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

pkg/front_end/lib/src/base/syntactic_entity.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.
 
 /**
  * Interface representing a syntactic entity (either a token or an AST node)
  * which has a location and extent in the source file.
  */
 abstract class SyntacticEntity {
   /**
    * Return the offset from the beginning of the file to the character after the
    * last character of the syntactic entity.
    */
   int get end;
 
   /**
    * Return the number of characters in the syntactic entity's source range.
    */
   int get length;
 
   /**
    * Return the offset from the beginning of the file to the first character in
    * the syntactic entity.
    */
   int get offset;
 }