Show actualy FD for files in Observatory.

sdk/lib/io/file_impl.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;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 568 matching lines @@
     var r = {
       'id': _servicePath,
       'type': _serviceType(ref),
       'name': '$path',
       'user_name': '$path',
     };
     if (ref) {
       return r;
     }
     r['asyncDispatched'] = _asyncDispatched;
-    r['fd'] = _id;
+    r['fd'] = _getFD(_id);
     return r;
   }
 
   void _maybePerformCleanup() {
     if (closed) {
       _files.remove(_serviceId);
     }
   }
 
+  external static int _getFD(int id);
+
   Future<RandomAccessFile> close() {
     return _dispatch(_FILE_CLOSE, [_id], markClosed: true).then((result) {
       if (result != -1) {
         _id = result;
         _maybePerformCleanup();
         return this;
       } else {
         throw new FileSystemException("Cannot close file", path);
       }
     });
@@ skipping 324 matching lines @@
   void _checkAvailable() {
     if (_asyncDispatched) {
       throw new FileSystemException("An async operation is currently pending",
                                     path);
     }
     if (closed) {
       throw new FileSystemException("File closed", path);
     }
   }
 }