Forward cancel subscription in stream pipes.

tests/lib/async/wait_for_cancel_test.dart

Index: tests/lib/async/wait_for_cancel_test.dart
diff --git a/tests/language/async_star_stream_take_test.dart b/tests/lib/async/wait_for_cancel_test.dart
similarity index 50%
copy from tests/language/async_star_stream_take_test.dart
copy to tests/lib/async/wait_for_cancel_test.dart
index 095082b0a36bca0fda34f50b2a6a6bcce58da1d4..7c400849ff9413e45a299459db2945c62b9f2a04 100644
--- a/tests/language/async_star_stream_take_test.dart
+++ b/tests/lib/async/wait_for_cancel_test.dart
@@ -6,21 +6,20 @@ 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() {
-  f(Stream s) async {
-    var r = 0;
-    await for(var v in s.take(5)) r += v;
-    return r;
-  }
-
   asyncStart();
-  f(makeStream(10)).then((v) {
-    Expect.equals(10, v);
+
+  bool waitedForCancel = false;
+
+  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();
   });
 }
-