anchor.diff:        }
         // Append parts.
         for (String partUri in contentSource.partUris) {
  -        Source partSource =
  -            context.sourceFactory.resolveUri(librarySource, partUri);
  +        Source partSource = _resolveUri(librarySource, partUri);
           if (partSource == null) {
             throw new StateError('Unable to resolve $partUri in $librarySource');
           }
  
positive.code:       }
        // Append imports and exports.
        void appendLibrarySources(String refUri) {
          Source refSource =
              context.sourceFactory.resolveUri(librarySource, refUri);
          if (refSource == null) {
            throw new StateError('Unable to resolve $refUri in $librarySource');
          }
  
negative.code:   final Map<Source, CacheSourceContent> _sourceContentMap =
        <Source, CacheSourceContent>{};
    final Map<Source, List<Source>> _libraryClosureMap = <Source, List<Source>>{};
    final Map<Source, List<int>> _libraryClosureHashMap = <Source, List<int>>{};
    final Map<Source, List<int>> _sourceContentHashMap = <Source, List<int>>{};
  
    /**
     * Mapping from a library closure key to its [PackageBundle].
     */
    final Map<String, PackageBundle> _bundleMap = <String, PackageBundle>{};
  
    final Map<String, Source> _absoluteUriMap = <String, Source>{};
  
    IncrementalCache(this.storage, this.context, this.configSalt);
  
  