The execution of an async* should only be scheduled when the stream is

sdk/lib/_internal/compiler/js_lib/js_helper.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.
 
 library _js_helper;
 
 import 'dart:_js_embedded_names' show
     GET_TYPE_FROM_NAME,
     GET_ISOLATE_TAG,
     INTERCEPTED_NAMES,
@@ skipping 3532 matching lines @@
 ///
 /// If the async* function wants to do an await it calls this function with
 /// [object] not and [IterationMarker].
 ///
 /// If [object] is not a [Future], it is wrapped in a `Future.value`.
 /// The [helperCallback] is called on successfull completion of the
 /// future.
 ///
 /// If [helperCallback] or [errorCallback] throws the error is added to the
 /// stream.
-dynamic streamHelper(dynamic object,
+void streamHelper(dynamic object,
                      dynamic /* js function */ helperCallback,
                      AsyncStarStreamController controller,
                      dynamic /* js function */ errorCallback) {
   if (helperCallback == null) {
     // This happens on return from the async* function.
     controller.close();
-    return null;
+    return;
   }
 
   if (object is IterationMarker) {
     if (controller.stopRunning) {
       _wrapJsFunctionForStream(errorCallback, controller)();
-      return null;
+      return;
     }
     if (object.state == IterationMarker.YIELD_SINGLE) {
       controller.add(object.value);
       // If the controller is paused we stop producing more values.
       if (controller.isPaused) {
-        return null;
+        return;
       }
       // TODO(sigurdm): We should not suspend here according to the spec.
       scheduleMicrotask(() {
           _wrapJsFunctionForStream(helperCallback, controller)(null);
       });
       return;
     } else if (object.state == IterationMarker.YIELD_STAR) {
       Stream stream = object.value;
       controller.isAdding = true;
       // Errors of [stream] are passed though to the main stream. (see
       // [AsyncStreamController.addStream].
       // TODO(sigurdm): The spec is not very clear here. Clarify with Gilad.
       controller.addStream(stream).then((_) {
         controller.isAdding = false;
         _wrapJsFunctionForStream(helperCallback, controller)(null);
       });
-      return null;
+      return;
     }
   }
 
   Future future = object is Future ? object : new Future.value(object);
   future.then(_wrapJsFunctionForStream(helperCallback, controller),
               onError: errorCallback == null
                   ? null
                   : _wrapJsFunctionForStream(errorCallback, controller));
+}
+
+Stream streamOfController(AsyncStarStreamController controller) {
   return controller.stream;
 }
 
 /// A wrapper around a [StreamController] that remembers if that controller
 /// got a cancel.
 ///
 /// Also has a subSubscription that when not null will provide events for the
 /// stream, and will be paused and resumed along with this controller.
 class AsyncStarStreamController {
   StreamController controller;
   Stream get stream => controller.stream;
   bool stopRunning = false;
   bool isAdding = false;
   bool get isPaused => controller.isPaused;
   add(event) => controller.add(event);
   addStream(Stream stream) {
     return controller.addStream(stream, cancelOnError: false);
   }
   addError(error, stackTrace) => controller.addError(error, stackTrace);
   close() => controller.close();
 
   AsyncStarStreamController(helperCallback) {
     controller = new StreamController(
+      onListen: () {
+        scheduleMicrotask(() => JS('', '#(null)', helperCallback));
+      },
       onResume: () {
         if (!isAdding) {
           streamHelper(null, helperCallback, this, null);
         }
       }, onCancel: () {
         stopRunning = true;
       });
   }
 }
 
@@ skipping 83 matching lines @@
   // This is a function that will return a helper function that does the
   // iteration of the sync*.
   //
   // Each invocation should give a helper with fresh state.
   final dynamic /* js function */ _outerHelper;
 
   SyncStarIterable(this._outerHelper);
 
   Iterator get iterator => new SyncStarIterator(JS('', '#()', _outerHelper));
 }