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:    * Write the content based information about the given [source].
     */
    void _writeCacheSourceContent(Source source, CacheSourceContentBuilder b) {
      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);
    }
  
    /**
  
negative.code:   void _writeCacheSourceContents(LibraryElement library,
        [Set<LibraryElement> writtenLibraries]) {
      Source librarySource = library.source;
      // Stop recursion cycle.
      writtenLibraries ??= new Set<LibraryElement>();
      if (!writtenLibraries.add(library)) {
  