Wait for InstrumentationService.shutdown() before exit(0).

pkg/analyzer/test/instrumentation/instrumentation_test.dart

 // Copyright (c) 2015, 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.instrumentation;
 
+import 'dart:async';
+
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:unittest/unittest.dart';
 
 import '../reflective_tests.dart';
 
 main() {
   group('instrumentation', () {
     runReflectiveTests(InstrumentationServiceTest);
     runReflectiveTests(MulticastInstrumentationServerTest);
   });
@@ skipping 150 matching lines @@
   void log(String message) {
     normalChannel.writeln(message);
   }
 
   @override
   void logWithPriority(String message) {
     priorityChannel.writeln(message);
   }
 
   @override
-  void shutdown() {
+  Future shutdown() async {
     // Ignored
   }
 }