Remove Signal class and use Future/.whenComplete instead.

tests/lib/async/stream_controller_test.dart

Index: tests/lib/async/stream_controller_test.dart
diff --git a/tests/lib/async/stream_controller_test.dart b/tests/lib/async/stream_controller_test.dart
index 1115e4ad9554d6b71d097147665084f3bde90bef..5b63dc9f51baa97161a4822855f406567f54e935 100644
--- a/tests/lib/async/stream_controller_test.dart
+++ b/tests/lib/async/stream_controller_test.dart
@@ -231,7 +231,7 @@ testSingleController() {
   c = new StreamController.singleSubscription();
   var list = <int>[];
   c.pipeInto(new CollectionSink<int>(list))
-   .then(() { Expect.listEquals(<int>[1,2,9,3,9], list); });
+   .whenComplete(() { Expect.listEquals(<int>[1,2,9,3,9], list); });
   c.add(1);
   c.add(2);
   c.add(9);