Pipe the stack traces of errors through Future operations.

corelib/src/future.dart

Index: corelib/src/future.dart
diff --git a/corelib/src/future.dart b/corelib/src/future.dart
index 9c84680d8e03d9f328d01e355f8da2a25a826b1a..6b2fa19e3e82b3c5abb094af4f21570117fab0a5 100644
--- a/corelib/src/future.dart
+++ b/corelib/src/future.dart
@@ -217,7 +217,9 @@ class Futures {
         }
       });
       future.handleException((exception) {
-        if (!result.isComplete) completer.completeException(exception);
+        if (!result.isComplete) {
+          completer.completeException(exception, future.stackTrace);
+        }
         return true;
       });
     }