anchor.diff:      }
       return new Credentials(parameters['oauth_token'], parameters['oauth_token_secret']);
     }
  +  factory Credentials.fromJSON(String json) {
  +    return new Credentials.fromMap(JSON.decode(json));
  +  }
   
     String get token => _token;
     String get tokenSecret => _tokenSecret;
  
positive.code:   String toString() {
      return 'oauth_token=$token&oauth_token_secret=$tokenSecret';
    }
  }
  
negative.code: library credentials;
  
  import 'dart:convert';
  
  /**
   * A class describing OAuth credentials except for client credential
   */
  class Credentials {
    final String _token;
  