Revert "Use relative URIs for core library part-of statements."

sdk/lib/io/file_system_entity.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 "io.dart";
+part of dart.io;
 
 /**
  * The type of an entity on the file system, such as a file, directory, or link.
  *
  * These constants are used by the [FileSystemEntity] class
  * to indicate the object's type.
  *
  */
 
 class FileSystemEntityType {
@@ skipping 803 matching lines @@
     buffer.write(')');
     return buffer.toString();
   }
 }
 
 class _FileSystemWatcher {
   external static Stream<FileSystemEvent> _watch(
       String path, int events, bool recursive);
   external static bool get isSupported;
 }