Add fromUri constructor to File, Directory and Link

sdk/lib/io/file.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;
 
 /**
  * FileMode describes the modes in which a file can be opened.
  */
 class FileMode {
@@ skipping 23 matching lines @@
  *    this.[openWrite]().
  *  * Open the file for random access operations using [open].
  */
 abstract class File implements FileSystemEntity {
   /**
    * Create a File object.
    */
   factory File(String path) => new _File(path);
 
   /**
+   * Create a File object from a URI.
+   *
+   * If [uri] cannot reference a file this throws [UnsupportedError].
+   */
+  factory File.fromUri(Uri uri) => new File(uri.toFilePath());
+
+  /**
    * Create the file. Returns a [:Future<File>:] that completes with
    * the file when it has been created.
    *
    * If [recursive] is false, the default, the file is created only if
    * all directories in the path exist. If [recursive] is true, all
    * non-existing path components are created.
    *
    * Existing files are left untouched by [create]. Calling [create] on an
    * existing file might fail if there are restrictive permissions on
    * the file.
@@ skipping 471 matching lines @@
       }
     } else if (osError != null) {
       sb.write(": osError");
       if (path != null) {
         sb.write(", path = $path");
       }
     }
     return sb.toString();
   }
 }