Make common file mode and encoding constants available on the top level in dart:io

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 {
   static const READ = const FileMode._internal(0);
   static const WRITE = const FileMode._internal(1);
   static const APPEND = const FileMode._internal(2);
   const FileMode._internal(int this._mode);
   final int _mode;
 }
 
+const READ = FileMode.READ;
+const WRITE = FileMode.WRITE;
+const APPEND = FileMode.APPEND;
 
 /**
  * [File] objects are references to files.
  *
  * If [path] is a symbolic link, rather than a file, then
  * the methods of [File] operate on the ultimate target of the
  * link, except for File.delete and File.deleteSync, which operate on
  * the link.
  *
  * To operate on the underlying file data there are two options:
@@ skipping 494 matching lines @@
         sb.write(" ($osError)");
       }
     } else if (osError != null) {
       sb.write(": osError");
     }
     return sb.toString();
   }
   final String message;
   final OSError osError;
 }