Dart: Avoid MojoResult and MojoHandleSignals constructors.

mojo/dart/test/validation_test.dart

Index: mojo/dart/test/validation_test.dart
diff --git a/mojo/dart/test/validation_test.dart b/mojo/dart/test/validation_test.dart
index 0d344e2e25d6de68d5e3053342a152e4b5d21741..296377493082912d022ada56b63e34a7d79ae363 100644
--- a/mojo/dart/test/validation_test.dart
+++ b/mojo/dart/test/validation_test.dart
@@ -96,8 +96,8 @@ Future runTest(
   });
 
   var length = (test.data == null) ? 0 : test.data.lengthInBytes;
-  var r = pipe.endpoints[1].write(test.data, length, handles);
-  assert(r.isOk);
+  int r = pipe.endpoints[1].write(test.data, length, handles);
+  assert(r == MojoResult.kOk);
 
   return completer.future.then((_) {
     assert(expected == "PASS");