Show actualy FD for files in Observatory.

sdk/lib/io/file_impl.dart

Index: sdk/lib/io/file_impl.dart
diff --git a/sdk/lib/io/file_impl.dart b/sdk/lib/io/file_impl.dart
index d5e4cfc2950e1f203fa63a091a43800537f87a78..c43b4cf60e9db3a5cc97ec38e8f6566b76bb02cb 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -586,7 +586,7 @@ class _RandomAccessFile
       return r;
     }
     r['asyncDispatched'] = _asyncDispatched;
-    r['fd'] = _id;
+    r['fd'] = _getFD(_id);
     return r;
   }
 
@@ -596,6 +596,8 @@ class _RandomAccessFile
     }
   }
 
+  external static int _getFD(int id);
+
   Future<RandomAccessFile> close() {
     return _dispatch(_FILE_CLOSE, [_id], markClosed: true).then((result) {
       if (result != -1) {