Get rid of join() and encapsulate File and Directory in io.dart.

utils/pub/system_cache.dart

 // Copyright (c) 2012, 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 system_cache;
 
 import 'dart:io';
 import 'dart:async';
 
+import '../../pkg/path/lib/path.dart' as path;
+
 import 'git_source.dart';
 import 'hosted_source.dart';
 import 'io.dart';
 import 'io.dart' as io show createTempDir;
 import 'log.dart' as log;
 import 'package.dart';
 import 'path_source.dart';
 import 'pubspec.dart';
 import 'sdk_source.dart';
 import 'source.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
 
 /// The system-wide cache of installed packages.
 ///
 /// This cache contains all packages that are downloaded from the internet.
 /// Packages that are available locally (e.g. from the SDK) don't use this
 /// cache.
 class SystemCache {
   /// The root directory where this package cache is located.
   final String rootDir;
 
-  String get tempDir => join(rootDir, '_temp');
+  String get tempDir => path.join(rootDir, '_temp');
 
   /// Packages which are currently being asynchronously installed to the cache.
   final Map<PackageId, Future<Package>> _pendingInstalls;
 
   /// The sources from which to install packages.
   final SourceRegistry sources;
 
   /// Creates a new package cache which is backed by the given directory on the
   /// user's file system.
   SystemCache(this.rootDir)
@@ skipping 54 matching lines @@
     return future;
   }
 
   /// Create a new temporary directory within the system cache. The system
   /// cache maintains its own temporary directory that it uses to stage
   /// packages into while installing. It uses this instead of the OS's system
   /// temp directory to ensure that it's on the same volume as the pub system
   /// cache so that it can move the directory from it.
   String createTempDir() {
     var temp = ensureDir(tempDir);
-    return io.createTempDir(join(temp, 'dir'));
+    return io.createTempDir(path.join(temp, 'dir'));
   }
 
   /// Delete's the system cache's internal temp directory.
   Future deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     return defer(() {
       if (!dirExists(tempDir)) return;
       return deleteDir(tempDir);
     });
   }
 }