anchor.diff:      if (result == null) return null;
       var stylesheet = result.item2;
       var canonicalUrl = stylesheet.span.sourceUrl;
  -    if (!_moduleScopes.containsKey(canonicalUrl)) {
  -      var oldScope = _scope;
  -      _scope = Scope();
  -      _moduleScopes[canonicalUrl] = _scope;
  -      var oldSources = _declarationSources;
  -      _declarationSources = {};
  -      _moduleSources[canonicalUrl] = _declarationSources;
  -      var oldImporter = _importer;
  -      _importer = result.item1;
  -      visitStylesheet(stylesheet);
  -      _checkUnresolvedReferences(_scope);
  -      _importer = oldImporter;
  -      _scope = oldScope;
  -      _declarationSources = oldSources;
  -    }
  +    if (_moduleScopes.containsKey(canonicalUrl)) return canonicalUrl;
  +
  +    var oldScope = _scope;
  +    _scope = Scope();
  +    _moduleScopes[canonicalUrl] = _scope;
  +    var oldSources = _declarationSources;
  +    _declarationSources = {};
  +    _importers[canonicalUrl] = result.item1;
  +    _moduleSources[canonicalUrl] = _declarationSources;
  +    var oldImporter = _importer;
  +    _importer = result.item1;
  +    visitStylesheet(stylesheet);
  +    _checkUnresolvedReferences(_scope);
  +    _importer = oldImporter;
  +    _scope = oldScope;
  +    _declarationSources = oldSources;
       return canonicalUrl;
     }
   
  
positive.code:         _functions,
          _getFunctionReferences,
          _globalDeclarations,
          _sources);
    }
  
    /// Checks any remaining [_unresolvedReferences] to see if they match a
  
negative.code:   References build(Stylesheet stylesheet, Importer importer) {
      _importer = importer;
      _scope = Scope();
      _importers[stylesheet.span.sourceUrl] = importer;
      _moduleScopes[stylesheet.span.sourceUrl] = _scope;
      _declarationSources = {};
      _moduleSources[stylesheet.span.sourceUrl] = _declarationSources;
  