Add Server.isClosed and Client.isClosed.

lib/src/two_way_stream.dart

Index: lib/src/two_way_stream.dart
diff --git a/lib/src/two_way_stream.dart b/lib/src/two_way_stream.dart
index 5914173876f6d21799bfdcc1c34cd45471c71852..f2d5b5eb6929ab8c46ea405bd94a45984fc38e51 100644
--- a/lib/src/two_way_stream.dart
+++ b/lib/src/two_way_stream.dart
@@ -38,6 +38,9 @@ class TwoWayStream {
   Future get done => _doneCompleter.future;
   final _doneCompleter = new Completer();
 
+  /// Whether the stream has been closed.
+  bool get isClosed => _doneCompleter.isCompleted;
+
   /// Creates a two-way stream.
   ///
   /// [input] and [output] should emit and take (respectively) JSON-encoded
@@ -103,12 +106,12 @@ class TwoWayStream {
     _inputSubscription = _input.listen(handleInput,
         onError: (error, stackTrace) {
       if (_doneCompleter.isCompleted) return;
-      _output.close();
       _doneCompleter.completeError(error, stackTrace);
+      _output.close();
     }, onDone: () {
       if (_doneCompleter.isCompleted) return;
-      _output.close();
       _doneCompleter.complete();
+      _output.close();
     }, cancelOnError: true);
 
     return _doneCompleter.future;
@@ -123,6 +126,7 @@ class TwoWayStream {
       throw new StateError("Can't call $_name.close before $_name.listen.");
     }
 
+    _isClosed = true;
     if (!_doneCompleter.isCompleted) _doneCompleter.complete();
 
     var inputFuture = _inputSubscription.cancel();