Modify scanner and parser to be standalone packages.

pkg/dart_scanner/lib/src/precedence.dart

Index: pkg/dart_scanner/lib/src/precedence.dart
diff --git a/pkg/dart_scanner/lib/src/precedence.dart b/pkg/dart_scanner/lib/src/precedence.dart
index 08475ec6754be67a8899ba73387212f0956f5875..46cee9117c76ebc055456693aa0bc7b112f4fcdc 100644
--- a/pkg/dart_scanner/lib/src/precedence.dart
+++ b/pkg/dart_scanner/lib/src/precedence.dart
@@ -2,11 +2,20 @@
 // 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.precedence.constants;
+library scanner.precedence;
 
-import 'precedence.dart' show PrecedenceInfo;
 import 'token_constants.dart';
 
+class PrecedenceInfo {
+  final String value;
+  final int precedence;
+  final int kind;
+
+  const PrecedenceInfo(this.value, this.precedence, this.kind);
+
+  toString() => 'PrecedenceInfo($value, $precedence, $kind)';
+}
+
 // TODO(ahe): The following are not tokens in Dart.
 const PrecedenceInfo BACKPING_INFO =
     const PrecedenceInfo('`', 0, BACKPING_TOKEN);