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

lib/core/future.dart

Index: lib/core/future.dart
diff --git a/lib/core/future.dart b/lib/core/future.dart
index 8c7207a3bea14eaa47a7271dafb6c5fc44eb866d..99af2415180efcd019af18bce4ad6a7800ef087b 100644
--- a/lib/core/future.dart
+++ b/lib/core/future.dart
@@ -132,7 +132,9 @@ abstract class Future<T> {
    * 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));
 }