Forward cancel subscription in stream pipes.

tests/lib/async/wait_for_cancel_test.dart

 // 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 makeStream(int n) async* {
-  for (int i = 0; i < n; i++) yield i;
-}
+main() {
+  asyncStart();
 
-main() {
-  f(Stream s) async {
-    var r = 0;
-    await for(var v in s.take(5)) r += v;
-    return r;
-  }
+  bool waitedForCancel = false;
 
-  asyncStart();
-  f(makeStream(10)).then((v) {
-    Expect.equals(10, v);
+  var controller = new StreamController(
+      onCancel: () => new Future(() => waitedForCancel = true));
+  var sub = controller.stream.take(1).listen((x) {
+    Expect.fail("onData should not be called");
+  });
+  var cancelFuture = sub.cancel();
+  Expect.isNotNull(cancelFuture);
+  cancelFuture.then((_) {
+    Expect.isTrue(waitedForCancel);
     asyncEnd();
   });
 }
-