anchor.diff:  
     LibraryBuilder coreLibrary;
   
  +  /// The first library that we've been asked to compile. When compiling a
  +  /// program (aka script), this is the library that should have a main method.
     LibraryBuilder first;
   
     int byteCount = 0;
  
positive.code:   /// compile-time error.
    LibraryBuilder read(Uri uri, int charOffset,
        {Uri fileUri, LibraryBuilder accessor, bool isPatch: false}) {
      firstSourceUri ??= uri;
      LibraryBuilder builder = builders.putIfAbsent(uri, () {
        if (fileUri == null) {
          switch (uri.scheme) {
  
negative.code:         coreLibrary = library;
          target.loadExtraRequiredLibraries(this);
        }
        if (library.loader != this) {
          // This library isn't owned by this loader, so not further processing
          // should be attempted.
          return library;
        }
  
        {
          // Add any additional logic after this block. Setting the
          // firstSourceUri and first library should be done as early as
          // possible.
          firstSourceUri ??= uri;
          first ??= library;
        }
        if (target.backendTarget.mayDefineRestrictedType(uri)) {
          library.mayImplementRestrictedTypes = true;
        }
        if (uri.scheme == "dart") {
          target.readPatchFiles(library);
        }
        unparsedLibraries.addLast(library);
        return library;
      });
      if (accessor != null &&
  