If transformException() returns a future, act like chain().

lib/core/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.
 
 /**
  * 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 114 matching lines @@
 
   /**
    * 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].
+   * with the value returned by [transformation]. If the returned value is
+   * itself a future, then the future returned by [transformException] will
+   * complete with the value that that future completes to.
    */
   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:
@@ skipping 83 matching lines @@
       future.handleException((exception) {
         if (!result.isComplete) {
           completer.completeException(exception, future.stackTrace);
         }
         return true;
       });
     }
     return result;
   }
 }