anchor.diff:    ConstFieldElementImpl.forNode(Identifier name) : super.forNode(name);
   
     @override
  -  DartObject get constantValue => _result.value;
  +  DartObject get constantValue => _result?.value;
   
     @override
     EvaluationResultImpl get evaluationResult => _result;
  
positive.code:   ConstLocalVariableElementImpl.forNode(Identifier name) : super.forNode(name);
  
    @override
    DartObject get constantValue => _result.value;
  
    @override
    EvaluationResultImpl get evaluationResult => _result;
  
negative.code:       : super.forNode(name);
  
    @override
    DartObject get constantValue => _result?.value;
  
    @override
    EvaluationResultImpl get evaluationResult => _result;
  