Modify scanner and parser to be standalone packages.

pkg/dart_scanner/lib/src/token_constants.dart

Index: pkg/dart_scanner/lib/src/token_constants.dart
diff --git a/pkg/dart_scanner/lib/src/token_constants.dart b/pkg/dart_scanner/lib/src/token_constants.dart
index c1078f0a4d1739440010b45a900a8b224ecf64b8..178e73fc8adc7b5df8e213aed479725fc76156dc 100644
--- a/pkg/dart_scanner/lib/src/token_constants.dart
+++ b/pkg/dart_scanner/lib/src/token_constants.dart
@@ -2,9 +2,9 @@
 // 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 dart2js.tokens.constants;
+library scanner.token_constants;
 
-import '../util/characters.dart';
+import 'characters.dart';
 
 const int EOF_TOKEN = 0;