Dart: Better handle leak checks. close() is async.

services/dart/test/pingpong/main.dart

Index: services/dart/test/pingpong/main.dart
diff --git a/services/dart/test/pingpong/main.dart b/services/dart/test/pingpong/main.dart
index 5c10d354e18ee8c50b0931dc549a9a6d7c1341ab..ceb7c2da1928e1f89f16bdff5f95dd07e24bae7b 100644
--- a/services/dart/test/pingpong/main.dart
+++ b/services/dart/test/pingpong/main.dart
@@ -51,7 +51,8 @@ class PingPongServiceImpl implements PingPongService {
     }
   }
 
-  Future pingTargetUrl(String url, int count, Function responseFactory) async {
+  Future pingTargetUrl(String url, int count,
+      [Function responseFactory]) async {
     if (_application == null) {
       return responseFactory(false);
     }
@@ -72,8 +73,8 @@ class PingPongServiceImpl implements PingPongService {
     return responseFactory(true);
   }
 
-  Future pingTargetService(
-      ProxyBase proxyBase, int count, Function responseFactory) async {
+  Future pingTargetService(ProxyBase proxyBase, int count,
+      [Function responseFactory]) async {
     var pingPongService = proxyBase;
     var completer = new Completer();
     var client = new PingPongClientImpl.unbound(count, completer);
@@ -117,5 +118,8 @@ class PingPongApplication extends Application {
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   String url = args[1];
-  new PingPongApplication.fromHandle(appHandle);
+  new PingPongApplication.fromHandle(appHandle)
+    ..onError = (() {
+      assert(MojoHandle.reportLeakedHandles());
+    });
 }