Dart: Use a RawReceivePort to receive events for Mojo handles.

services/dart/test/echo/lib/main.dart

Index: services/dart/test/echo/lib/main.dart
diff --git a/services/dart/test/echo/lib/main.dart b/services/dart/test/echo/lib/main.dart
index 4b5cce07ded9e8a708388d76d7b6654210c717f9..92f2771f35f0a2b675ffffb842a8b0d96ec95ae4 100644
--- a/services/dart/test/echo/lib/main.dart
+++ b/services/dart/test/echo/lib/main.dart
@@ -49,7 +49,7 @@ main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   String url = args[1];
   new EchoApplication.fromHandle(appHandle)
-    ..onError = (() {
+    ..onError = ((_) {
       MojoHandle.reportLeakedHandles();
     });
 }