Move parser and token related libraries into their own subfolder.

pkg/compiler/lib/src/tree/tree.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 tree;
 
 import 'dart:collection';
 
 import '../diagnostics/spannable.dart' show
     Spannable,
     SpannableAssertionFailure;
-import '../scanner/token.dart' show
+import '../tokens/token.dart' show
     BeginGroupToken,
     FUNCTION_INFO,
     IDENTIFIER_TOKEN,
     KEYWORD_TOKEN,
     PLUS_TOKEN,
     Token;
 import '../util/util.dart';
 import '../util/characters.dart';
 
 import '../resolution/secret_tree_element.dart'
     show StoredTreeElementMixin, NullTreeElementMixin;
 
 import '../elements/elements.dart' show MetadataAnnotation;
 
 part 'dartstring.dart';
 part 'nodes.dart';
 part 'prettyprint.dart';
 part 'unparser.dart';