Make dartdoc compile.

dart/frog/leg/lib/io.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.
 
 // This is a copy of the VM's dart:io library. This API is not usable
 // when running inside a web browser. Nevertheless, Leg provides a
 // mock version of the dart:io library so that it can statically
 // analyze programs that use dart:io.
 
 // TODO(ahe): Separate API from implementation details.
@@ skipping 100 matching lines @@
   }
 
   static _EventHandler get _eventHandler() {
     throw new UnsupportedOperationException('_EventHandler._eventhandler');
   }
 
   static void set _eventHandler(_EventHandler e) {
     throw new UnsupportedOperationException('_EventHandler._eventhandler = $e');
   }
 }
+
+final InputStream stdin = null;
+
+final OutputStream stdout = null;
+
+final OutputStream stderr = null;