Add Server.isClosed and Client.isClosed.

lib/src/server.dart

Index: lib/src/server.dart
diff --git a/lib/src/server.dart b/lib/src/server.dart
index 4458e0e5919ee65e5ea3ed2a6ce6f20820a6d714..4188e78cd5c724bc17ef6ca7e9a3e39656adfab5 100644
--- a/lib/src/server.dart
+++ b/lib/src/server.dart
@@ -43,6 +43,9 @@ class Server {
   /// This is the same future that's returned by [listen].
   Future get done => _streams.done;
 
+  /// Whether the connection is closed.
+  bool get isClosed => _streams.isClosed;
+
   /// Creates a [Server] that reads requests from [requests] and writes
   /// responses to [responses].
   ///
@@ -141,7 +144,7 @@ class Server {
         return nonNull.isEmpty ? null : nonNull.toList();
       });
     }).then((response) {
-      if (response != null) _streams.add(response);
+      if (!_streams.isClosed && response != null) _streams.add(response);
     });
   }