anchor.diff:  
     void dispose() {
       _isolateNow = null;
  -    if (!_isolate.isCompleted) {
  -      _isolate.complete(null);
  +    if (!_completer.isCompleted) {
  +      _completer.complete(null);
       } else {
  -      _isolate = Completer()..complete(null);
  +      _completer = Completer()..complete(null);
       }
     }
   
  -  void handleDebugEvent(String kind) {
  +  void handleDebugEvent(String? kind) {
       switch (kind) {
         case EventKind.kResume:
           _isPaused.value = false;
  
positive.code: // Copyright 2018 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  // @dart=2.9
  
  import 'dart:async';
  import 'dart:core';
  
negative.code: class IsolateState {
    IsolateState(this.isolateRef);
  
    ValueListenable<bool?> get isPaused => _isPaused;
  
    final IsolateRef isolateRef;
  
    Future<Isolate?> get isolate => _completer.future;
    Completer<Isolate?> _completer = Completer();
  
    Isolate? get isolateNow => _isolateNow;
    Isolate? _isolateNow;
  
    /// Paused is null until we know whether the isolate is paused or not.
    final _isPaused = ValueNotifier<bool?>(null);
  
    void onIsolateLoaded(Isolate isolate) {
      _isolateNow = isolate;
      _completer.complete(isolate);
      if (_isPaused.value == null) {
        if (isolate.pauseEvent != null &&
            isolate.pauseEvent!.kind != EventKind.kResume) {
          _isPaused.value = true;
        } else {
          _isPaused.value = false;
  