Clean up the semantics of io.dart operations w.r.t. symlinks.

utils/pub/path_source.dart

 // Copyright (c) 2013, 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 path_source;
 
 import 'dart:async';
 import 'dart:io';
 
 import '../../pkg/pathos/lib/path.dart' as path;
@@ skipping 90 matching lines @@
   }
 
   /// Ensures that [description] is a valid path description. It must be a map,
   /// with a "path" key containing a path that points to an existing directory.
   /// Throws a [FormatException] if the path is invalid.
   void _validatePath(String name, description) {
     var dir = description["path"];
 
     if (dirExists(dir)) return;
 
-    // Check this after dirExists() so that symlinks to directories don't get
-    // confused as files.
     if (fileExists(dir)) {
       throw new FormatException(
           "Path dependency for package '$name' must refer to a "
           "directory, not a file. Was '$dir'.");
     }
 
     throw new FormatException("Could not find package '$name' at '$dir'.");
   }
 }