (searchId == null) {
+        return null;
+      } else {
+        return waitForSearchResults();
+      }
     });
   }
 
   test_constructor_named() {
     addTestFile('''
 class A {
   A.named(p);
 }
 main() {
   new A.named(1);
@@ skipping 526 matching lines @@
 }
 ''');
     return findElementReferences('T> {', false).then((_) {
       expect(searchElement.kind, ElementKind.TYPE_PARAMETER);
       expect(results, hasLength(2));
       assertHasResult(SearchResultKind.REFERENCE, 'T f;');
       assertHasResult(SearchResultKind.REFERENCE, 'T m()');
     });
   }
 }