Fix field override error

lib/runtime/dart/async.js

 dart_library.library('dart/async', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
   'dart/_isolate_helper'
 ], function(exports, dart, core, _internal, collection, _isolate_helper) {
   'use strict';
   let dartx = dart.dartx;
@@ skipping 1242 matching lines @@
   let _removeAfterFiring = Symbol('_removeAfterFiring');
   let _BroadcastSubscription$ = dart.generic(function(T) {
     class _BroadcastSubscription extends _ControllerSubscription$(T) {
       _BroadcastSubscription(controller, onData, onError, onDone, cancelOnError) {
         this[_eventState] = null;
         this[_next] = null;
         this[_previous] = null;
         super._ControllerSubscription(dart.as(controller, _StreamControllerLifecycle$(T)), onData, onError, onDone, cancelOnError);
         this[_next] = this[_previous] = this;
       }
-      get [_controller]() {
-        return dart.as(super[_controller], _BroadcastStreamController$(T));
-      }
       [_expectsEvent](eventId) {
         return (dart.notNull(this[_eventState]) & dart.notNull(_BroadcastSubscription$()._STATE_EVENT_ID)) == eventId;
       }
       [_toggleEventId]() {
         this[_eventState] = dart.notNull(this[_eventState]) ^ dart.notNull(_BroadcastSubscription$()._STATE_EVENT_ID);
       }
       get [_isFiring]() {
         return (dart.notNull(this[_eventState]) & dart.notNull(_BroadcastSubscription$()._STATE_FIRING)) != 0;
       }
       [_setRemoveAfterFiring]() {
@@ skipping 4638 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;
 });