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

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;
 
 /** The onValue and onError handlers return either a value or a future */
 typedef dynamic _FutureOnValue<T>(T value);
 /** Test used by [Future.catchError] to handle skip some errors. */
 typedef bool _FutureErrorTest(var error);
@@ skipping 526 matching lines @@
       source = listener;
     }
   }
 
   Future timeout(Duration timeLimit, {void onTimeout()}) {
     if (_isComplete) return new _Future.immediate(this);
     _Future result = new _Future();
     Timer timer;
     if (onTimeout == null) {
       timer = new Timer(timeLimit, () {
-        result._completeError(new TimeoutException(timeLimit));
+        result._completeError(new TimeoutException("Future not completed",
+                                                   timeLimit));
       });
     } else {
       Zone zone = Zone.current;
       onTimeout = zone.registerCallback(onTimeout);
       timer = new Timer(timeLimit, () {
         try {
           result._complete(zone.run(onTimeout));
         } catch (e, s) {
           result._completeError(e, s);
         }
       });
     }
     this.then((T v) {
       if (timer.isActive) {
         timer.cancel();
         result._complete(v);
       }
     }, onError: (e, s) {
       if (timer.isActive) {
         timer.cancel();
         result._completeError(e, s);
       }
     });
     return result;
   }
 }