Make findElementReferences' "id" result optional.

pkg/analysis_server/test/integration/integration_test_methods.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.
 //
 // This file has been automatically generated.  Please do not edit it manually.
 // To regenerate the file, use the script
 // "pkg/analysis_server/spec/generate_files".
 
 /**
  * Convenience methods for running integration tests
@@ skipping 746 matching lines @@
    *
    *   The offset within the file of the declaration of or reference to the
    *   element.
    *
    * includePotential ( bool )
    *
    *   True if potential matches are to be included in the results.
    *
    * Returns
    *
-   * id ( SearchId )
+   * id ( optional SearchId )
    *
    *   The identifier used to associate results with this search request.
    *
+   *   If no element was found at the given location, this field will be
+   *   absent, and no results will be reported via the search.results
+   *   notification.
+   *
    * element ( optional Element )
    *
    *   The element referenced or defined at the given offset and whose
    *   references will be returned in the search results.
    *
    *   If no element was found at the given location, this field will be
    *   absent.
    */
   Future sendSearchFindElementReferences(String file, int offset, bool includePotential, {bool checkTypes: true}) {
     Map<String, dynamic> params = {};
@@ skipping 673 matching lines @@
       case "debug.launchData":
         expect(params, isDebugLaunchDataParams);
         _onDebugLaunchData.add(params);
         break;
       default:
         fail('Unexpected notification: $event');
         break;
     }
   }
 }