Change the zone that late-added stream subscription callbacks are registered in.

sdk/lib/async/stream_impl.dart

Index: sdk/lib/async/stream_impl.dart
diff --git a/sdk/lib/async/stream_impl.dart b/sdk/lib/async/stream_impl.dart
index fd104b0edc6ef9a30fa5f41c709d440268899391..3d7a2b377535a320b1cfa4581d2c76bb32006b38 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -144,17 +144,17 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
 
   void onData(void handleData(T event)) {
     if (handleData == null) handleData = _nullDataHandler;
-    _onData = Zone.current.registerUnaryCallback(handleData);
+    _onData = _zone.registerUnaryCallback(handleData);
   }
 
   void onError(Function handleError) {
     if (handleError == null) handleError = _nullErrorHandler;
-    _onError = _registerErrorHandler(handleError, Zone.current);
+    _onError = _registerErrorHandler(handleError, _zone);
   }
 
   void onDone(void handleDone()) {
     if (handleDone == null) handleDone = _nullDoneHandler;
-    _onDone = Zone.current.registerCallback(handleDone);
+    _onDone = _zone.registerCallback(handleDone);
   }
 
   void pause([Future resumeSignal]) {