Add FileSystemEntity.identical, to test if two file system objects are the same.

sdk/lib/io/file_system_entity.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.
 
 part of dart.io;
 
 class FileSystemEntityType {
   static const FILE = const FileSystemEntityType._internal(0);
   static const DIRECTORY = const FileSystemEntityType._internal(1);
   static const LINK = const FileSystemEntityType._internal(2);
@@ skipping 17 matching lines @@
  * [FileSystemEntity] objects are returned from directory listing
  * operations. To determine if a FileSystemEntity is a [File] or a
  * [Directory], perform a type check:
  *
  *     if (entity is File) (entity as File).readAsStringSync();
  */
 abstract class FileSystemEntity {
   String get path;
 
   external static int _getType(String path, bool followLinks);
+  external static bool _identical(String path1, String path2);
 
   static int _getTypeSync(String path, bool followLinks) {
     var result = _getType(path, followLinks);
     _throwIfError(result, 'Error getting type of FileSystemEntity');
     return result;
   }
 
+  /**
+   * Do two paths refer to the same object in the file system?
+   * Links are not identical to their targets, and two links
+   * are not identical just because they point to identical targets.
+   * Links in intermediate directories in the paths are followed, though.
+   *
+   * Throws an error if one of the paths points to an object that does not
+   * exist.
+   * The target of a link can be compared by first getting it with Link.target.
+   */
+  static bool identicalSync(String path1, String path2) {
+    var result = _identical(path1, path2);
+    _throwIfError(result, 'Error in FileSystemEntity.identical');
+    return result;
+  }
+
   static FileSystemEntityType typeSync(String path, {bool followLinks: true})
       => FileSystemEntityType._lookup(_getTypeSync(path, followLinks));
 
   static bool isLinkSync(String path) =>
       (_getTypeSync(path, false) == FileSystemEntityType.LINK._type);
 
   static bool isFileSync(String path) =>
       (_getTypeSync(path, true) == FileSystemEntityType.FILE._type);
 
   static bool isDirectorySync(String path) =>
       (_getTypeSync(path, true) == FileSystemEntityType.DIRECTORY._type);
 
   static _throwIfError(Object result, String msg) {
     if (result is OSError) {
       throw new FileIOException(msg, result);
+    } else if (result is ArgumentError) {
+      throw result;
     }
   }
 }