Dart: Adds a content handler and a test.

mojo/dart/embedder/test/dart_to_cpp_tests.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:mojo_bindings' as bindings;
 import 'dart:mojo_core' as core;
 import 'dart:typed_data';
 
 import 'package:mojo/dart/embedder/test/dart_to_cpp.mojom.dart';
@@ skipping 79 matching lines @@
     }
     callTestFinished();
     _completer.complete(null);
   }
 
   Future<bool> get future => _completer.future;
 }
 
 
 main(List args) {
-  assert(args.length == 1);
+  assert(args.length == 2);
   int mojoHandle = args[0];
   var rawHandle = new core.MojoHandle(mojoHandle);
   var endpoint = new core.MojoMessagePipeEndpoint(rawHandle);
   var dartSide = new DartSide(endpoint);
   dartSide.listen();
   dartSide.callStartTest();
   dartSide.future.then((_) {
     print('Success');
   });
 }