Ensure multi channels aren't double-closed.

lib/src/util/multi_channel.dart

 // Copyright (c) 2015, 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 test.multi_channel;
 
 import 'dart:async';
 
 import 'stream_channel.dart';
 
@@ skipping 75 matching lines @@
   Stream _innerStream;
 
   /// The inner sink over which all communication is sent.
   ///
   /// This will be `null` if the underlying communication channel is closed.
   StreamSink _innerSink;
 
   /// The subscription to [_innerStream].
   StreamSubscription _innerStreamSubscription;
 
+  /// Whether this channel has been closed.
+  bool _closed = false;
+
   Stream get stream => _streamController.stream;
   final _streamController = new StreamController(sync: true);
 
   StreamSink get sink => _sinkController.sink;
   final _sinkController = new StreamController(sync: true);
 
   /// A map from virtual channel ids to [StreamController]s that should be used
   /// to write messages received from those channels.
   final _streamControllers = new Map<int, StreamController>();
 
@@ skipping 85 matching lines @@
         (message) => _innerSink.add([outputId, message]),
         onDone: () => _closeChannel(inputId, outputId));
 
     return new VirtualChannel._(
         this, outputId, streamController.stream, sinkController.sink);
   }
 
   /// Closes the virtual channel for which incoming messages have [inputId] and
   /// outgoing messages have [outputId].
   void _closeChannel(int inputId, int outputId) {
+    if (_closed) return;
+    _closed = true;
+
     // A message without data indicates that the virtual channel has been
     // closed.
     _streamControllers.remove(inputId).close();
     _sinkControllers.remove(inputId).close();
 
     if (_innerSink == null) return;
     _innerSink.add([outputId]);
     if (_streamControllers.isEmpty) _closeInnerChannel();
   }
 
@@ skipping 25 matching lines @@
   /// except that it will be JSON-serializable.
   final id;
 
   final Stream stream;
   final StreamSink sink;
 
   VirtualChannel._(this._parent, this.id, this.stream, this.sink);
 
   VirtualChannel virtualChannel([id]) => _parent.virtualChannel(id);
 }