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

utils/pub/sdk_source.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 sdk_source;
 
 import 'dart:async';
+
+import '../../pkg/path/lib/path.dart' as path;
+
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
 import 'sdk.dart' as sdk;
 import 'source.dart';
 import 'utils.dart';
 import 'version.dart';
 
 /// A package source that uses libraries from the Dart SDK.
 class SdkSource extends Source {
@@ skipping 20 matching lines @@
       var path = _getPackagePath(id);
       if (path == null) return false;
 
       return createPackageSymlink(id.name, path, destPath).then((_) => true);
     });
   }
 
   /// Gets the path in the SDK's "pkg" directory to the directory containing
   /// package [id]. Returns `null` if the package could not be found.
   String _getPackagePath(PackageId id) {
-    var pkgPath = join(sdk.rootDirectory, "pkg", id.description);
+    var pkgPath = path.join(sdk.rootDirectory, "pkg", id.description);
     return dirExists(pkgPath) ? pkgPath : null;
   }
 }