Enable asynchrony test.

pkg/analysis_server/test/integration/test_all.dart

Index: pkg/analysis_server/test/integration/test_all.dart
diff --git a/pkg/analysis_server/test/integration/test_all.dart b/pkg/analysis_server/test/integration/test_all.dart
index be609ceb607670c256e667145c270405e3d4f707..54d3fb246ba9962554dbbc84cd7fa0c719c04fd9 100644
--- a/pkg/analysis_server/test/integration/test_all.dart
+++ b/pkg/analysis_server/test/integration/test_all.dart
@@ -12,10 +12,6 @@ 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.
  */
@@ -23,9 +19,7 @@ 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();