anchor.diff:     */
     void makeValid() {
       var library = entityMap[owner];
  -    if (!library.classes.containsKey(name)) {
  +    if (library != null && !library.classes.containsKey(name)) {
         this.isPrivate = true;
         // Since we are now making the mixin a private class, make all elements 
         // with the mixin as an owner private too. 
  
positive.code: /// Current member being documented to be used for comment links.
  MemberMirror _currentMember;
  
  /// Resolves reference links in doc comments.
  markdown.Resolver linkResolver;
  
  
negative.code:   var reference;
    var memberScope = currentMember == null ?
        null : currentMember.lookupInScope(name);
    if (memberScope != null) {
      reference = memberScope.qualifiedName;
    } else {
      var classScope = currentClass == null ?
          null : currentClass.lookupInScope(name);
      if (classScope != null) {
        reference = classScope.qualifiedName;
      } else {
        var libraryScope = currentLibrary == null ?
            null : currentLibrary.lookupInScope(name);
        reference = libraryScope != null ? libraryScope.qualifiedName : name;
      }
    }
    return new markdown.Element.text('a', reference);
  }
  