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

sdk/lib/async/future_impl.dart

 // Copyright (c) 2012, 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.
 
 part of dart.async;
 
 deprecatedFutureValue(_FutureImpl future) =>
   future._isComplete ? future._resultOrListeners : null;
 
 
@@ skipping 15 matching lines @@
     _isComplete = true;
     AsyncError asyncError;
     if (error is AsyncError) {
       asyncError = error;
     } else {
       asyncError = new AsyncError(error, stackTrace);
     }
     _FutureImpl future = this.future;
     future._setError(asyncError);
   }
+
+  bool get isCompleted => _isComplete;
 }
 
 /**
  * A listener on a future.
  *
  * When the future completes, the [_sendValue] or [_sendError] method
  * is invoked with the result.
  *
  * Listeners are kept in a linked list.
  */
@@ skipping 478 matching lines @@
   Future catchError(function(AsyncError error), {bool test(var error)}) {
     return _future.catchError(function, test: test);
   }
 
   Future<T> whenComplete(action()) {
     return _future.whenComplete(action);
   }
 
   Stream<T> asStream() => new Stream.fromFuture(_future);
 }