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

examples/dart/console_example/main.dart

Index: examples/dart/console_example/main.dart
diff --git a/examples/dart/console_example/main.dart b/examples/dart/console_example/main.dart
index 940c83285861f9cffc8239717f3a0684bd8a8ed5..1c38fb6eb11dd934a439fb75a3094f21a8e17c2f 100644
--- a/examples/dart/console_example/main.dart
+++ b/examples/dart/console_example/main.dart
@@ -25,7 +25,8 @@ class ConsoleApplication extends Application {
     await _console.ptr.printLines([result.line]);
 
     _console.close();
-    close();
+    await close();
+    assert(MojoHandle.reportLeakedHandles());
   }
 }