anchor.diff:        TypeSystem typeSystem, AstNode node, bool retainDataForTesting) {
       return FlowAnalysisHelper._(
           const AnalyzerNodeOperations(),
  -        _TypeSystemTypeOperations(typeSystem),
  +        TypeSystemTypeOperations(typeSystem),
           computeAssignedVariables(node),
           retainDataForTesting ? FlowAnalysisResult() : null);
     }
  
positive.code:   final List<AstNode> unassignedNodes = [];
  }
  
  /// The visitor that gathers local variables that are potentially assigned
  /// in corresponding statements, such as loops, `switch` and `try`.
  class _AssignedVariablesVisitor extends RecursiveAstVisitor<void> {
  
negative.code:     return promotedType ?? variable.type;
    }
  }
  