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

pkg/analyzer/test/instrumentation/instrumentation_test.dart

Index: pkg/analyzer/test/instrumentation/instrumentation_test.dart
diff --git a/pkg/analyzer/test/instrumentation/instrumentation_test.dart b/pkg/analyzer/test/instrumentation/instrumentation_test.dart
index 290b71a296f9f6521be57392c220a642b45ae827..ac8755e3cd640e1da51b7ab1db1763a93c08845b 100644
--- a/pkg/analyzer/test/instrumentation/instrumentation_test.dart
+++ b/pkg/analyzer/test/instrumentation/instrumentation_test.dart
@@ -4,6 +4,8 @@
 
 library test.instrumentation;
 
+import 'dart:async';
+
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:unittest/unittest.dart';
 
@@ -174,7 +176,7 @@ class TestInstrumentationServer implements InstrumentationServer {
   }
 
   @override
-  void shutdown() {
+  Future shutdown() async {
     // Ignored
   }
 }