anchor.diff:        Map<String, EpubTextContentFileRef> textContentFileRefs) async {
       Map<String, EpubTextContentFile> result =
           new Map<String, EpubTextContentFile>();
  -    await textContentFileRefs
  -        .forEach((String key, EpubContentFileRef value) async {
  +
  +    await Future.forEach(textContentFileRefs.keys, (key) async {
  +      EpubContentFileRef value = textContentFileRefs[key];
         EpubTextContentFile textContentFile = new EpubTextContentFile();
         textContentFile.FileName = value.FileName;
         textContentFile.ContentType = value.ContentType;
  
positive.code:       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);
      });
      return result;
    }
  
negative.code:   static Future<List<EpubChapter>> readChapters(
        List<EpubChapterRef> chapterRefs) async {
      List<EpubChapter> result = new List<EpubChapter>();
      await Future.forEach(chapterRefs, (EpubChapterRef chapterRef) async {
        EpubChapter chapter = new EpubChapter();
  
        chapter.Title = chapterRef.Title;
  