Move parser and token related libraries into their own subfolder.

pkg/compiler/lib/src/dart_backend/dart_backend.dart

 // 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.
 
 library dart_backend;
 
 import 'dart:async' show Future;
 import 'dart:math' show max;
 
 import '../../compiler.dart' show
@@ skipping 31 matching lines @@
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer,
     WorldImpact;
 import '../library_loader.dart' show
     LoadedLibraries;
 import '../mirror_renamer/mirror_renamer.dart';
 import '../resolution/tree_elements.dart' show
     TreeElements,
     TreeElementMapping;
-import '../scanner/keyword.dart' show
+import '../tokens/keyword.dart' show
     Keyword;
 import '../tree/tree.dart';
 import '../universe/universe.dart' show
     Selector,
     UniverseSelector;
 import '../util/util.dart';
 import 'backend_ast_to_frontend_ast.dart' as backend2frontend;
 
 part 'backend.dart';
 part 'renamer.dart';
 part 'placeholder_collector.dart';
 part 'outputter.dart';