dart:io | Add Link.update, and change Link.updateSync.

sdk/lib/io/link.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;
 
 /**
  * [Link] objects are references to filesystem links.
  *
  */
@@ skipping 37 matching lines @@
    * On other platforms, the posix symlink() call is used to make a symbolic
    * link containing the string [target].  If [target] is a relative path,
    * it will be interpreted relative to the directory containing the link.
    */
   void createSync(String target);
 
   /**
    * Synchronously updates the link. Calling [updateSync] on a non-existing link
    * will throw an exception.
    *
-   * If [linkRelative] is true, the target argument should be a relative path,
-   * and the link will interpret the target as a path relative to the link's
-   * directory.
-   *
    * On the Windows platform, this will only work with directories, and the
    * target directory must exist.
    */
-  void updateSync(String target, {bool linkRelative: false });
+  void updateSync(String target);
+
+  /**
+   * Updates the link. Returns a [:Future<Link>:] that completes with the
+   * link when it has been updated.  Calling [update] on a non-existing link
+   * will complete its returned future with an exception.
+   *
+   * On the Windows platform, this will only work with directories, and the
+   * target directory must exist.
+   */
+  Future<Link> update(String target);
 
   /**
    * Deletes the link. Returns a [:Future<Link>:] that completes with
    * the link when it has been deleted. This does not delete, or otherwise
    * affect, the target of the link. It also works on broken links, but if
    * the link does not exist or is not actually a link, it completes the
    * future with a LinkException.
    */
   Future<Link> delete();
 
@@ skipping 115 matching lines @@
     if (target.length > 3 && target[1] == ':' && target[2] == '\\') {
       target = '\\??\\$target';
     } else {
       throw new LinkException(
           'Target $target of Link.create on Windows cannot be converted' +
           ' to start with a drive letter.  Unexpected error.');
     }
     return target;
   }
 
-  void updateSync(String target, {bool linkRelative: false }) {
-    // TODO(whesse): Replace with atomic update, where supported by platform.
+  void updateSync(String target) {
+    // TODO(12414): Replace with atomic update, where supported by platform.
+    // Atomically changing a link can be done by creating the new link, with
+    // a different name, and using the rename() posix call to move it to
+    // the old name atomically.
     deleteSync();
     createSync(target);
   }
 
+  Future<Link> update(String target) {
+    // TODO(12414): Replace with atomic update, where supported by platform.
+    // Atomically changing a link can be done by creating the new link, with
+    // a different name, and using the rename() posix call to move it to
+    // the old name atomically.
+    return delete().then((_) => create(target));
+  }
+
   Future<Link> delete() {
     _ensureFileService();
     List request = new List(2);
     request[0] = _DELETE_LINK_REQUEST;
     request[1] = path;
     return _fileService.call(request).then((response) {
       if (_isErrorResponse(response)) {
         throw _exceptionFromResponse(response, "Cannot delete link", path);
       }
       return this;
@@ skipping 98 matching lines @@
       if (path != null) {
         sb.write(", path = $path");
       }
     }
     return sb.toString();
   }
   final String message;
   final String path;
   final OSError osError;
 }