anchor.diff:  
     DartdocResults(this.packageMeta, this.packageGraph, this.outDir);
   }
  +
  +String get _dartdocFailedMessage =>
  +    'dartdoc $packageVersion (${Platform.script.path}) failed';
  
positive.code: 
  import 'dart:async';
  import 'dart:convert';
  import 'dart:io' show exitCode, stderr;
  
  import 'package:analyzer/file_system/file_system.dart';
  import 'package:dartdoc/options.dart';
  
negative.code:       },
        (e, chain) {
          if (e is DartdocFailure) {
            stderr.writeln('\n$_dartdocFailedMessage: $e.');
            exitCode = 1;
          } else {
            stderr.writeln('\n$_dartdocFailedMessage: $e\n$chain');
            exitCode = 255;
          }
        },
  