Make synchronous broadcast StreamController throw if adding event while adding event.

pkg/async/test/stream_zip_zone_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 "dart:async";
 import "package:unittest/unittest.dart";
 
 // Test that stream listener callbacks all happen in the zone where the
 // listen occurred.
 
@@ skipping 29 matching lines @@
           sub.onData(expectAsync((v) {
             expect(v, 37);
             expect(Zone.current, newZone1);
             runZoned(() {
               Zone newZone2 = Zone.current;
               sub.onData(expectAsync((v) {
                 expect(v, 87);
                 expect(Zone.current, newZone1);
               }));
             });
-            controller.add(87);
+            if (controller is SynchronousStreamController) {
+              scheduleMicrotask(() => controller.add(87));
+            } else {
+              controller.add(87);
+            }
           }));
         });
-        controller.add(37);
+        if (controller is SynchronousStreamController) {
+          scheduleMicrotask(() => controller.add(37));
+        } else {
+          controller.add(37);
+        }
       }));
     });
     controller.add(42);
   });
 }