Add Server.isClosed and Client.isClosed.

test/client/stream_test.dart

Index: test/client/stream_test.dart
diff --git a/test/client/stream_test.dart b/test/client/stream_test.dart
index 20b21bde030f1e550bc269172ff6c05afc22f9ad..ba5db5bc32c4fbb5544348b8386b2761f24bab64 100644
--- a/test/client/stream_test.dart
+++ b/test/client/stream_test.dart
@@ -79,7 +79,10 @@ void main() {
         responseController.stream, requestController.sink);
 
     expect(client.listen(), completes);
+
+    expect(client.isClosed, isFalse);
     expect(client.close(), completes);
+    expect(client.isClosed, isTrue);
 
     expect(() => responseController.stream.listen((_) {}), throwsStateError);
     expect(requestController.isClosed, isTrue);