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

examples/dart/hello_world/hello/main.dart

Index: examples/dart/hello_world/hello/main.dart
diff --git a/examples/dart/hello_world/hello/main.dart b/examples/dart/hello_world/hello/main.dart
index bd896a118a9304bd79d7fd21085ea59be96cab7a..0f828a86a496936f77c3606a1d912bc8416e3ec8 100644
--- a/examples/dart/hello_world/hello/main.dart
+++ b/examples/dart/hello_world/hello/main.dart
@@ -5,6 +5,7 @@
 // To run this app:
 // mojo_shell mojo:hello
 
+import 'dart:async';
 import 'mojo:application';
 import 'mojo:bindings';
 import 'mojo:core';
@@ -23,7 +24,12 @@ class Hello extends Application {
     // app has a chance to come up. Instead, we wait to close this app until
     // the "world" app comes up, does its print, and closes its end of the
     // connection.
-    c.onError = close;
+    c.onError = closeApplication;
+  }
+
+  Future closeApplication() async {
+    await close();
+    assert(MojoHandle.reportLeakedHandles());
   }
 }