Dart: Avoid MojoResult and MojoHandleSignals constructors.

mojo/dart/test/handle_finalizer_test.dart

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:isolate';
 import 'dart:typed_data';
 
 import 'package:_testing/expect.dart';
 import 'package:mojo/core.dart';
@@ skipping 18 matching lines @@
   var l = new List.filled(1000000, 7);
   return l;
 }
 
 test() {
   MojoHandle handle = leakMojoHandle();
   triggerGC();
 
   // The handle will be closed by the MojoHandle finalizer during GC, so the
   // attempt to close it again will fail.
-  MojoResult result = handle.close();
-  Expect.isTrue(result.isInvalidArgument);
+  int result = handle.close();
+  Expect.isTrue(result == MojoResult.kInvalidArgument);
 }
 
 main() {
   test();
 }