Support transforming the exception in Future.transform().

lib/core/future.dart

 // Copyright (c) 2011, 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.
 
 /**
  * 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:
  *
  *     Future<int> future = getFutureFromSomewhere();
  *     future.then((value) {
@@ skipping 105 matching lines @@
     * will receive it.
     *
     * If an exception occurs (received by this future, thrown by
     * [transformation], or received by the future returned by [transformation])
     * then the returned future will receive the exception.
     *
     * You must not add exception handlers to [this] future prior to calling
     * chain, and any you add afterwards will not be invoked.
     */
    Future chain(Future transformation(T value));
+
+  /**
+   * A future representing [transformation] applied to this future's exception.
+   * This can be used to "catch" an exception coming from `this` and translate
+   * it to a more appropriate result.
+   *
+   * If this future gets a value, it simply completes to that same value. If an
+   * exception occurs, then [transformation] will be called with the exception
+   * value. If [transformation] itself throws an exception, then the returned
+   * future completes with that exception. Otherwise, the future will complete
+   * with the value returned by [transformation].
+   */
+  Future transformException(transformation(Object exception));
 }
 
-
 /**
  * A [Completer] is used to produce [Future]s and supply their value when it
  * becomes available.
  *
  * A service that provides values to callers, and wants to return [Future]s can
  * use a [Completer] as follows:
  *
  *   Completer completer = new Completer();
  *   // send future object back to client...
  *   return completer.future;
@@ skipping 80 matching lines @@
       future.handleException((exception) {
         if (!result.isComplete) {
           completer.completeException(exception, future.stackTrace);
         }
         return true;
       });
     }
     return result;
   }
 }