anchor.diff:  
     @override
     DartType build() {
  -    if (_type != null) {
  -      return _type!;
  +    var type = _type;
  +    if (type != null) {
  +      return type;
       }
   
       for (var typeParameter in typeFormals) {
  
positive.code: import 'package:analyzer/src/dart/ast/extensions.dart';
  import 'package:analyzer/src/dart/element/element.dart';
  import 'package:analyzer/src/dart/element/type.dart';
  import 'package:analyzer/src/dart/element/type_visitor.dart';
  import 'package:analyzer/src/summary2/type_builder.dart';
  
  
negative.code:     }
  
      var builtReturnType = _buildType(returnType);
      type = FunctionTypeImpl(
        typeFormals: typeFormals,
        parameters: parameters,
        returnType: builtReturnType,
        nullabilitySuffix: nullabilitySuffix,
      );
  
      var fresh = getFreshTypeParameters(typeFormals);
      type = fresh.applyToFunctionType(type) as FunctionTypeImpl;
  
      _type = type;
      node?.type = type;
      return type;
    }
  
    @override
  