Move parser and token related libraries into their own subfolder.

tests/compiler/dart2js/scanner_offset_length_test.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.
 
 import "package:expect/expect.dart";
 import 'package:compiler/src/scanner/string_scanner.dart';
-import 'package:compiler/src/scanner/token.dart';
+import 'package:compiler/src/tokens/token.dart';
 
 Token scan(String text) =>
     new StringScanner.fromString(text, includeComments: true).tokenize();
 
 check(String text) {
   Token token = scan(text);
   while (token.kind != EOF_TOKEN) {
     Expect.equals(token.value.length, token.charCount);
 
     var start = token.charOffset;
@@ skipping 22 matching lines @@
 main() {
     check('foo'); // identifier
     check('\'\''); // empty string
     check('\'foo\''); // simple string
     check('\'\$foo\''); // interpolation, identifier
     check('\'\${foo}\''); // interpolation, expression
 
     check('//'); // single line comment
     check('/**/'); // multi line comment
 }