Get symlinks working in pub on Windows.

utils/pub/io.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.
 
 /**
  * Helper functionality to make working with IO easier.
  */
 #library('io');
 
 #import('dart:io');
@@ skipping 237 matching lines @@
     }
   });
 }
 
 /**
  * Creates a new symlink that creates an alias from [from] to [to], both of
  * which can be a [String], [File], or [Directory]. Returns a [Future] which
  * completes to the symlink file (i.e. [to]).
  */
 Future<File> createSymlink(from, to) {
-  // TODO(rnystrom): What should this do on Windows?
   from = _getPath(from);
   to = _getPath(to);
 
-  return runProcess('ln', ['-s', from, to]).transform((result) {
+  var command = 'ln';
+  var args = ['-s', from, to];
+
+  if (Platform.operatingSystem == 'windows') {
+    // Call mklink on Windows to create an NTFS junction point. Only works on
+    // Vista or later. (Junction points are available earlier, but the "mklink"
+    // command is not.) I'm using a junction point (/j) here instead of a soft
+    // link (/d) because the latter requires some privilege shenanigans that
+    // I'm not sure how to specify from the command line.
+    command = 'cmd';
+    args = ['/c', 'mklink', '/j', to, from];
+  }
+
+  return runProcess(command, args).transform((result) {
     // TODO(rnystrom): Check exit code and output?
     return new File(to);
   });
 }
 
 /**
  * Given [entry] which may be a [String], [File], or [Directory] relative to
  * the current working directory, returns its full canonicalized path.
  */
 // TODO(rnystrom): Should this be async?
@@ skipping 198 matching lines @@
   return new Directory(entry);
 }
 
 /**
  * Gets a [Uri] for [uri], which can either already be one, or be a [String].
  */
 Uri _getUri(uri) {
   if (uri is Uri) return uri;
   return new Uri.fromString(uri);
 }