Support some file operations in dart:io under mojo

services/dart/dart_apptests/lib/main.dart

 // Copyright 2015 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 'package:mojo_apptest/apptest.dart';
 
 import 'src/connect_to_loader_apptests.dart' as connect_to_loader_apptests;
 import 'src/echo_apptests.dart' as echo;
+import 'src/file_apptests.dart' as file;
 import 'src/pingpong_apptests.dart' as pingpong;
 import 'src/io_http_apptests.dart' as io_http;
 import 'src/io_internet_address_apptests.dart' as io_internet_address;
 import 'src/versioning_apptests.dart' as versioning;
 
 main(List args) {
   final tests = [
     connect_to_loader_apptests.connectToLoaderApptests,
     echo.echoApptests,
+    file.tests,
     io_internet_address.tests,
     io_http.tests,
     pingpong.pingpongApptests,
     versioning.tests
   ];
   runAppTests(args[0], tests);
 }