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:   void _writeCacheSourceContents(LibraryElement library,
        [Set<LibraryElement> writtenLibraries]) {
      Source librarySource = library.source;
      // Do nothing if already cached.
      if (_sourceContentMap.containsKey(librarySource)) {
        return;
      }
      // Stop recursion cycle.
      writtenLibraries ??= new Set<LibraryElement>();
      if (!writtenLibraries.add(library)) {
  
negative.code:       List<Source> closureSources = _getLibraryClosure(librarySource);
        List<LibraryBundleWithId> closureBundles = <LibraryBundleWithId>[];
        for (Source source in closureSources) {
          if (getSourceKind(source) == SourceKind.PART) {
            continue;
          }
  