anchor.diff:  // Copyright 2014 The Flutter 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 = 2.8
   
   import 'dart:async';
   
  
positive.code:   Object addHandler(ProcessSignal signal, SignalHandler handler) {
      final Object token = Object();
      _handlersTable.putIfAbsent(signal, () => <Object, SignalHandler>{});
      _handlersTable[signal][token] = handler;
  
      _handlersList.putIfAbsent(signal, () => <SignalHandler>[]);
      _handlersList[signal].add(handler);
  
      // If we added the first one, then call signal.watch(), listen, and cache
      // the stream controller.
      if (_handlersList[signal].length == 1) {
        _streamSubscriptions[signal] = signal.watch().listen(
          _handleSignal,
          onError: (Object e) {
            _handlersTable[signal].remove(token);
            _handlersList[signal].remove(handler);
          },
        );
      }
  
negative.code:       return false;
      }
      // We don't know about this token.
      if (!_handlersTable[signal]!.containsKey(token)) {
        return false;
      }
      final SignalHandler? handler = _handlersTable[signal]!.remove(token);
      if (handler == null) {
        return false;
      }
      final bool removed = _handlersList[signal]!.remove(handler);
      if (!removed) {
        return false;
      }
  