Support transforming the exception in Future.transform().

lib/core/future.dart

Index: lib/core/future.dart
diff --git a/lib/core/future.dart b/lib/core/future.dart
index 6b2fa19e3e82b3c5abb094af4f21570117fab0a5..e4ab4cd107b68c48ff542735014a830bd91cdd1f 100644
--- a/lib/core/future.dart
+++ b/lib/core/future.dart
@@ -123,6 +123,23 @@ interface Future<T> default FutureImpl<T> {
     * 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 exception which will then be sent to the
+   * returned future.
+   *
+   * 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. If it returns `true`, then the exception is
+   * considered handled and is not propagated to the returned future. If it
+   * returns anything else, the original exception will be propagated.

[Siggi Cherem (dart-lang), 2012/09/04 20:17:52]

thinking about the control flow of this and how similar it is to try-catch -
maybe the return value should be propagated as the result of the future?

[Bob Nystrom, 2012/09/04 21:14:43]

Works for me. Done.

+   */
+  Future transformException(bool transformation(Object exception));

[nweiz, 2012/09/04 20:26:14]

It seems like this should just be the default behavior of handleException.

[Bob Nystrom, 2012/09/04 21:14:43]

I'm trying to keep this change as minimally invasive as possible, so I don't
want to change the behavior of existing methods. How about I ignore this for now
and after M1 we look at a revamp?

[nweiz, 2012/09/04 21:35:35]

Sure, seems fine.

 }