anchor.diff:        Map<String, EpubByteContentFileRef> byteContentFileRefs) async {
       Map<String, EpubByteContentFile> result =
           new Map<String, EpubByteContentFile>();
  -    byteContentFileRefs
  -        .forEach((String key, EpubByteContentFileRef value) async {
  -      result[key] = await readByteContentFile(value);
  +    await Future.forEach(byteContentFileRefs.keys, (key) async {
  +      result[key] = await readByteContentFile(byteContentFileRefs[key]);
       });
       return result;
     }
  
positive.code:       result.AllFiles[key] = value;
      });
  
      await contentRef.AllFiles
          .forEach((String key, EpubContentFileRef value) async {
        if (!result.AllFiles.containsKey(key)) {
          result.AllFiles[key] = await readByteContentFile(value);
        }
      });
  
  
negative.code:       Map<String, EpubTextContentFileRef> textContentFileRefs) async {
      Map<String, EpubTextContentFile> result =
          new Map<String, EpubTextContentFile>();
  
      await Future.forEach(textContentFileRefs.keys, (key) async {
        EpubContentFileRef value = textContentFileRefs[key];
        EpubTextContentFile textContentFile = new EpubTextContentFile();
        textContentFile.FileName = value.FileName;
        textContentFile.ContentType = value.ContentType;
  