Add Server.isClosed and Client.isClosed.

test/server/stream_test.dart

Index: test/server/stream_test.dart
diff --git a/test/server/stream_test.dart b/test/server/stream_test.dart
index a7a2d8652701a8a945acc2f49c06528bbac7da94..8fd12396f859643a807f9c83cdd4bfc4ec817766 100644
--- a/test/server/stream_test.dart
+++ b/test/server/stream_test.dart
@@ -81,7 +81,10 @@ void main() {
         requestController.stream, responseController.sink);
 
     expect(server.listen(), completes);
+
+    expect(server.isClosed, isFalse);
     expect(server.close(), completes);
+    expect(server.isClosed, isTrue);
 
     expect(() => requestController.stream.listen((_) {}), throwsStateError);
     expect(responseController.isClosed, isTrue);