anchor.diff:  
     void _addMutatedFieldElement(Expression expression) {
       final element =
  -    DartTypeUtilities.getCanonicalElementFromIdentifier(expression);
  +        DartTypeUtilities.getCanonicalElementFromIdentifier(expression);
       if (element is FieldElement) {
         _mutatedFields.add(element);
       }
  
positive.code: }
  
  bool _containedInInitializer(
      Element element, ConstructorInitializer initializer) =>
      initializer is ConstructorFieldInitializer &&
          DartTypeUtilities.getCanonicalElementFromIdentifier(
              initializer.fieldName) ==
              element;
  
  bool _containedInFormal(Element element, FormalParameter formal) =>
      formal is FieldFormalParameter &&
          formal.declaredElement.toString() == element.toString();
  
negative.code: class PreferFinalFields extends LintRule implements NodeLintRule {
    PreferFinalFields()
        : super(
              name: 'prefer_final_fields',
              description: _desc,
              details: _details,
              group: Group.style);
  
    @override
    void registerNodeProcessors(NodeLintRegistry registry,
  