anchor.diff:     * all its directly or indirectly imported or exported libraries.
      */
     void _appendLibraryClosure(Set<Source> closure, Source librarySource) {
  +    if (librarySource.isInSystemLibrary) {
  +      return;
  +    }
       if (closure.add(librarySource)) {
         CacheSourceContent contentSource = _getCacheSourceContent(librarySource);
         if (contentSource == null) {
  
positive.code:       List<Source> closureSources = _getLibraryClosure(librarySource);
        List<LibraryBundleWithId> closureBundles = <LibraryBundleWithId>[];
        for (Source source in closureSources) {
          if (source.isInSystemLibrary) {
            continue;
          }
          if (getSourceKind(source) == SourceKind.PART) {
            continue;
          }
  
negative.code:    * Write the content based information about the given [source].
     */
    void _writeCacheSourceContent(Source source, CacheSourceContentBuilder b) {
      if (!_sourceContentMap.containsKey(source)) {
        String key = _getCacheSourceContentKey(source);
        List<int> bytes = b.toBuffer();
        storage.put(key, bytes);
        // Put into the cache to avoid reading it later.
        _sourceContentMap[source] = new CacheSourceContent.fromBuffer(bytes);
      }
    }
  
    /**
  