Unify dart:async and pub/io.dart TimeoutException classes.

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;
 
 /**
  * An object representing a delayed computation.
  *
  * A [Future] is used to obtain a not yet
@@ skipping 411 matching lines @@
    * If `onTimeout` is omitted, a timeout will cause the returned future to
    * complete with a [TimeoutException].
    */
   Future timeout(Duration timeLimit, {void onTimeout()});
 }
 
 /**
  * Thrown when a scheduled timeout happens while waiting for an async result.
  */
 class TimeoutException implements Exception {
-  /** The duration that was exceeded without a result. */
+  /** Description of the cause of the timeout. */
+  final String message;
+  /** The duration that was exceeded. */
   final Duration duration;
 
-  TimeoutException(this.duration);
+  TimeoutException(this.message, [this.duration]);
 
-  String toString() => "Timeout after $duration";
+  String toString() {
+    if (message != null) {
+      if (duration != null) return "TimeoutException after $duration: $message";
+      return "TimeoutException: $message";
+    }
+    if (duration != null) return "TimeoutException after $duration";
+    return "TimeoutException";
+  }
 }
 
 /**
  * A way to produce Future objects and to complete them later
  * with a value or error.
  *
  * If you already have a Future, you probably don't need a Completer.
  * Instead, you can usually use [Future.then], which returns a Future:
  *
  *     Future doStuff(){
@@ skipping 113 matching lines @@
    *     theFuture.catchError(thisCompleter.completeError);
    *
    */
   void completeError(Object error, [StackTrace stackTrace]);
 
   /**
    * Whether the future has been completed.
    */
   bool get isCompleted;
 }