Forward stack traces through stream transformers.

R=lrn@google.com

Review URL: https://codereview.chromium.org//1302363003 .

CHANGELOG.md

@@ -11,6 +11,11 @@
     The `HttpServer` still gracefully receives fragments, but discards them
     before delivering the request.
 
+* `dart:async`
+  * `StreamTransformer`s created with `fromHandlers` with no `handleError`
+    callback now forward stack traces along with errors to the resulting
+    streams.
+
 ## 1.12.0
 
 ### Language changes

sdk/lib/async/stream_transformers.dart

@@ -250,7 +250,7 @@ class _StreamHandlerTransformer<S, T> extends _StreamSinkTransformer<S, T> {
   /** Default error handler forwards all errors. */
   static void _defaultHandleError(error, StackTrace stackTrace,
                                   EventSink sink) {
-    sink.addError(error);
+    sink.addError(error, stackTrace);
   }
 
   /** Default done handler forwards done. */