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

sdk/lib/async/stream_controller.dart

Index: sdk/lib/async/stream_controller.dart
diff --git a/sdk/lib/async/stream_controller.dart b/sdk/lib/async/stream_controller.dart
index e5b8b457c13bc4416b5b8001c34420939c70b20d..8394c007ae78108820a4307cd8a070efde2a7244 100644
--- a/sdk/lib/async/stream_controller.dart
+++ b/sdk/lib/async/stream_controller.dart
@@ -63,6 +63,14 @@ class StreamController<T> implements StreamSink<T> {
    */
   StreamSink<T> get sink => new StreamSinkView<T>(this);
 
+  /**
+   * Whether the stream is closed for adding more events.
+   *
+   * If true, the "done" event might not have fired yet, but it has been
+   * scheduled, and it is too late to add more events.
+   */
+  bool get isClosed => stream._isClosed;
+
   /** Whether one or more active subscribers have requested a pause. */
   bool get isPaused => stream._isPaused;