Create pkg/front_end/src/base and begin populating with general use classes.

pkg/front_end/lib/src/scanner/token.dart

 // Copyright (c) 2016, 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.
 
 /**
  * Defines the tokens that are produced by the scanner, used by the parser, and
  * referenced from the [AST structure](ast.dart).
  */
-
 import 'dart:collection';
 
+import 'package:front_end/src/base/syntactic_entity.dart';
 import 'package:front_end/src/scanner/string_utilities.dart';
-import 'package:front_end/src/scanner/syntactic_entity.dart';
 
 /**
  * The opening half of a grouping pair of tokens. This is used for curly
  * brackets ('{'), parentheses ('('), and square brackets ('[').
  */
 class BeginToken extends SimpleToken {
   /**
    * The token that corresponds to this token.
    */
   Token endToken;
@@ skipping 1261 matching lines @@
  */
 class _EndOfFileTokenType extends TokenType {
   /**
    * Initialize a newly created token.
    */
   const _EndOfFileTokenType() : super._('EOF', TokenClass.NO_CLASS, '');
 
   @override
   String toString() => '-eof-';
 }