anchor.diff:            imports.putIfAbsent(library, () => generateUniqueName('p'));
         return '$prefix.$originalName';
       }
  -    return renamed[element] = generateUniqueName(originalName);
  +
  +    return renamed.putIfAbsent(library, () => <String>{})
  +        .putIfAbsent(originalName, () => generateUniqueName(originalName));
     }
   }
  
positive.code:   }
  
    String renameElement(Element element) {
      // TODO(smok): Make sure that the new name does not conflict with existing
      // local identifiers.
      if (renamed[element] !== null) return renamed[element];
  
      generateUniqueName(name) {
        while (usedTopLevelIdentifiers.contains(name)) {
          name = "p_$name";
  
negative.code:     }
  
      String originalName = element.name.slowToString();
  
      // TODO(antonm): we should rename lib private names as well.
      if (!element.isTopLevel()) return originalName;
      final library = element.getLibrary();
  