Add test for whether a completer or stream controller has been closed.

tests/lib/async/stream_controller_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test the basic StreamController and StreamController.singleSubscription.
 library stream_controller_test;
 
 import 'dart:async';
 import 'event_helper.dart';
 
@@ skipping 374 matching lines @@
   sentEvents = new Events()
       ..add(5)..add(6)..add(4)..add(6)..add(8)..add(3)..add(4)..add(1)..close();
   expectedEvents = new Events()
       ..add(5)..add(4)..add(3)..add(1)..close();
   // Use 'distinct' as a filter with access to the previously emitted event.
   actualEvents = new Events.capture(c.stream.distinct((a, b) => a < b));
   sentEvents.replay(c);
   Expect.listEquals(expectedEvents.events, actualEvents.events);
 }
 
+testClosed() {
+  for (StreamController c in [new StreamController(),
+                              new StreamController.broadcast()]) {
+    Expect.isFalse(c.isClosed);
+    c.add(42);
+    Expect.isFalse(c.isClosed);
+    c.signalError("bad");
+    Expect.isFalse(c.isClosed);
+    c.close();
+    Expect.isTrue(c.isClosed);
+  }
+}
+
 main() {
   testMultiController();
   testSingleController();
   testExtraMethods();
+  testClosed();
 }