Add fromUri constructor to File, Directory and Link

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);
 
   /**
+   * Create a Lint object from a URI.
+   *
+   * If [uri] cannot reference a link this throws [UnsupportedError].
+   */
+  factory Link.fromUri(Uri uri) => new Link(uri.toFilePath());
+
+  /**
    * 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.
    *
    * If [recursive] is false, the default, the link is created
    * only if all directories in its path exist.
    * If [recursive] is true, all non-existing path
    * components are created. The directories in the path of [target] are
    * not affected, unless they are also in [path].
    *
@@ skipping 108 matching lines @@
 class _Link extends FileSystemEntity implements Link {
   final String path;
 
   _Link(String this.path) {
     if (path is! String) {
       throw new ArgumentError('${Error.safeToString(path)} '
                               'is not a String');
     }
   }
 
-
   String toString() => "Link: '$path'";
 
   Future<bool> exists() => FileSystemEntity.isLink(path);
 
   bool existsSync() => FileSystemEntity.isLinkSync(path);
 
   Link get absolute => new Link(_absolutePath);
 
   Future<FileStat> stat() => FileStat.stat(path);
 
@@ skipping 128 matching lines @@
         return new ArgumentError();
       case _OSERROR_RESPONSE:
         var err = new OSError(response[_OSERROR_RESPONSE_MESSAGE],
                               response[_OSERROR_RESPONSE_ERROR_CODE]);
         return new FileSystemException(message, path, err);
       default:
         return new Exception("Unknown error");
     }
   }
 }