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 123 matching lines @@
  *
  *     // alternatively, if the service cannot produce the value, it
  *     // can provide an exception:
  *     completer.completeException(exception);
  *
  */
 abstract class Completer<T> {
 
   factory Completer() => new _CompleterImpl<T>();
 
-  /** The future that will contain the value produced by this completer. */
+  /** The future that will contain the result provided to this completer. */
   Future get future;
 
-  /** Supply a value for [future]. */
-  void complete(T value);
+  /**
+   * Completes [future] with the supplied values.
+   *
+   * All listeners on the future will be immediately informed about the value.
+   */
+  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
+   * Complete [future] with an error.
+   *
+   * Completing a future with an error indicates that an exception was thrown
+   * while trying to produce a 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]);
 }
 
 class Futures {
   /**
    * Returns a future which will complete once all the futures in a list are
    * complete. If any of the futures in the list completes with an exception,
    * the resulting future also completes with an exception. (The value of the
@@ skipping 13 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);
   }
 }