Clean up observatory testing processes when we are done with them.

runtime/bin/vmservice/vmservice_io.dart

Index: runtime/bin/vmservice/vmservice_io.dart
diff --git a/runtime/bin/vmservice/vmservice_io.dart b/runtime/bin/vmservice/vmservice_io.dart
index 75aa56c24d060a5bd59d369a341c11b5727838df..409bc5972a0c110d842c0da006e319a958bef8ff 100644
--- a/runtime/bin/vmservice/vmservice_io.dart
+++ b/runtime/bin/vmservice/vmservice_io.dart
@@ -31,7 +31,9 @@ Future<Server> serverFuture;
 
 _onShutdown() {
   if (server != null) {
-    server.close(true).catchError((e, st) { assert(e); });
+    server.close(true).catchError((e, st) {
+      print("Error in vm-service shutdown: $e\n$st\n");
+    });
   }
   if (_signalSubscription != null) {
     _signalSubscription.cancel();