Remove Signal class and use Future/.whenComplete instead.

sdk/lib/async/future.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;
 
 /**
  * A [Future] is used to obtain a value sometime in the future.  Receivers of a
  * [Future] can obtain the value by passing a callback to [then]. For example:
  *
@@ skipping 126 matching lines @@
  *     completer.completeException(exception);
  *
  */
 abstract class Completer<T> {
 
   factory Completer() => new _CompleterImpl<T>();
 
   /** The future that will contain the value produced by this completer. */
   Future get future;
 
-  /** Supply a value for [future]. */
-  void complete(T value);
+  /** Supply a value for [future], or supply [:null:] if a value is omitted. */

[floitsch, 2013/01/08 12:24:40]

dartdoc comments should be third person:

/// Completes the [future] with the [value].
///
/// All subscriber of the [future] will be immediately informed of the
completion.

(or something similar).

I'm not sure we need to specify that the default value is `null`.

+  void complete([T value]);
 
   /**
    * Indicate in [future] that an exception occured while trying to produce its
    * value. The argument [exception] should not be [:null:]. A [stackTrace]
    * object can be provided as well to give the user information about where
    * the error occurred. If omitted, it will be [:null:].
    */
   void completeError(Object exception, [Object stackTrace]);
 }
 
@@ skipping 18 matching lines @@
    */
   static Future forEach(Iterable input, Future f(element)) {
     var iterator = input.iterator;
     Future nextElement(_) {
       if (!iterator.moveNext()) return new Future.immediate(null);
       return f(iterator.current).then(nextElement);
     }
     return nextElement(null);
   }
 }