Fix cancel behaviour of async* in dart2js.

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:_async_await_error_codes' as async_error_codes;
 
 import 'dart:_js_embedded_names' show
     DEFERRED_LIBRARY_URIS,
@@ skipping 3667 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 [asyncBody] is called on completion of the future (see [asyncHelper].
 void asyncStarHelper(dynamic object,
                      dynamic /* int | js function */ bodyFunctionOrErrorCode,
                      AsyncStarStreamController controller) {
   if (identical(bodyFunctionOrErrorCode, async_error_codes.SUCCESS)) {
     // This happens on return from the async* function.
-    controller.close();
+    if (controller.cancelationCompleter != null) {
+      controller.cancelationCompleter.complete();
+    } else {
+      controller.close();
+    }
     return;
   } else if (identical(bodyFunctionOrErrorCode, async_error_codes.ERROR)) {
     // The error is a js-error.
-    controller.addError(unwrapException(object),
-                        getTraceFromException(object));
-    controller.close();
+    if (controller.cancelationCompleter != null) {
+      controller.cancelationCompleter.completeError(
+          unwrapException(object),
+          getTraceFromException(object));
+    } else {
+      controller.addError(unwrapException(object),
+                          getTraceFromException(object));
+      controller.close();
+    }
     return;
   }
 
   if (object is IterationMarker) {
-    if (controller.stopRunning) {
+    if (controller.cancelationCompleter != null) {
       _wrapJsFunctionForAsync(bodyFunctionOrErrorCode,
           async_error_codes.STREAM_WAS_CANCELED)(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;
       }
@@ skipping 36 matching lines @@
 }
 
 /// 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;
+  Completer cancelationCompleter = null;
+  bool get isCanceled => cancelationCompleter != null;
   bool isAdding = false;
   bool isPaused = false;
   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(body) {
     controller = new StreamController(
       onListen: () {
         scheduleMicrotask(() {
           Function wrapped = _wrapJsFunctionForAsync(body,
                                                      async_error_codes.SUCCESS);
           wrapped(null);
         });
       },
       onPause: () {
         isPaused = true;
       }, onResume: () {
         isPaused = false;
         if (!isAdding) {
           asyncStarHelper(null, body, this);
         }
       }, onCancel: () {
-        stopRunning = true;
-        if (isPaused) asyncStarHelper(null, body, this);
+        if (!controller.isClosed) {
+          cancelationCompleter = new Completer();
+          if (isPaused) asyncStarHelper(null, body, this);
+
+          return cancelationCompleter.future;
+        }
       });
   }
 }
 
 makeAsyncStarController(body) {
   return new AsyncStarStreamController(body);
 }
 
 class IterationMarker {
   static const YIELD_SINGLE = 0;
@@ skipping 93 matching lines @@
   // This is a function that will return a helper function that does the
   // iteration of the sync*.
   //
   // Each invocation should give a body with fresh state.
   final dynamic /* js function */ _outerHelper;
 
   SyncStarIterable(this._outerHelper);
 
   Iterator get iterator => new SyncStarIterator(JS('', '#()', _outerHelper));
 }