Update homebrew updater script to use 64-bit dartium

tools/apps/update_homebrew/bin/update_homebrew.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library update_homebrew;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:googleapis/common/common.dart' show DownloadOptions, Media;
 import 'package:googleapis/storage/v1.dart' as storage;
 import 'package:http/http.dart' as http;
 import 'package:stack_trace/stack_trace.dart';
 
 String repository; // The path to the temporary git checkout of dart-homebrew.
 Map gitEnvironment; // Pass a wrapper script for SSH to git in the environment.
 
 const GITHUB_REPO = 'dart-lang/homebrew-dart';
 
 const CHANNELS = const ['dev', 'stable'];
 
 const FILES = const [x64File, ia32File, dartiumFile, contentShellFile];
 
 const urlBase = 'https://storage.googleapis.com/dart-archive/channels';
 const x64File = 'sdk/dartsdk-macos-x64-release.zip';
 const ia32File = 'sdk/dartsdk-macos-ia32-release.zip';
-const dartiumFile = 'dartium/dartium-macos-ia32-release.zip';
-const contentShellFile = 'dartium/content_shell-macos-ia32-release.zip';
+const dartiumFile = 'dartium/dartium-macos-x64-release.zip';
+const contentShellFile = 'dartium/content_shell-macos-x64-release.zip';
 
 Future<String> getHash256(
     String channel, String revision, String download) async {
   var client = new http.Client();
   try {
     var api = new storage.StorageApi(client);
     var media = await api.objects.get('dart-archive',
         'channels/$channel/release/$revision/$download.sha256sum',
         downloadOptions: DownloadOptions.FullMedia);
 
@@ skipping 219 matching lines @@
 
       await runGit(['push']);
     } finally {
       await tempDir.delete(recursive: true);
     }
   }, onError: (error, chain) {
     print(error);
     print(chain.terse);
   });
 }