anchor.diff:  /// will be deleted and a new [ToolEnvRef] with a new directory will be created.
   const _maxSize = 500 * 1024 * 1024; // 500 MB
   
  +/// The id of the next [ToolEnvRef] to be created.
  +int _nextId = 0;
  +
   ToolEnvRef _current;
   Completer _ongoing;
   
  
positive.code:   while (result == null) {
      if (_current != null && _current._isAvailable) {
        result = _current;
        result._aquire();
        break;
      }
  
  
negative.code: import 'dart:async';
  import 'dart:io';
  
  import 'package:logging/logging.dart';
  import 'package:pana/pana.dart';
  
  import 'configuration.dart';
  
  final _logger = Logger('tool_env');
  
  /// Subsequent calls of the analyzer or dartdoc job can use the same [ToolEnvRef]
  /// instance [_maxCount] times.
  