Add error-intercept for Completer.completeError and StreamController.addError.

sdk/lib/async/broadcast_stream_controller.dart

Index: sdk/lib/async/broadcast_stream_controller.dart
diff --git a/sdk/lib/async/broadcast_stream_controller.dart b/sdk/lib/async/broadcast_stream_controller.dart
index a9879e0e20d28b9a9f1c764ee4aef9b0bbbe6da7..cda7c0f960c7ce16effca089b555da257c8c7bc8 100644
--- a/sdk/lib/async/broadcast_stream_controller.dart
+++ b/sdk/lib/async/broadcast_stream_controller.dart
@@ -239,6 +239,11 @@ abstract class _BroadcastStreamController<T>
 
   void addError(Object error, [StackTrace stackTrace]) {
     if (!_mayAddEvent) throw _addEventError();
+    AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
+    if (replacement != null) {
+      error = replacement.error;
+      stackTrace = replacement.stackTrace;
+    }
     _sendError(error, stackTrace);
   }
 
@@ -269,7 +274,6 @@ abstract class _BroadcastStreamController<T>
   }
 
   void _addError(Object error, StackTrace stackTrace) {
-    assert(_isAddingStream);
     _sendError(error, stackTrace);
   }
 
@@ -459,7 +463,8 @@ class _AsBroadcastStreamController<T>
       _addPendingEvent(new _DelayedError(error, stackTrace));
       return;
     }
-    super.addError(error, stackTrace);
+    if (!_mayAddEvent) throw _addEventError();
+    _sendError(error, stackTrace);
     while (_hasPending) {
       _pending.handleNext(this);
     }