Revert "Change some platform libraries to use `part of URI`."

sdk/lib/io/directory.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.
 
-part of "dart:io";
+part of dart.io;
 
 /**
  * A reference to a directory (or _folder_) on the file system.
  *
  * A Directory instance is an object holding a [path] on which operations can
  * be performed. The path to the directory can be [absolute] or relative.
  * You can get the parent directory using the getter [parent],
  * a property inherited from [FileSystemEntity].
  *
  * In addition to being used as an instance to access the file system,
@@ skipping 288 matching lines @@
    * directories, files, and links.
    */
   List<FileSystemEntity> listSync(
       {bool recursive: false, bool followLinks: true});
 
   /**
    * Returns a human readable string for this Directory instance.
    */
   String toString();
 }