Delete dart2js specific private library code

lib/runtime/dart/async.js

 var async = dart.defineLibrary(async, {});
 var core = dart.import(core);
 var _internal = dart.import(_internal);
-var _js_helper = dart.lazyImport(_js_helper);
 var _isolate_helper = dart.lazyImport(_isolate_helper);
+var _js_helper = dart.import(_js_helper);
 var collection = dart.import(collection);
-(function(exports, core, _internal, _js_helper, _isolate_helper, collection) {
+(function(exports, core, _internal, _isolate_helper, _js_helper, collection) {
   'use strict';
   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, core.Object, [core.Function, core.Object, core.StackTrace]);
   function _registerErrorHandler(errorHandler, zone) {
@@ skipping 2744 matching lines @@
       return;
     }
     Zone.current.scheduleMicrotask(Zone.current.bindCallback(callback, {runGuarded: true}));
   }
   dart.fn(scheduleMicrotask, dart.void, [dart.functionType(dart.void, [])]);
   class _AsyncRun extends core.Object {
     static _scheduleImmediate(callback) {
       dart.dcall(_AsyncRun.scheduleImmediateClosure, callback);
     }
     static _initializeScheduleImmediate() {
-      _js_helper.requiresPreamble();
       if (self.scheduleImmediate != null) {
         return _AsyncRun._scheduleImmediateJsOverride;
       }
       if (dart.notNull(self.MutationObserver != null) && dart.notNull(self.document != null)) {
         let div = self.document.createElement("div");
         let span = self.document.createElement("span");
         let storedCallback = null;
         let internalCallback = _ => {
           _isolate_helper.leaveJsAsync();
           let f = storedCallback;
@@ skipping 3117 matching lines @@
   exports.ErrorCallbackHandler = ErrorCallbackHandler;
   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;
-})(async, core, _internal, _js_helper, _isolate_helper, collection);
+})(async, core, _internal, _isolate_helper, _js_helper, collection);