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 fce5972af0aef4ce4ba79bb52cf93ae95224fd86..3de27b41cbe3ae9d8bf0bd136b2b1a00c2d6d991 100644
--- a/services/dart/test/pingpong/main.dart
+++ b/services/dart/test/pingpong/main.dart
@@ -118,5 +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());
+    });
 }