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

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index b28b2c5c53ef2b1a6ffa46559084e6a812cf553b..64118842b700d2549d6bf055434ea7d8fb2efed4 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -363,4 +363,9 @@ abstract class Completer<T> {
    * [AsyncError] and sent to this future's listeners.
    */
   void completeError(Object exception, [Object stackTrace]);
+
+  /**
+   * Whether the future has been completed.
+   */
+  bool get isCompleted;
 }