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

sdk/lib/core/future_impl.dart

 // Copyright 2012 Google Inc. All Rights Reserved.
 // Dart core library.
 
 class _FutureImpl<T> implements Future<T> {
 
   bool _isComplete = false;
 
   /**
    * Value that was provided to this Future by the Completer
    */
@@ skipping 217 matching lines @@
         final result = transformation(ex);
 
         // If the transformation itself returns a future, then we will
         // complete to what that completes to.
         if (result is Future) {
           _forward(result, completer);
         } else {
           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;
     });
 
     _handleSuccess(completer.complete);
 
     return completer.future;
   }
 
   /**
@@ skipping 27 matching lines @@
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(Object exception, [Object stackTrace]) {
     _futureImpl._setException(exception, stackTrace);
   }
 }