anchor.diff:     */
     ExportElement _findExport(
         ExportDirective node, List<ExportElement> exports, Source source) {
  -    if (source == null || !_enclosingUnit.context.exists(source)) {
  +    if (source == null) {
         return null;
       }
       for (ExportElement export in exports) {
  
positive.code:    */
    ImportElement _findImport(
        ImportDirective node, List<ImportElement> imports, Source source) {
      if (source == null || !_enclosingUnit.context.exists(source)) {
        return null;
      }
      SimpleIdentifier prefix = node.prefix;
  
negative.code:         return export;
        }
      }
      if (!_enclosingUnit.context.exists(source)) {
        return null;
      }
      _mismatch("Could not find export element for '$source'", node);
      return null; // Never reached
    }
  