anchor.diff:          if (libraryElement == null) {
             continue;
           }
  +        _allImports.add(directive);
           _unusedImports.add(directive);
           //
           // Initialize prefixElementMap
  
positive.code:       if (_unusedImports.isEmpty && _unusedShownNamesMap.isEmpty) {
          return;
        }
        // Prepare import directives for this element's library.
        LibraryElement library = element.library;
        List<ImportDirective> importsLibrary = _libraryMap[library];
        if (importsLibrary == null) {
          // element's library is not imported. Must be the current library.
          continue;
        }
        // If there is only one import directive for this library, then it must be
        // the directive that this element is imported with, remove it from the
        // unusedImports list.
        if (importsLibrary.length == 1) {
          ImportDirective usedImportDirective = importsLibrary[0];
          _unusedImports.remove(usedImportDirective);
          _removeFromUnusedShownNamesMap(element, usedImportDirective);
          continue;
        }
        // Otherwise, find import directives using namespaces.
        String name = element.displayName;
        for (ImportDirective importDirective in importsLibrary) {
          Namespace namespace = _computeNamespace(importDirective);
          if (namespace?.get(name) != null) {
            _unusedImports.remove(importDirective);
            _removeFromUnusedShownNamesMap(element, importDirective);
          }
        }
      }
    }
  
    /**
     * Recursively add any exported library elements into the [libraryMap].
     */
    void _addAdditionalLibrariesForExports(LibraryElement library,
        ImportDirective importDirective, Set<LibraryElement> visitedLibraries) {
      if (library == null || !visitedLibraries.add(library)) {
        return;
      }
      List<ExportElement> exports = library.exports;
      int length = exports.length;
      for (int i = 0; i < length; i++) {
        ExportElement exportElt = exports[i];
        LibraryElement exportedLibrary = exportElt.exportedLibrary;
        _putIntoLibraryMap(exportedLibrary, importDirective);
        _addAdditionalLibrariesForExports(
            exportedLibrary, importDirective, visitedLibraries);
      }
    }
  
  
negative.code:       }
      }
      return namespace;
    }
  
    /**
  