anchor.diff:  }
   
   class Visitor extends SimpleAstVisitor {
  -  Visitor(this.rule);
  +  final LintRule rule;
   
  -  final LintRule rule;
     DartType boolType;
   
  -  @override
  -  visitCompilationUnit(CompilationUnit node) {
  -    boolType = node.element.context.typeProvider.boolType;
  -  }
  +  Visitor(this.rule);
   
     @override
     visitAssertStatement(AssertStatement node) {
  
positive.code: import 'package:analyzer/dart/ast/visitor.dart';
  import 'package:analyzer/dart/element/type.dart';
  import 'package:linter/src/analyzer.dart';
  import 'package:linter/src/util/dart_type_utilities.dart';
  
  const _desc = r'Prefer using a boolean as the assert condition.';
  
  
negative.code:     }
    }
  
    @override
    visitCompilationUnit(CompilationUnit node) {
      boolType = node.element.context.typeProvider.boolType;
    }
  
    DartType _unbound(DartType type) {
      DartType t = type;
      while (t is TypeParameterType) t = (t as TypeParameterType).bound;
  