Remove Signal class and use Future/.whenComplete instead.

sdk/lib/async/future_impl.dart

 // Copyright (c) 2012, 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.
 
 // part of dart.async;
 
 deprecatedFutureValue(_FutureImpl future) =>
   future._isComplete ? future._resultOrListeners : null;
 
 
 class _CompleterImpl<T> implements Completer<T> {
   final Future<T> future;
   bool _isComplete = false;
 
   _CompleterImpl() : future = new _FutureImpl<T>();
 
-  void complete(T value) {
+  void complete([T value]) {
     if (_isComplete) throw new StateError("Future already completed");
     _isComplete = true;
     _FutureImpl future = this.future;
     future._setValue(value);
   }
 
   void completeError(Object error, [Object stackTrace = null]) {
     if (_isComplete) throw new StateError("Future already completed");
     _isComplete = true;
     new Timer(0, (_) {
@@ skipping 423 matching lines @@
   Future catchError(function(AsyncError error), {bool test(var error)}) {
     return _future.catchError(function, test: test);
   }
 
   Future whenComplete(void action()) {
     return _future.whenComplete(action);
   }
 
   Stream<T> asStream() => new Stream.fromFuture(this);
 }