dart:io | Change creation of symbolic links with relative targets on Windows.

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.
  *
  */
 abstract class Link implements FileSystemEntity {
   /**
    * Creates a Link object.
    */
   factory Link(String path) => new _Link(path);
 
   /**
    * Creates a symbolic link. Returns a [:Future<Link>:] that completes with
    * the link when it has been created. If the link exists,
    * the future will complete with an error.
    *
    * On the Windows platform, this will only work with directories, and the
    * target directory must exist. The link will be created as a Junction.
    * Only absolute links will be created, and relative paths to the target
-   * will be converted to absolute paths.
+   * will be converted to absolute paths by joining them with the path of the
+   * directory the link is contained in.
    *
    * 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.
    */
   Future<Link> create(String target);
 
   /**
    * Synchronously create the link. Calling [createSync] on an existing link
    * will throw an exception.
@@ skipping 127 matching lines @@
   void createSync(String target) {
     if (Platform.operatingSystem == 'windows') {
       target = _makeWindowsLinkTarget(target);
     }
     var result = _File._createLink(path, target);
     throwIfError(result, "Cannot create link", path);
   }
 
   // Put target into the form "\??\C:\my\target\dir".
   String _makeWindowsLinkTarget(String target) {
-    if (target.startsWith('\\??\\')) {
-      return target;
-    }
-    if (!(target.length > 3 && target[1] == ':' && target[2] == '\\')) {
-      try {
-        target = new File(target).fullPathSync();
-      } on FileException catch (e) {
-        throw new LinkException('Could not locate target', target, e.osError);
-      }
-    }
-    if (target.length > 3 && target[1] == ':' && target[2] == '\\') {
-      target = '\\??\\$target';
+    Uri base = new Uri.file('${Directory.current.path}\\');
+    Uri link = new Uri.file(path);
+    Uri destination = new Uri.file(target);
+    String result = base.resolveUri(link).resolveUri(destination).toFilePath();
+    if (result.length > 3 && result[1] == ':' && result[2] == '\\') {
+      return '\\??\\$result';
     } else {
       throw new LinkException(
-          'Target $target of Link.create on Windows cannot be converted' +
+          'Target $result of Link.create on Windows cannot be converted' +
           ' to start with a drive letter.  Unexpected error.');
     }
     return target;
   }
 
   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.
@@ skipping 126 matching lines @@
       if (path != null) {
         sb.write(", path = $path");
       }
     }
     return sb.toString();
   }
   final String message;
   final String path;
   final OSError osError;
 }