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

services/dart/test/pingpong_target/main.dart

Index: services/dart/test/pingpong_target/main.dart
diff --git a/services/dart/test/pingpong_target/main.dart b/services/dart/test/pingpong_target/main.dart
index a91d04022112b579c4f05582826594c4785e18bd..8be497896fc8b453a1c8a4f93cb56802741dfe58 100644
--- a/services/dart/test/pingpong_target/main.dart
+++ b/services/dart/test/pingpong_target/main.dart
@@ -32,6 +32,13 @@ class PingPongServiceImpl implements PingPongService {
     }
     _stub.close();
   }
+
+  Future pingTargetUrl(String url, int count, [Function responseFactory]) {}
+
+  Future pingTargetService(ProxyBase proxyBase, int count,
+      [Function responseFactory]) {}
+
+  getPingPongService(PingPongServiceStub serviceStub) {}
 }
 
 class PingPongApplication extends Application {
@@ -43,7 +50,12 @@ class PingPongApplication extends Application {
     connection.provideService(PingPongServiceName,
         (endpoint) => new PingPongServiceImpl(this, endpoint));
     // Close the application when the first connection goes down.
-    connection.onError = close;
+    connection.onError = closeApplication;
+  }
+
+  Future closeApplication() async {
+    await close();
+    assert(MojoHandle.reportLeakedHandles());
   }
 }