Standalone: Null out _packagesPort after close.

runtime/bin/builtin.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.
 
 library builtin;
 // NOTE: Do not import 'dart:io' in builtin.
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_internal';
 import 'dart:isolate';
@@ skipping 378 matching lines @@
          "${_pendingPackageLoads.length} packages pending.");
   }
 }
 
 
 RawReceivePort _packagesPort;
 
 void _handlePackagesReply(msg) {
   // Make sure to close the _packagePort before any other action.
   _packagesPort.close();
+  _packagesPort = null;
 
   if (_traceLoading) {
     _log("Got packages reply: $msg");
   }
   if (msg is String) {
     if (_traceLoading) {
       _log("Got failure response on package port: '$msg'");
     }
     throw msg;
   }
@@ skipping 418 matching lines @@
 }
 
 
 // Register callbacks and hooks with the rest of the core libraries.
 _setupHooks() {
   _setupCompleted = true;
   VMLibraryHooks.resourceReadAsBytes = _resourceReadAsBytes;
   VMLibraryHooks.getPackageRoot = _getPackageRoot;
   VMLibraryHooks.getPackageMap = _getPackageMap;
 }