Use relative URIs for core library part-of statements.

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;
+part of "io.dart";
 
 /**
  * The modes in which a File can be opened.
  */
 class FileMode {
   /// The mode for opening a file only for reading.
   static const READ = const FileMode._internal(0);
 
   /// Mode for opening a file for reading and writing. The file is
   /// overwritten if it already exists. The file is created if it does not
@@ skipping 933 matching lines @@
       sb.write(": $osError");
       if (path != null) {
         sb.write(", path = '$path'");
       }
     } else if (path != null) {
       sb.write(": $path");
     }
     return sb.toString();
   }
 }