Ensure multi channels aren't double-closed.

lib/src/util/multi_channel.dart

Index: lib/src/util/multi_channel.dart
diff --git a/lib/src/util/multi_channel.dart b/lib/src/util/multi_channel.dart
index f897de9994160afadc57822a3102aed302c3fce2..34358488a80ae9de5b0bcfd64ab067e9e45cb521 100644
--- a/lib/src/util/multi_channel.dart
+++ b/lib/src/util/multi_channel.dart
@@ -93,6 +93,9 @@ class _MultiChannel extends StreamChannelMixin implements MultiChannel {
   /// 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);
 
@@ -198,6 +201,9 @@ class _MultiChannel extends StreamChannelMixin implements MultiChannel {
   /// 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();