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

pkg/compiler/lib/src/js/rewrite_async.dart

 // Copyright (c) 2015, 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 rewrite_async;
 
 // TODO(sigurdm): Throws in catch-handlers are handled wrong.
 // TODO(sigurdm): Avoid using variables in templates. It could blow up memory
 // use.
 
@@ skipping 168 matching lines @@
   /// Contructor used to initialize the [completerName] variable.
   ///
   /// Specific to async methods.
   final js.Expression newCompleter;
 
   /// Contructor used to initialize the [controllerName] variable.
   ///
   /// Specific to async* methods.
   final js.Expression newController;
 
+  /// Used to get the `Stream` out of the [controllerName] variable.
+  ///
+  /// Specific to async* methods.
+  final js.Expression streamOfController;
+
   /// Contructor creating the Iterable for a sync* method. Called with
   /// [helperName].
   final js.Expression newIterable;
 
   /// A JS Expression that creates a marker showing that iteration is over.
   ///
   /// Called without arguments.
   final js.Expression endOfIteration;
 
   /// A JS Expression that creates a marker indicating a 'yield' statement.
@@ skipping 26 matching lines @@
 
   bool get isSync => async == const js.AsyncModifier.sync();
   bool get isAsync => async == const js.AsyncModifier.async();
   bool get isSyncStar => async == const js.AsyncModifier.syncStar();
   bool get isAsyncStar => async == const js.AsyncModifier.asyncStar();
 
   AsyncRewriter(this.diagnosticListener,
                 spannable,
                 {this.thenHelper,
                  this.streamHelper,
+                 this.streamOfController,
                  this.newCompleter,
                  this.newController,
                  this.endOfIteration,
                  this.newIterable,
                  this.yieldExpression,
                  this.yieldStarExpression,
                  this.safeVariableName})
       : _spannable = spannable;
 
   /// Main entry point.
@@ skipping 337 matching lines @@
   /// Returns the [Future]/[Stream] coming from [completerName]/
   /// [controllerName].
   js.Statement generateInitializer() {
     if (isAsync) {
       return js.js.statement(
           "return #thenHelper(null, $helperName, $completerName, null);", {
         "thenHelper": thenHelper
       });
     } else if (isAsyncStar) {
       return js.js.statement(
-          "return #streamHelper(null, $helperName, $controllerName, null);", {
-        "streamHelper": streamHelper
+          "return #streamOfController($controllerName);", {
+        "streamOfController": streamOfController
       });
     } else {
       throw diagnosticListener.internalError(
           spannable, "Unexpected asyncModifier: $async");
     }
   }
 
   /// Rewrites an async/sync*/async* function to a normal Javascript function.
   ///
   /// The control flow is flattened by simulating 'goto' using a switch in a
@@ skipping 1540 matching lines @@
     loopsAndSwitches.removeLast();
     return condition || body;
   }
 
   @override
   bool visitDartYield(js.DartYield node) {
     visit(node.expression);
     return true;
   }
 }