Dart: Adds optional named arguments for creating bindings.

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 4dcbd4c86614081d699517f8d177cc5131559b46..40849f976169e06c497de1aff2a927f40f3bf442 100644
--- a/examples/dart/hello_world/hello/main.dart
+++ b/examples/dart/hello_world/hello/main.dart
@@ -16,8 +16,8 @@ class Hello extends Application {
     print("$url Hello");
     // We expect to find the world mojo application at the same
     // path as this application.
-    connectToApplication(url.replaceAll("hello", "world"));
-    close();
+    var c = connectToApplication(url.replaceAll("hello", "world"));
+    c.listen(onClosed: close);
   }
 }