Enable asynchrony test.

pkg/analysis_server/test/integration/test_all.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.integration.all;
 
 import 'package:unittest/unittest.dart';
 
 import 'analysis/test_all.dart' as analysis_test_all;
 import 'asynchrony_test.dart' as asynchrony_test;
 import 'completion/test_all.dart' as completion_test_all;
 import 'search/test_all.dart' as search_test_all;
 import 'server/test_all.dart' as server_test_all;
 
-// Disable asynchrony test for now.
-// TODO(paulberry): re-enable when issue 21252 is fixed.
-const bool _ENABLE_ASYNCHRONY_TEST = false;
-
 /**
  * Utility for manually running all integration tests.
  */
 main() {
   groupSep = ' | ';
   group('analysis_server_integration', () {
     analysis_test_all.main();
-    if (_ENABLE_ASYNCHRONY_TEST) {
-      asynchrony_test.main();
-    }
+    asynchrony_test.main();
     completion_test_all.main();
     search_test_all.main();
     server_test_all.main();
   });
 }