Revert "Don't use `LinkedList` in the core libraries anymore."

sdk/lib/internal/internal_sources.gypi

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # This file contains all sources for the dart:_internal library.
 {
   'sources': [
     'internal.dart',
     # The above file needs to be first as it lists the parts below.
     'iterable.dart',
     'list.dart',
-    'linked_list.dart',
     'print.dart',
     'sort.dart',
     'symbol.dart',
   ],
 }