Remove Signal class and use Future/.whenComplete instead.

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index c787f79c1ea3d10fed0bd29304505fa3f397ed67..23fb06af87f0a1f4a98face0d19eedd71a35f300 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -144,8 +144,8 @@ abstract class Completer<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