Dart: Reorganize files

mojo/dart/unittests/embedder_tests/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:typed_data';
 
 import 'package:_mojo_for_test_only/dart_to_cpp/dart_to_cpp.mojom.dart';
 import 'package:mojo/bindings.dart' as bindings;
 import 'package:mojo/core.dart' as core;
@@ skipping 99 matching lines @@
 main(List args) {
   assert(args.length == 3);
   int mojoHandle = args[0];
   var rawHandle = new core.MojoHandle(mojoHandle);
   var endpoint = new core.MojoMessagePipeEndpoint(rawHandle);
   var dartSide = new DartSideImpl(endpoint);
   dartSide.future.then((_) {
     print('Success');
   });
 }