Make search.findElementReferences.result.element field optional.

pkg/analysis_server/test/integration/protocol_matchers.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".
 
 /**
  * Matchers for data types defined in the analysis server API
@@ skipping 399 matching lines @@
     "file": isFilePath,
     "offset": isInt,
     "includePotential": isBool
   }));
 
 /**
  * search.findElementReferences result
  *
  * {
  *   "id": SearchId
- *   "element": Element
+ *   "element": optional Element
  * }
  */
 final Matcher isSearchFindElementReferencesResult = new LazyMatcher(() => new MatchesJsonObject(
   "search.findElementReferences result", {
-    "id": isSearchId,
+    "id": isSearchId
+  }, optionalFields: {
     "element": isElement
   }));
 
 /**
  * search.findMemberDeclarations params
  *
  * {
  *   "name": String
  * }
  */
@@ skipping 1453 matching lines @@
  *
  * {
  *   "newName": String
  * }
  */
 final Matcher isRenameOptions = new LazyMatcher(() => new MatchesJsonObject(
   "rename options", {
     "newName": isString
   }));