Add an integration test to verify that server I/O is asynchronous.

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 d9251f0ec4609ddc8feb70cc82f6ed8900bbd693..be609ceb607670c256e667145c270405e3d4f707 100644
--- a/pkg/analysis_server/test/integration/test_all.dart
+++ b/pkg/analysis_server/test/integration/test_all.dart
@@ -7,10 +7,15 @@ 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.
  */
@@ -18,6 +23,9 @@ main() {
   groupSep = ' | ';
   group('analysis_server_integration', () {
     analysis_test_all.main();
+    if (_ENABLE_ASYNCHRONY_TEST) {
+      asynchrony_test.main();
+    }
     completion_test_all.main();
     search_test_all.main();
     server_test_all.main();