Make findElementReferences' "id" result optional.

pkg/analysis_server/test/integration/integration_test_methods.dart

Index: pkg/analysis_server/test/integration/integration_test_methods.dart
diff --git a/pkg/analysis_server/test/integration/integration_test_methods.dart b/pkg/analysis_server/test/integration/integration_test_methods.dart
index acf1619f10c8b5d88425a98082f750a16ed3fbdf..b608550893ccb54e2fa4d2e6c4ee98155aa111b6 100644
--- a/pkg/analysis_server/test/integration/integration_test_methods.dart
+++ b/pkg/analysis_server/test/integration/integration_test_methods.dart
@@ -764,10 +764,14 @@ abstract class IntegrationTestMixin {
    *
    * 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