Consider a thrown AsyncError from a future/stream handler a rethrow.

sdk/lib/async/stream_controller.dart

Index: sdk/lib/async/stream_controller.dart
diff --git a/sdk/lib/async/stream_controller.dart b/sdk/lib/async/stream_controller.dart
index 61d0ad8ea999f620c959d0a3598a498ea3643dd8..eb36698c43e68ce34936f9abd41efa8798e4ac1e 100644
--- a/sdk/lib/async/stream_controller.dart
+++ b/sdk/lib/async/stream_controller.dart
@@ -85,10 +85,24 @@ class StreamController<T> extends Stream<T> implements StreamSink<T> {
   /**
    * Send or enqueue an error event.
    *
+   * If the [error] is an [AsyncError], it is used directly as the
+   * error object reported to listeners, and the [stackTrace] is ignored.

[floitsch, 2013/01/10 16:38:10]

I would mention the special case after the normal case.
Maybe:

If the [exception] is not an [AsyncError], combines the [exception] and the
optional [stackTrace] into an [AsyncError] and sends the result to [this]
stream's listeners.

If the [exception] is already an [AsyncError], then the [stackTrace] is ignored
and the [exception] is sent directly to the listeners.

[Lasse Reichstein Nielsen, 2013/01/11 07:02:05]

Done.

+   *
+   * Otherwise the [exception] and an optional [stackTrace] is combined and sent
+   * this stream's listeners.
+   *
    * If a subscription has requested to be unsubscribed on errors,
    * it will be unsubscribed after receiving this event.
    */
-  void signalError(AsyncError error) { _stream._signalError(error); }
+  void signalError(Object error, [Object stackTrace]) {
+    AsyncError asyncError;
+    if (error is AsyncError) {
+      asyncError = error;
+    } else {
+      asyncError = new AsyncError(error, stackTrace);
+    }
+    _stream._signalError(asyncError);
+  }
 
   /**
    * Send or enqueue a "done" message.