Avoid crashing when a closed StreamIterator is cancelled

tests/lib/async/stream_iterator_double_cancel_test.dart

Index: tests/lib/async/stream_iterator_double_cancel_test.dart
diff --git a/tests/lib/async/stream_iterator_double_cancel_test.dart b/tests/lib/async/stream_iterator_double_cancel_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4b88802d811c588eb9fbc2ac9a548bca1bac5825
--- /dev/null
+++ b/tests/lib/async/stream_iterator_double_cancel_test.dart
@@ -0,0 +1,38 @@
+// Copyright (c) 2015, 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.
+
+import "dart:async";
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+Stream timedStream(int n) {
+  int i = 0;
+  StreamController controller;
+  tick() {
+    if (i >= n) {
+      controller.close();
+      return;
+    }
+    controller.add(i);
+    i++;
+    new Future.delayed(new Duration(milliseconds: 10), tick);
+  }
+  controller = new StreamController(onListen: tick);
+  return controller.stream;
+}
+
+void main() {
+  asyncStart();
+  StreamIterator iterator = new StreamIterator(timedStream(10));
+  helper(more) {
+    if (!more) {
+      // Canceling the already closed iterator should not lead to a crash.
+      iterator.cancel();
+      asyncEnd();
+    } else {
+      iterator.moveNext().then(helper);
+    }
+  }
+  iterator.moveNext().then(helper);
+}