Restore the test for an unknown element search.

pkg/analysis_server/test/services/search/search_engine_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.services.src.search.search_engine;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/index/index.dart';
 import 'package:analysis_server/src/services/index/local_memory_index.dart';
@@ skipping 200 matching lines @@
 ''');
     ConstructorElement element = findElement('named');
     Element mainElement = findElement('main');
     var expected = [
       _expectId(mainElement, MatchKind.REFERENCE, '.named();', length: 6)
     ];
     return _verifyReferences(element, expected);
   }
 
   Future test_searchReferences_Element_unknown() {
-    // TODO(scheglov) implement some test
-//    return _verifyReferences(UniverseElement.INSTANCE, []);
+    return _verifyReferences(DynamicElementImpl.instance, []);
   }
 
   Future test_searchReferences_FieldElement() {
     _indexTestUnit('''
 class A {
   var field;
   A({this.field});
   main() {
     new A(field: 1);
     // getter
@@ skipping 410 matching lines @@
         .then((List<SearchMatch> matches) {
       _assertMatches(matches, expectedMatches);
     });
   }
 
   static void _assertMatches(
       List<SearchMatch> matches, List<ExpectedMatch> expectedMatches) {
     expect(matches, unorderedEquals(expectedMatches));
   }
 }