Remove Signal class and use Future/.whenComplete instead.

tests/lib/async/stream_controller_test.dart

 // Copyright (c) 2011, 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.
 
 // Test the basic StreamController and StreamController.singleSubscription.
 library stream_controller_test;
 
 import 'dart:async';
 import 'event_helper.dart';
 
@@ skipping 213 matching lines @@
     for (int i = 0; i < v; i++) l.add(i + 1);
     return l;
   }));
   sentEvents.replay(c);
   Expect.listEquals(expectedEvents.events, actualEvents.events);
 
   // pipe is tested asynchronously and therefore not in this file.
   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);
   c.add(3);
   c.add(9);
   c.close();
 
   // Test transform.
   c = new StreamController.singleSubscription();
   sentEvents = new Events()..add("a")..error(42)..add("b")..close();
@@ skipping 107 matching lines @@
   actualEvents = new Events.capture(c.distinct((a, b) => a < b));
   sentEvents.replay(c);
   Expect.listEquals(expectedEvents.events, actualEvents.events);
 }
 
 main() {
   testController();
   testSingleController();
   testExtraMethods();
 }