Preserve the stack trace for rethrown exceptions in Future.transformException.

sdk/lib/core/future_impl.dart

Index: sdk/lib/core/future_impl.dart
diff --git a/sdk/lib/core/future_impl.dart b/sdk/lib/core/future_impl.dart
index 2c22e46123115e6b3dd852f5ed4fc5333f1975c7..95ee9831f9e6119f10c2090feb9a77d0c45d58e0 100644
--- a/sdk/lib/core/future_impl.dart
+++ b/sdk/lib/core/future_impl.dart
@@ -235,7 +235,11 @@ class _FutureImpl<T> implements Future<T> {
           completer.complete(result);
         }
       } catch (innerException, stackTrace) {
-        completer.completeException(innerException, stackTrace);
+        if (identical(ex, innerException)) {
+          completer.completeException(innerException, this.stackTrace);
+        } else {
+          completer.completeException(innerException, stackTrace);
+        }
       }
       return false;
     });