Dart: Removes need to call listen().

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 fda17418b3dba3bb2b60dbdc58f74eb4c1dd34fc..bd896a118a9304bd79d7fd21085ea59be96cab7a 100644
--- a/examples/dart/hello_world/hello/main.dart
+++ b/examples/dart/hello_world/hello/main.dart
@@ -23,7 +23,7 @@ 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.listen(onClosed: close);
+    c.onError = close;
   }
 }