Don't consider controller.addError to be a zone-crossing.

tests/lib/async/catch_errors24_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'catch_errors.dart';
 
 main() {
   asyncStart();
   Completer done = new Completer();
 
   var events = [];
   StreamController controller;
   Stream stream;
   // Test that the first listen on a `asBroadcastStream` determines the
   // zone the subscription lives in. In this case the outer listen happens first
   // and the error reaches `handleError`.
   catchErrors(() {
     catchErrors(() {
       controller = new StreamController();
+
+      // Assign to the "global" `stream`.
       stream = controller.stream
         .map((x) {
           events.add("map $x");
           return x + 100;
         })
         .transform(new StreamTransformer.fromHandlers(
             handleError: (e, st, sink) { sink.add("error $e"); }))
         .asBroadcastStream();
+
+      // Listen to the `stream` in the inner zone (but wait in a microtask).
       scheduleMicrotask(() {
         stream.listen((x) {
           events.add("stream $x");
           if (x == "error 2") done.complete(true);
         });
       });
     }).listen((x) { events.add(x); })
       .asFuture().then((_) { Expect.fail("Unexpected callback"); });
+
+    // Listen to `stream` from the outer zone.
     stream.listen((x) { events.add("stream2 $x"); });
+
+    // Feed the controller, but wait in a microtask.
     scheduleMicrotask(() {
       controller.add(1);
-      // Errors are not allowed to traverse boundaries, but in this case the
-      // first listener of the broadcast stream is in the same error-zone. So
-      // this should work.
       controller.addError(2);
       controller.close();
     });
   }).listen((x) { events.add("outer: $x"); },
             onDone: () { Expect.fail("Unexpected callback"); });
 
   done.future.whenComplete(() {
     // Give handlers time to complete.
     Timer.run(() {
       Expect.listEquals(["map 1",
-                          "stream2 101",
-                          "stream 101",
-                          "stream2 error 2",
-                          "stream error 2",
+                         "stream2 101",
+                         "stream 101",
+                         "stream2 error 2",
+                         "stream error 2",
                         ],
                         events);
       asyncEnd();
     });
   });
 }