anchor.diff:  ///
   /// [message] can be a String or a [Matcher].
   Matcher isClientException([message]) => predicate((error) {
  -      expect(error, new isInstanceOf<http.ClientException>());
  +      expect(error, const isInstanceOf<http.ClientException>());
         if (message != null) {
           expect(error.message, message);
         }
  
positive.code: 
  import 'dart:convert';
  
  import 'package:async/async.dart';
  import 'package:http/http.dart' as http;
  import 'package:http_parser/http_parser.dart';
  import 'package:test/test.dart';
  
negative.code:     return _matcher.matches(parsed, matchState);
    }
  
    Description describe(Description description) =>
        description.add('parses to a value that ').addDescriptionOf(_matcher);
  }
  
  /// A matcher that validates the body of a multipart request after finalization.
  