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

utils/pub/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 source;
 
 import 'dart:async';
 
 import '../../pkg/path/lib/path.dart' as path;
 
@@ skipping 29 matching lines @@
     assert(_systemCache != null);
     return _systemCache;
   }
 
   /// The system cache variable. Set by [_bind].
   SystemCache _systemCache;
 
   /// The root directory of this source's cache within the system cache.
   ///
   /// This shouldn't be overridden by subclasses.
-  String get systemCacheRoot => join(systemCache.rootDir, name);
+  String get systemCacheRoot => path.join(systemCache.rootDir, name);
 
   /// Records the system cache to which this source belongs.
   ///
   /// This should only be called once for each source, by
   /// [SystemCache.register]. It should not be overridden by base classes.
   void bind(SystemCache systemCache) {
     assert(_systemCache == null);
     this._systemCache = systemCache;
   }
 
@@ skipping 76 matching lines @@
   /// that symlink.
   ///
   /// This tries to determine if the cached package at [packageDir] has been
   /// corrupted. The heuristics are it is corrupted if any of the following are
   /// true:
   ///
   ///   * It has an empty "lib" directory.
   ///   * It has no pubspec.
   Future<bool> _isCachedPackageCorrupted(String packageDir) {
     return defer(() {
-      if (!fileExists(join(packageDir, "pubspec.yaml"))) return true;
+      if (!fileExists(path.join(packageDir, "pubspec.yaml"))) return true;
 
-      var libDir = join(packageDir, "lib");
+      var libDir = path.join(packageDir, "lib");
       if (dirExists(libDir)) {
         return listDir(libDir).then((contents) => contents.length == 0);
       }
 
       // If we got here, it's OK.
       return false;
     });
   }
 
   /// Returns the directory in the system cache that the package identified by
@@ skipping 42 matching lines @@
   /// according to [validateDescription], although it must still be serializable
   /// to JSON and YAML. It must also be equal to [id] according to
   /// [descriptionsEqual].
   ///
   /// By default, this just returns [id].
   Future<PackageId> resolveId(PackageId id) => new Future.immediate(id);
 
   /// Returns the source's name.
   String toString() => name;
 }