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

examples/dart/hello_world/world/main.dart

Index: examples/dart/hello_world/world/main.dart
diff --git a/examples/dart/hello_world/world/main.dart b/examples/dart/hello_world/world/main.dart
index b1343feee44ab6f1c59db0b919b29a7527dc6d8d..a9c2b92c5a78b0a8b388b56c60392449d2b573e2 100644
--- a/examples/dart/hello_world/world/main.dart
+++ b/examples/dart/hello_world/world/main.dart
@@ -4,6 +4,7 @@
 
 // This app is run by examples/dart/hello_world/hello.
 
+import 'dart:async';
 import 'mojo:application';
 import 'mojo:bindings';
 import 'mojo:core';
@@ -13,7 +14,12 @@ class World extends Application {
 
   void initialize(List<String> args, String url) {
     print("$url World");
-    close();
+    closeApplication();
+  }
+
+  Future closeApplication() async {
+    await close();
+    assert(MojoHandle.reportLeakedHandles());
   }
 }