fixes #168, dart:js implementation with a test

lib/runtime/dart/async.js

 dart_library.library('dart/async', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
-  'dart/_isolate_helper',
-  'dart/_js_helper'
-], function(exports, dart, core, _internal, collection, _isolate_helper, _js_helper) {
+  'dart/_isolate_helper'
+], function(exports, dart, core, _internal, collection, _isolate_helper) {
   'use strict';
   let dartx = dart.dartx;
   function _invokeErrorHandler(errorHandler, error, stackTrace) {
     if (dart.is(errorHandler, ZoneBinaryCallback)) {
       return dart.dcall(errorHandler, error, stackTrace);
     } else {
       return dart.dcall(errorHandler, error);
     }
   }
   dart.fn(_invokeErrorHandler, dart.dynamic, [core.Function, core.Object, core.StackTrace]);
@@ skipping 2761 matching lines @@
         let span = self.document.createElement("span");
         let storedCallback = null;
         let internalCallback = _ => {
           _isolate_helper.leaveJsAsync();
           let f = storedCallback;
           storedCallback = null;
           dart.dcall(f);
         };
         dart.fn(internalCallback);
         ;
-        let observer = new self.MutationObserver(_js_helper.convertDartClosureToJS(internalCallback, 1));
+        let observer = new self.MutationObserver(internalCallback);
         observer.observe(div, {childList: true});
         return dart.fn(callback => {
           dart.assert(storedCallback == null);
           _isolate_helper.enterJsAsync();
           storedCallback = callback;
           div.firstChild ? div.removeChild(span) : div.appendChild(span);
         }, dart.dynamic, [dart.functionType(dart.void, [])]);
       } else if (self.setImmediate != null) {
         return _AsyncRun._scheduleImmediateWithSetImmediate;
       }
       return _AsyncRun._scheduleImmediateWithTimer;
     }
     static _scheduleImmediateJsOverride(callback) {
       let internalCallback = () => {
         _isolate_helper.leaveJsAsync();
         callback();
       };
       dart.fn(internalCallback);
       ;
       _isolate_helper.enterJsAsync();
-      self.scheduleImmediate(_js_helper.convertDartClosureToJS(internalCallback, 0));
+      self.scheduleImmediate(internalCallback);
     }
     static _scheduleImmediateWithSetImmediate(callback) {
       let internalCallback = () => {
         _isolate_helper.leaveJsAsync();
         callback();
       };
       dart.fn(internalCallback);
       ;
       _isolate_helper.enterJsAsync();
-      self.setImmediate(_js_helper.convertDartClosureToJS(internalCallback, 0));
+      self.setImmediate(internalCallback);
     }
     static _scheduleImmediateWithTimer(callback) {
       Timer._createTimer(core.Duration.ZERO, callback);
     }
   }
   dart.setSignature(_AsyncRun, {
     statics: () => ({
       _scheduleImmediate: [dart.void, [dart.functionType(dart.void, [])]],
       _initializeScheduleImmediate: [core.Function, []],
       _scheduleImmediateJsOverride: [dart.void, [dart.functionType(dart.void, [])]],
@@ skipping 3079 matching lines @@
   exports.ScheduleMicrotaskHandler = ScheduleMicrotaskHandler;
   exports.CreateTimerHandler = CreateTimerHandler;
   exports.CreatePeriodicTimerHandler = CreatePeriodicTimerHandler;
   exports.PrintHandler = PrintHandler;
   exports.ForkHandler = ForkHandler;
   exports.ZoneSpecification = ZoneSpecification;
   exports.ZoneDelegate = ZoneDelegate;
   exports.Zone = Zone;
   exports.runZoned = runZoned;
 });